checking piston to valve clearances


Staff member
this is an old post I got asked to repost, naturally the first step is to install and correctly degree in your cam,


if you have a minimum of .120 on both the intake and exhaust valve on that #1 and #2 cylinder (one on each bank) I think youll be ok, measuring just that cylinder , if either the intake or exhaust valve has less than .100 ID measure all the cylinders just to be sure!
don,t forget to verify the valve edge to valve pocket clearance along with the valve face too piston deck clearances





this is a common warning posted , on the subject of checking piston to valve clearance, this is from AFR website
if the end user builds an engine without properly verifying he has enough piston to valve clearance (depth and radial clearance both checked). We recommend a minimum depth of .080 on the intake valve and .100 on the exhaust with a radial clearance of .200 minimum around the perimeter of each valve. Clay is recommended to visually verify both depth as well as radial clearance. Just checking depth is not enough...the position of the valve pocket must also be addressed.


it should be rather obvious that youll need to know the exact distance the piston deck sits at TDC ,above or below the block deck surface and the valve notch recess or pop-up dome volume of the piston to do accurate quench or compression calculations
heres a few links with that info.
like always reading thru all the links gives you much more complete info




you need .090 clearance between the retainer to valve seal and a TOTAL of .060 clearance MINIMUM on the valve spring coils from spring bind, or solid stacking the spring coils






inspect carefully and use modeling clay over the whole piston dome to verify potential piston to valve contact areas

Position the cylinder head on the block and use enough head bolts to tighten snugly. Install the lifters and pushrods. If the engine is to be built with hydraulic lifters, they cannot be used for checking clearances. Hydraulic lifters will bleed down and corrupt the measurements using both the clay or indicator methods. A solid lifter must be used for measurements. Install the rocker arms and zero the valve lash. Finger-tight is okay for securing the rocker arm. Note the checker springs. Regular valve springs can be used for the clay method, but it’s much tougher to rotate the crank. With a ratchet and crank socket--or in our case, the timing wheel--rotate the crankshaft in the normal direction several times to ensure that the valves make a good clean impression in the clay. Remember, it takes two crankshaft rotations to completely rotate the camshaft once. There’s a chance the clay may adhere to a valve, so carefully remove the cylinder head so that the clay is not disturbed. Now you can observe the distinct impressions in the clay on top of the piston. Use a razor blade or thin, sharp knife to cut carefully through the center of the impression made by the intake valve.



Carefully lift away the outer half of the clay to reveal the clearance between the piston and valve. Use a machinist’s scale or the depth probe on a dial caliper to get an accurate measurement of piston to valve clearance at the thinnest area of the clay impression.
check clearances especially radial or edge to valve pocket clearances carefully , I try for .100" -to-.120" on both intake and exhaust but Ive run engines with as little as .080 at times with no problems




keep in mind that the intake valve usually starts to open well before the piston reaches TDC on its exhaust stroke, and continues to extend out towards the piston after it passes TDC and follows it down into the bore as it descends on the intake stroke, generally closest contact points are in the 20 degees btdc too 20 degrees atdc, obviously exact potential clearances are dependent on both cam timing and cam indexing
it should be rather obvious that youll need to know the exact distance the piston deck sits at TDC ,above or below the block deck surface and the valve notch recess or pop-up dome volume of the piston to do accurate quench or compression calculations


keep in mind any valve clearance recessed areas must have the areas shrouding flow blended to increase rather than restrict air flow and to reduce the potential for detonation that sharp exposed edges tend to have


flowridge2.png ... g-144.html ... _n9406663/ ... &Itemid=56 ... index.html ... &ct=title#
IF your like most guys you wonder how a cam can have a .600-.700 plus lift and theres obviously no way theres sufficient piston to valve clearance with the piston at TOP DEAD CENTER... TDC in its stroke,
keep in mind that the valve never reaches its full lift figures until the pistons between .400"-.800" away from TDC,
heres a typical radical cam s index card ... e=camshaft

opens Closes Max Lift Duration
Intake 25.0 BTDC 53.0 ABDC
max lift near 104degs past tdc
103 258.0 °
Exhaust 65.0 BBDC 21.0 ATDC 113 266.0 °
max lift near 68degs past bdc
0607phr_03_z+camshaft_basics+example_cam_card.jpg ... ewall.html


isky cams sells piston notching tools

read thru these LINKS CAREFULLY


IF your looking for a quick answer on whats the correct valve to piston clearance, Ive seen guys get by with as tight as .060 intake/.090 exhaust, but in my opinion thats rather tight and allows little to no safety if you ever get into valve float , and you can be 100% certain youll bust parts, miss a shift or get into condition's of valve float eventually, if you race the car.
its just a mater of time before you do.

