cleaning piston ring grooves, and related info


Staff member
now if there's any job that's frequently ignored or avoided during an engine rebuild , its more than likely ,cleaning pistons and cleaning the grooves its high on the list!
(take advantage of millions of hours of previous engine builds, by skilled and experienced people, and the knowledge gained)

A couple days of
reading the linked and sub-linked info
could save you a great deal of wasted time and money
and money spent on wrong or un-necessary parts
read the links and sub links below
a day or so spent doing reading and research,
(reading links and threads)
will frequently save you thousands of dollars and weeks of wasted work.
if you want a fast dependable car you will need to either do the research required to know exactly how and why things should work, or pay someone else to do the work that has taken that time and effort.

one factor I find amazing is how few guys realize that the rings MUST have space both above the ring and behind the ring in the piston grooves simply because its the hundreds of PSI of cylinder pressure that first forces the ring into the bottom of its groove then the pressure gets behind the ring and tends to expand it and hold it