Ive always tried for .100 intake/.120 exhaust ... g-144.html ... arance.asp ... ear_WS.pdf ... index.html ... index.html

if your using the low pressure check springs its best to roll clay into several 1/4" diam. 3" long strips, resembling soda straws and lay several parallel across the piston valve notches as its far easier to compress that a solid layer of clay. strips tend to work better than small blobs like this picture below, spray both the piston and clay strips with WD40 to prevent sticking


if your thinking, well my new cam has only a bit more lift and duration than the current cam I don,t need to verify clearances, keep in mind pistons vary a good deal in shape as do valve angles so clearance issues are common, do yourself a favor and verify clearances before you start busting parts, by making random assumptions












I still use the strips of modeling clay about 1" square and .2-.25" (two tenths too a 1/4" thick) but one thing everyone forgot to mention so far is that you need to spray the piston and valve and clay strips with WD-40 to ensure the clay does not stick to any parts, otherwise the clay will tend to stick to the valve and piston allowing them to push the clay between them during the compression of its surface by the valve (exactly what its there for) and PULL ON THE SURFACE of the clay as the valve moves away during separation (because the clay tends to stick ever so slightly as the parts pull away from each other if you don,t)which tends to give a false slightly greater than correct clearance measurement
most people tend to tell me Im wrong about that until they try it both ways :grin: yeah the difference is usually minor but five to 10 thousands difference is not rare if the parts are clean and dry versus sprayed with an oil mist


don,t forget you check BOTH the depth the valve extends down into the valve notch on the piston and how close the radial outside edge of the valve extends out towards the edge of the valve clearance notch
Advance cam timing Decreases intake valve-to-piston clearance, increases exhaust valve-to-piston clearance
Retard cam timing Decreases exhaust valve-to-piston clearance, increases intake valve-to-piston clearance
Intake and exhaust: Minimum 0.100-inch depth clearance preferred
Minimum 0.050-inch radially
Piston dome to head: 0.050-inch minimum*
Piston to spark plug: 0.050-inch minimum*
*Steel-connecting-rod engines only

Variables That Affect VALVE-TO-Piston Clearance
Intake center line (cam position)
Valve lift
Rocker ratio
Valve diameter
Piston top (domed, dish, or flat)
Shape and angle of piston valve reliefs
Valve angle
Piston deck height
Head-gasket thickness
Valve float at high rpm (loss of spring control)
Pushrod deflection at high rpm
Angle milling heads

One of the easiest and quickest methods thats a bunch
more accurate than the clay-method , would be to use

Acid-core solder (usually .120" thick )
Resin-core solder (usually .090" thick )
if your wondering how to turn a crank over to do test and diagnosing and cam installs you will benefit from the proper tools
IF you remove the spark plugs and take the car out of gear its fairly safe to turn the engine over manually,
using a breaker bar on the cranks damper retainer bolt,the problem is 99.9% of us are LAZY,
and just try it as the engine sits and eventually we strip the crank bolt or the crank snout threads
If the damper been removed the crank snout socket thats designed for your crank to turn the crank.
and hold the degree wheel while degreeing in the cam will be the route to take

shop carefully and ask questions the sockets and tools don,t fit universally, you'll need an assortment of several OF EACH TYPE ONCE YOU GET INTO ENGINE BUILDING SERIOUSLY
the crank socket like this that can turn the crank safely and firmly and accuratly mount a degree wheel is prefered


there are also crank turning nuts that fit individual crank sizes
most of us are too lazy to remove the damper/balancers


there are over size extra strength damper bolts for sale that are less likely to strip

they sell an engine damper bolt on tool that fits some dampers that allows you to use a 1/2" braker bar rather easily

IT MAKES LITTLE SENSE TO BUY the sockets that don.t have the provision for mounting the cam degree wheel to save a few dollars in my opinion



With the solder-method , you don't actually need a degree wheel
..just the harmonic balancer timing marks and a 6" dial caliper

Turn the engine over till you are coming up to TDC-Overlap
with both the exhaust valve on its way to closing, and
the intake valve beginning to open

Turn the engine till you are about 1/2 inch from TDC ,
then roll out and straighten a piece of acid-core solder about
6 to 8 inches long ....then with headers off , look thru
#1 Cylinder's exhaust port with a pen lite...take the solder
and place it thru the spark plug hole , placing solder
across the Exhaust valve piston notch...then hold solder at that
angle while someone slowly turns engine over to TDC-Overlap
and then past TDC until you "feel" you can pull out solder .

as you turn the engine over at TDC the exhaust valve will
touch or squeeze the solder to the valve-to-piston clearance you keep turning engine past TDC-Overlap,
the solder will be released

remove the solder and look for indentation ...measure with
dial-caliper ..and this is the valve-to-piston clearance !
No clay mess , no clay spring-back , very much accurate than clay-method

Cut a new piece of solder ...and just repeat for intake side !

Note=>can use solder method to check total deck heights accurately !

the best method would be to use a 1.000" dial indicator and
magnetic stand ....bolt a 1/8 thick small plate to valve cover
bolt hole then stick the indicator in place on the steel plate .
(sometimes a SBC fuel-pump cover works great)

attach a degree wheel and pointer and find true TDC ,
then turn engine over till 10 degrees BEFORE TDC-Overlap
to measure Exhaust clearance . (8 -to- 12 deg closest points)

at 10 deg BTDC the 1.000" dial indicator's point on
the flat part of the spring retainer , zero the indicator,
and with the set-screw backed out of the adjuster nut, take
a wrench and turn the adjuster nut till you force the valve to
bottom out against the piston's exhaust notch how much
the dial indicator traveled ...that's your Exhaust clearance

back-off Exhaust adjuster nut back to ZERO point on dial indicator

now, repeat the same procedure on the Intake side ...but this
time turn engine past TDC-Overlap to 10 degrees AFTER TDC
then check Intake clearance .

Note : You should always check valve-to-piston clearance with
a fully assembled valvetrain with the real springs in place
and every rocker lashed ...and ONLY turning the engine over in
the direction of rotation (ClockWise).

using light checker type springs will make you flycut pistons
approx. .030" more than necessary other words, what ever
valve-to-piston clearance you check with lite-springs, when the
engine is fully assembled with the real springs, it will have
approx. .030" more clearance !

using lite-checker springs will be a "SAFER" way to check and will
be a good method to use for a beginner engine builder !!!

A professional engine builder would use the real springs and watch this
effect upon the cam-twist, Jesel belt distortion, push rod flex , ETC.

A professional engine builder would check each and every Cylinder's
clearance..and have detailed computer records of things like
1- Piston Intake and Exhaust fly cut valve notch depth
2- Piston Valve notch fly cut radius , angle + tilt, center-to-bore location
3- Intake and Exhaust valve seat depths on heads, valve margins, etc.

and other things like
4- Total Valve-Notch-Depths

to check the Total Notch Depths ...just place each piston at TDC ,
then place dial indicator zeroed on the top of the valve stem ,
push the valve down till it rests ontop of the piston notch ,
then record this distance !

(need to have springs off , and 2 rubber O-Rings holding up valve
in guides ...when you go to check distance)

As you gain experience and information, you can easily know in advance
what the ballpark valve-to-piston clearance will be with known cam lobes and
rocker ratios , along with cylinder head's valve depth readings , and
piston flycut data .

I try for 0.080 on the intake and 0.100 on the exhaust as absolute minimums but am far happier with 0.120 thousands (just under 1/8") or greater on both!
Ill always trade increased clearance to gain reliability for a slight loss in compression,keep in mind that if you get to tight on those clearances you will be locked into that cam timing and dropping it back (RETARDING the cam) for greater high rpm power or (advancing the cam) for more low rpm torque becomes next to impossible in some cases while you tune the engine combo!



"you'll need to consistently measure the piston quench area and the wrist pin center-line heights on the pistons, keeping in mind that rod and pistons dimensions vary slightly so you'll want to mix/match to minimize the combined differences, placing taller pin height pistons on shorter rods, etc."

pistons can and do rock in the bore very slightly so if you want the most consistent measurements between cylinders , youll use the yellow dot or center of the piston over the pin bore center line,and rock the piston on both edges measuring 90 degrees from the pin center line and average the two results, just be aware that your quench distance can change when the piston rocks slightly at higher rpms as theres definitely a thrust side and ring drag so the piston doesn,t always remain exactly strait in the bore center line









Drawing2.jpg ... -ASSORTED/


Proper push rod length is absolutely critical for peak performance—minimizing bent or broken valve stems, guide wear, and energy-wasting valve side-loading friction. IF YOU CHANGE VALVE TRAIN COMPONENTS VERIFY YOUR ROCKER GEOMETRY, AND ALL RELATED CLEARANCE ISSUES
With the lifter located on the round base circle, position the Push Rod length Checker (make sure you have the Checker with the proper diameter hole) over the stud. Ideally the Checker should contact the top of the push rod and the valve tip evenly at the same moment, should the Checker contact the push rod first, measure the gap between the front of the checker and the valve tip, and purchase a shorter push rod of the correct length. Should the Checker contact the valve tip first, measure the gap between the back of the Checker and the top of the push rod, and purchase a longer push rod

I don,t know many or in fact any engine builder that doesn,t have some good model clay in his tool box for checking clearances

or a can of wd 40 to spray on the valves and clay too prevent clay sticking to parts measured, use good quality modeling clay, some crap like kids PLAY DOW, is SPRINGY and won,t give exact and consistent measurements, I pick mine up at a local arts & craft supply



the cam rotates while indexed by the timing chain at 1/2 crank shaft speed , there are connecting rods designed to provide additional clearance.





don,t forget to verify the cam to connecting rod clearances
a cams VALVE LIFT is determined by the DISTANCE the lifter moves as the cam rotates under the lifter base as it moves from the cam lobe base circle
(the closest the lifter comes to the cams center line)
up to the cam lobes ramp to the lobes peak,
(the furthest the lifter up off or from the cams center line)

your cams lift, is the result of the lifter movement, or distance it travels from the cams base circle, where the valves seated, to the point in the cams rotation where the lifters moved along the ramp surface fully up on the nose of the cam lobe where the valves at full lift.

lets say in this case we compare two imaginary cams
a standard cams base circle is 1.125" and
your cams running on a .900 base circle
both cams have a .560 valve lift and run with 1.5:1 rockers
so both cams will need to move the lifter .374"
that means the standard cam lobe will be 1.125"+.374" or 1.499" from the cams base to the cam lobe nose
that means the small base cam lobe will be .900"+.374" or 1.274" from the cams base to the cam lobe nose
which is significantly smaller,
small base circle cams are generally only used when connecting rod clearance necessitates there use



the BASE CIRCLE IS NEVER A CLEARANCE ISSUE, its the cam LOBES that can potentially contact the edge of the connecting rods or bolts, this is why the rods or rod bolts are machined for additional clearance for the cam lobes as you rather obviously CAN,T machine the cam lobes themselves without destroying their function.
now think about it for a few seconds

the cams lobe lift and the cams LSA or LOBE SEPARATION ANGLE
both have an effect on the cam lobes potential interference with the connecting rods

notice how the rod bolts come close to the cam bearings as the pistons reach top dead canter in the bores







rods designed like the 3 SERIES generally won,t work with stroker cranks while the 2 series usually will

the connecting rods you sellect make a huge differance in the rod to cam lobe clearance, even a small base cam won,t clear some designs, it should be obvious that the connecting rod with the thru bolt has a great deal less cam lobe clearance potentially than the cap screw design next to it., and the cap screw rod probably clears the blocks oil pan rail area easier also
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Staff member
keep in mind swapping heads gives you the option to increase or decrease the compression ratio
of the combo

"(assume on a stock 350 small block)

Speaking in generalities and assuming no other changes, what's the relation between cylinder head combustion chamber size and overall engine compression?

Does just changing heads from a stock 76cc head, to a head with a smaller chamber make that much overall difference on engine c/ratio? Such as a 64cc or 58cc chamber. How about changes on necessary octane requirement?

Just curious to see what can be expected. I've read plenty on what performance gains can be had and what would work best for 350 TPI, but curious to see the other factors may play out.

Ok lets look at it a bit, theres two types of compression ratio, static and dynamic, keep in mind its MOSTLY dynamic compression ratio, that effects your results.
youll gain about 3% in hp increasing the effective static compression ratio one full point, so swapping from a 9:1 to a 10:1 cpr boost torque about 3%
swapping from a 76cc head to a 58cc head is a 1.84:1 cpr change so you can reasonably expect a 1.84 x 3% or a 5.52% boost in torque from that change alone.
if your current engine made 330hp that would jump to about 5.52% higher if the tq curve remained consistent, so you'll see about 350 hp.

one of the the main functions of compression is to pack the fuel/air mix into a tight area for both fast effective ignition and to provide a mechanical advantage for the piston & rod assembly to push against the crank throw,as that mass in the combustion chamber burns and rapidly expands.

lets look at your question, given identical 350 displacement engines with flat top pistons and a common .032 thick head gasket, a .023 deck and 5cc valve notches, heres what your going to see in STATIC COMPRESSION,as a result of combustion chamber changes


keep in mind you want to stay at about 8:1-8.5:1 in DYNAMIC compression

to run common pump gas without getting into detonation
that depends on the fuel octane, cylinder head temp. and several other variables but generally 8.0-8.5:1 dynamic works out well if your going to run mid grade pump gas



heres some differant calculators ... sure.shtml
average the results


viewtopic.php?f=54&t=10028&p=38941#p38941 ... -clearance

IDEALLY you would select the horsepower goal, and the displacement,you'll work with too reach that goal. then the cylinder heads, intake and exhaust are selected that supply the necessary flow rates,in that rpm band, you pick the cam too match the intended rpm band,and flow rates and power range for the application, you then match the compression ratio, to the cam timing too maintain the correct dynamic compression ratio, and you select the matching drive train and gearing to keep the engine IN the matched rpm band the vast majority of the time.
naturally if your limited to a set displacement or compression ratio the other factors must be selected with those limits in mind.






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I saw a neat tip for checking piston to valve clearance on HorsePower TV last weekend. Instead of using clay, they used a Tootsie Roll in the wrapper. The wrapper is wax so it doesn't stick to the piston or valve. They rolled the Tootsie Rolls in their hands for a bit to soften them, put them in the chamber and rolled the motor over, then removed them and put them in the freezer for a while to harden the candy. They then easily measured the rock hard candy with calipers.


Staff member
interesting,(strange acctually....)
every machinest I know has a few oz of non-hardening modeling clay to verift oil pump pick-up to oil pan floor and valve to piston, and piston dome to combustion chamber clearances.
but a bit of WD 40 sprayed on modeling clay strips ( about 1" wide and 1/4" thick and 3" long ),keeps them from sticking. and a razor can cut the valve indentation at its deepest point, cutting it cross wise, a dial caliper ... toview=sku

can meassure, that thickness and youve got the correct info in about 3 minutes.



once EITHER the duration exceeds about 225 @ .050 or the lift exceeds about .520 in a 383 SBC youll probably have clearance issues with stock connecting rods,and the cam lobes, aftermarket (H) or SOME (I) style stroker rods with 7/16" capscrew bolts are stronger and profiled to clear and use of both a small base billet roller cam and stroker profile rods will usually prevent that

actually measuring the piston to valve clearance in a temporarily assembled engine is the only reliable way you can be sure of your results, and that's best done once the cams been degree-ed into place,
Ive generally use a dial indicator,and a degree wheel, but its faster to use the 1/4" thick modeling clay, squares, sprayed with a bit of WD40 to prevent them from sticking, placed in the valve notches on the pistons and rotating the engine by hand with the head temporarily bolted in place, then use a dial caliper to measure the clay after sectioning it with a razor blade

these threads hold more info, and the sub links are very useful



viewtopic.php?f=52&t=528&p=654&hilit=+piston+valve+clearance#p654 ... arance.asp ... rance.html ... index.html ... g-144.html ... ear_WS.pdf










all valve train components made by any and all manufacturers eventually fail or have flaws
the higher the valve spring load rates, and the higher the rpm levels the more stress is induced and the faster wear may occur,
quality control and materials used matters.
sticking to the better name brands, reading and following the instructions, using matched components and checking clearances, use of the correct lubricants at the correct temps and pressure and flow rates, tends to reduce the percentage of failures.
Ill point out that I've done the forensics on quite a few failed cams over the years,
that guys have brought to my shop and Id say about
60% of the failed cam lobe & lifter problems were traced to a failure to check clearances or correct valve train geometry issues , like coil bind, rocker to rocker stud, or rocker to adjustment nut clearance, retainer to valve seal, clearances or rocker geometry, use of the wrong spring load rates for the application ,or failure to check valve train or push rods binding issues like rocker to retainer, push rods binding on guide plates or heads, etc. before they became an issue.
10% were traced to failure to remove metallic or other trash, generated by a previous cam failing from the engines internal oil passages, or failure to carefully clean the engine before installing the new cam, and components, ( use of shrapnel screens and magnets help a great deal in this but can,t remove all trash as some is non-magnetic)
5% to low quality components, or miss matched parts, like the wrong spring load rates for the application, and perhaps
15% of the failures due to using the wrong lubricants , or not nearly enough moly cam lube on the lobes and lifter bases or setting up the oil supply system correctly, or use of a high quality oil and filter, and a failure to change that oil and filter regularly after the first few hundred miles , the remaining
1o% were from unknown causes but more than likely due to a failure to correctly break in the cam, or properly adjust the valves before the engine break-in process or carefully check and re-adjust the lifters rapidly during the break-in process
cam quality varies a great deal
Isky claims that the Comp XE cams violate the 47.5% rule. The 47.5% rule applies to flat tappet cams for SBCs with 1.5 rockers but the concept is still the same for other configurations where the designs are "on the edge" or "over the edge" for lobe intensity. For 1.5 ratio SBCs, the duration at .50 must exceed 47.5% of the total valve lift or your asking valve train problems. For example, take a Comp Cams Magnum 280H, with 230 duration and, 480 lift...230/.480 = 47.9% which exceeds 47.5% therefore would not pose a threat to components. We do not regularly hear about the older, safer HE and Magnum designs rounding off lobes anywhere near as often as the XE cam designs. Unfortunately, some of the Comp Cams XE dual pattern lobes break this 47.5% rule on the intake side so they are likely to be problematic. The design has "steeper" ramps that are too quick for durability and reliability according to other cam manufacturers. They will wipe lobes in a heart beat especially if you have not followed the proper break-in procedure. Other designs are more forgiving during break-in and less likely to fail.
ones got 215 duration with .450 lift,215/450=.47.7%
the others got 215 duration with 510 lift/215/500=43%
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Staff member

youll need a few basic tools and a good understanding of what your doing, but its certainly not all that difficult.

I was asked where to get a CHEAP degree wheel

heres one you can print out and save for engine builds on the engine stand
(put curser on an click)

BTW you CAN advance or retard the roller timing chain its done bye drilling out the cam index pin hole in the timing gear and installing an off set bushing


you could buy these from summit racing or similar parts from jegs

this is 180 degrees out (the distrib rotor points at cylinder #6, so before you drop in the distrib rotate the crank 360 degrees bring both marks to the 12 o-clock location, then drop in the distrib pointing to cylinder #1, and adjust ignition timing from that point sells this KIT
Comp Cams #249-4796 Cams/249/4796/10002/-1

and you can buy these

MOR-62191 $44 (wheel)

MOR-61755 $47 (SBC)
MOR-61756 $47.(BBC)crank sockets

SUM-900188 $17 (piston stop, head off)
SUM-900189 $6.95(piston stop, head on)

TFS-90000 $94.95 (degree kit)

youll also want two flat tappet solid lifters and two weak check springs

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Staff member
keep in mind youll need to check the cam lobe to connecting rod clearances as they pass very close to each other on some cylinders at some points in thier rotation,finding out if youll need to grind clearances on the upper edge of some rod bolts needs to be verified, durring assembly,
the answer depends BOTH on the max lift, durration and LCA of the cam and the design of the connecting rods outer profile, IE wher it sticks out and how far it sticks out away from the crank journal. its the upper rod bolt area near the crank journal that tends to hit the cam lobes on sone cylinders, and you should verify about a .060 clearance between lobes and connecting rods durring the assembly process, AFTER degreeing in the cam, I usually use a large paper clip as a gauge, if it easily passes between the lobe and the rod bolt at thier closest point thier tangent rotational arcs youll be ok.

GOSFAST posted this great photo to illustrate the differance between rod designs


it should be obvious that the connecting rod on the right has some extra clearance to the cams lobe rotation arc.

rods designed like the 3 SERIES generally won,t work with stroker cranks while the 2 series usually will


Staff member
I posted this in the chit chat section and got a question
[color]"whats a valve spring mic and hows it used?"[/color]

I got what I requested from SANTA ,two, new valve spring mics, my old ones were fairly worn and hard to read



the valve spring compressor design you use and the use of a large strong magnet can significantly reduce the tendency of those little S.O.B,s from poping off to parts unknown, having the correct tools helps a great deal
$12 or so spent on a decent magnet placed next to the valve keepers during the removal process tends to significantly reduce the chances of lost valve keepers

valve spring compressors
this cheap type is well known for loosing valve keepers unless a strong magnets used










]"whats a valve spring mic and hows it used?"

your questions very common, its used to measure the distance or INSTALLED HEIGHT between the spring shims upper surface and the retainers lower surface that the valve spring occupys normally, the springs removed and the mics installed in its place, the retainer and valve keepers are put in place and the valve mics extended to raise the retainer and seat the valve in the head, thus you get the correct space or distance between the or INSTALLED HEIGHT between the spring shims upper surface and the retainers lower surface that the valve spring occupys normally,shims placed under the valve spring allow you to shorten the installed height, btw the thicker shims are placed on the upper end of the need to verify clearances and load rates to get the proper loads and not get into clearance issues.
most valve springs have a spec card that indicated their intended load rates and installed height an coil bind heights, and load rates are checked with this tool, and a dial indicator.
keep in mind coil bind is not the only clearance issue, retainer to valve seal, and retainer to rocker amoung other clearances along with coil bind and installed height must be verified, lets look at a spring, ... of_16.html

Premium Nitrided Beehive LS1 Spring
OD: 1.290"
130 lbs @ 1.800"
337 lbs @ 1.150"
Lift: 0.650"

notice coil bind is at 1.080 "
so youll install shims to get the height correct at 1.800 ... toview=sku


YOULL NEED to check ALL engine clearances very carefully,BEFORE YOU START AN ENGINE!!


IT takes HUNDREDS of LBS of force to bust off a valve,or rocker stud, or bend a valve, etc,


yet IVE seen guys start engines , bust parts and just replace the busted parts without looking for the cause and correcting the problem before trying to run the engine a second or third time,
ONE the definitions of insanity is doing the same thing over and over ,...... getting the same results .....but expecting DIFFERANT RESULTS......yet continueing, to expect differant results

start with checking COIL BIND height and max lift, clearances, you need in excess of .050 thousands of an inch more than the max lift on the cam compresses the valve train, before reaching coil bind at a MINIMUM

ROCKERs ARC of rotation clearance
retainer to valve seal
pushrod to pushrod slot or guide plates
rocker to spring retainer
and yes check the valve train geometry all along its rotational arc.
check piston to valve clearance
check piston to head clearance
check valve to cylinder wall clearance
check cam to connecting rod clearance
check crank counterweight to piston skirt clearance,
and be darn sure the cams DEGREED IN CORRECTLY, BEFORE you start the ENGINE.......
NO YOU CAN,T, JUST RELIE ON THE DOT-TO-DOT INDEX , on some timing gears, or the cam itself to be correct!
you WILL NOT be the first guy to be 100% sure you did everything correctly only to find out later YOU WERE WRONG! thats part of the LEARNING EXPERIANCE and WHY you learn to DEGREE IN CAMS with a dial indicator and degree wheel and youll LEARN WHY you check clearances,......experiance is expensive >>>LEARN FROM THE MISTAKES OTHER MAKE...IT MUCH CHEAPER THAT WAY!!!



just because YOU don,t feel it bind ,when its rotated by hand thru several rotations ,in no way indicates the clearances are correct!

BTW I only learned to start checking things carefully after screwing things up just like most of you gentelmen, IM certainly not immuned to screw ups, but I DO TAKE NOTES AND TRY NOT TO REPEAT MISTAKES
yes it tends to take a little more effort to do things correctly, sometimes some research, sometimes new tools, are needed, but you may be surprised at the satisfaction you get from knowing its been done correctly



minimum safe clearance?
that depends on the quench distance deck height and cam lift duration and lsa , how the cams degreed in and your rocker ratio
about .100 on the exhaust and about .080 on the intake valve to piston will normally be about MINIMUM required clearance and at lifts over about .480 and duration over about 220 degs @ .050 lift you better start checking carefully, and remember the valves come closest to the piston NEAR 10-20 degrees before and after TDC, not AT TDC

reading thru these threads should help





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Staff member
41- Security Clearance: Measuring Piston-to-Valve
Category: Tech Talk —
Published in National Dragster

Written by David Reher

With apologies to my sister the veterinarian, there is more than one way to skin a cat and more than one way to measure piston-to-valve clearance. I’m not an authority on cat skinning (nor do I want to be), but I do know about the importance of proper valve clearance in a racing engine. I see hundreds of engines come through our shop every year, and I’m alarmed by how many engines assembled by do-it-yourself builders have incorrect piston-to-valve clearance.
you might want to pull a spark plug and use a flex micro cam to inspect inside the cylinder

this is also a good inspection tool ... pv618.aspx

GOOGLE PV-618 and PV-636

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PV-618 and PV-636 models have .23” diameter, flexible, non-obedient cable. Durable carrying case included.
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Cable Length: 18" (457.2mm)
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Weight of Scope w/ Carrying Case: 1 lb. 5.7 oz (615 g)
Field of View: 40°
Optimal Viewing Distance: Min. .8" (20mm); Max. is dependent upon ambient lighting conditions.
Lamp Volts: 2.7 volts (Halogen)
Power Source: 2 AA batteries (not included)
Pressure Necessary to Operate Lamp: 2.9 avg. p/psi; 3.2 max. p/psi

heres a cheaper version that works with a lap top computer


GCA 5.5mm Diameter USB Waterproof 6 Led Endoscope Borescope Inspection Wire Camera (5m)
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There are two ways to get it wrong. The first is usually catastrophic: If you have insufficient clearance, the pistons hit the valves, followed by the predictable parts damage. On the other hand, if you have too much clearance, the engine will run, but it won’t achieve anything close to its performance potential. In either situation, you’ve wasted time and money that could have been saved by checking piston-to-valve clearance properly.

Here is the method we use at Reher-Morrison Racing Engines. Not everyone may agree with our technique, and that’s all right. I am convinced, however, that it is the best way to achieve repeatable, accurate results.

First, put away your clay and your light-tension checking springs. The time-honored practice of mocking up a motor and putting clay in the valve pockets to measure clearance introduces too many variables to be trustworthy. The amount of clay on the piston top, the density of the clay, the effect that the clay has on the relative positions of the valve and piston, and the difficulty of measuring the thickness of the compressed clay accurately are just a few of the sources of potential error with this method. You don’t use clay to measure piston-to-wall clearance and bearing clearance, and you shouldn’t use it to measure valve clearance either.

You must use the same components when checking valve clearance that you intend to use when you assemble the engine. This includes the same lifters, the same pushrods, the same rocker arms, and the same valve springs. Light-tension checking springs simply can’t duplicate the load and deflection that the valvetrain experiences with stiff race springs. The difference in actual valve clearance between checking springs and race springs is typically .020 to .030-inch. If you set up your engine with checking springs with .075-inch intake valve clearance, the actual clearance with race springs will be closer to .100-inch.

The first step in the Reher-Morrison method is to determine whether the valve pockets are located properly. A discarded valve that fits your cylinder head makes an ideal tool. Cut off the head of the valve and turn the stem to a point. Preassemble the engine with your bare cylinder heads (remember to use a previously compressed head gasket), put masking tape on the ring lands to center the piston in the bores, a bring the piston to 10 degrees before or after Top Dead Center (it doesn’t matter which at this point) and drop your homemade punch into the intake and exhaust guides. Give the punch a gentle tap to mark the valve stem centerline on the piston and then remove the head.

To check the valve pocket location, remove the cylinder head and set a pair of calipers to the radius of the valve head (for example, for a 2.500-inch diameter valve head, set the calipers at 1.250-inch). With one point centered on the punch mark, swing the other point around the valve eyebrow. If the caliper hits the edge of the valve pocket, so will the valve. I recommend a minimum of .050-inch radial clearance between the edge of the valve and the pocket.

If the valve relief is located properly, you must then check its angle. Again, a discarded valve with the proper stem diameter makes an excellent checking tool. Weld or epoxy a small steel ball onto the edge of a steel valve. Mark the tip of the valve stem with a notch in line with the ball as a reference point. If your engine has two different valve angles – a big-block Chevy or Cleveland Ford, for example – you will need to make intake and exhaust checking valves.

Insert the checking valves into the bare head and install the head on the preassembled short block. Bring the piston to 10 degrees before or after TDC. Put a dial indicator on the tip of the valve stem and slowly rotate the valve with your fingers. If the stem rises and falls as the ball travels around the valve notch, the angle of the relief is incorrect. You can draw a “road map” by noting the position of the reference notch as you turn the valve. For example, if the valve stem rises near the top of the dome and falls at the bottom of the valve notch, then the angle of the valve relief is too steep. Using this technique, you can precisely determine how much material must be machined to correct the angle of the valve relief.

After you have established that the valves have enough radial clearance in their respective notches and that the angles of the valve reliefs are correct, you are finally ready to check piston-to-valve clearance. Assemble the short-block and cylinder heads with the valvetrain components you intend to use. Adjust the valve lash, set up a dial indicator on the valve spring retainer so that its plunger is parallel to the valve stem, and bring the piston to 10 degrees BTDC. Compress the spring on the exhaust valve and measure the movement required for the valve to contact the piston. (We used a tool similar to a valve spring tester with a solid bar instead of a flat spring to compress the valve spring.) Move the dial indicator setup to the intake valve, rotate the crankshaft to 10 degrees ATDC, and repeat the procedure.

This procedure and a little patience will ensure that your engine’s piston-to-valve clearance is measured correctly. The ideal clearance dimension for your combination will depend on the weight of your engine’s valvetrain components (especially whether you use steel or titanium valves), the maximum rpm, the tension of the valve springs, the characteristics of the camshaft, and other factors.

In most instances, off-the-shelf pistons have valve pockets that are too deep and provide much more valve clearance than is really necessary. This is perfectly understandable, because the piston manufacturers can’t anticipate every possible combination of cylinder head, camshaft, block height, valve height, gasket thickness, etc. They don’t want to hear from an angry customer who crashed all the valves in a new engine, so the piston makers typically machine the reliefs in shelf-stock pistons with clearance for the worst case scenario. Then to compensate for the oversize valve reliefs, the piston dome is made taller to produce the advertised compression ratio.

The downside of this situation is that overly generous valve reliefs cost horsepower. For example, a 2.50-inch diameter valve pocket that is .100-inch deeper than it really needs to be has a volume of 8 cc’s. That much volume at TDC can significantly lower the compression ratio, reducing efficiency and power. It’s much better to have the proper piston-to-valve clearance and a shorter dome that doesn’t intrude as far into the combustion chamber.

Imagine two engines with the identical compression ratio. One has pistons with valve reliefs that are too deep and domes that resemble Mt. Everest; the second has pistons with optimized valve reliefs and shorter, rounded domes. Both engines have the same volume above the piston at TDC, but the engine with the proper valve reliefs and shorter domes will have a substantial horsepower advantage.

Measuring piston-to-valve clearance properly is one of the basic operations that every novice engine builder should master. It’s not as sexy as flow bench testing or as high-tech as running dyno simulations on your laptop, but it is an absolutely essential step in building a reliable and powerful racing engine.
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The Grumpy Grease Monkey mechanical engineer.
Staff member
Looking at the damage,and only having a few pictures..
it might be results of piston to valve contact,
or a rod bolt stretching ?
detonation? a valve spring breaking?
valve keepers popping out?
a timing chain snapping?
a cam gear coming loose ?
at high rpms??
theres little to suggest it was lack of oil flow?
Id need to inspect the damage carefully to determine the cause,
and once you start trying to compress , non-compressible valve train parts with pistons at higher rpms things get darn expensive and cascade into really ugly results in seconds
but you have to admit thats unlikely to buff out! pics

yet its a good reminder as to why the engines are best assembled with quality components, you can afford, and why you need to check clearances very carefully, the $60 you might save on an import timing chain, or the $150 on cheap valve springs, won,t save you a great deal if they fail after 6 12 months use, and the result is similar.

look at the two rod bolts the top one obviously stretched, this is a very common cause of rod bearings spinning if the engine is pushed to hard to higher rpms than the components can handle.
saving $60 by reusing stock rod bolts that stretch won,t seem like a great bargain, if something like this happens. once a rod bolt stretches and the bearing clearances open up the piston tends to impact the cylinder heads , bend valves, connecting rods bend and a rapid cascade of very expensive parts failure results




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The Grumpy Grease Monkey mechanical engineer.
Staff member

I recently helped one of the local guys assemble a 496 BBC 4.25" stroke
engine using 6.385: SCAT rods with 12.7:1 pistons thats being built
I watched him start to install the first piston with the dome facing the lifter gallery, or upper side of the cylinder... I waited until he had started to install the connecting rod cap on that first rod and asked him to rotate the crank to TDC I handed him a bridge and a dial indicator





and asked him to verify the deck height.....honestly I had a real hard time not laughing.....I think most of us realize that we all made similar mistakes.... no harm done (YET).
after a few seconds, I suggested he check the spark plug clearance with the head just laid on the block with an old head gasket...yeah, he caught the mistake then!

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Staff member
Is there software that could calculate piston-to-valve clearance? Who provides such software and does it require knowing the valve lift every couple of degrees to be accurate ???


Active Member
Most of us have made mistakes like he did with the reversed dome pistons! Maybe he had a plan to install the heads with the plugs pointing up for easy service??? LOL!

I recently helped one of the local guys assemble a 496 BBC 4.25" stroke
engine using 6.385: SCAT rods with 12.7:1 pistons thats being built
I watched him start to install the first piston with the dome facing the lifter gallery, or upper side of the cylinder... I waited until he had started to install the connecting rod cap on that first rod and asked him to rotate the crank to TDC I handed him a bridge and a dial indicator





and asked him to verify the deck height.....honestly I had a real hard time not laughing.....I think most of us realize that we all made similar mistakes.... no harm done (YET).
after a few seconds, I suggested he check the spark plug clearance with the head just laid on the block with an old head gasket...yeah, he caught the mistake then!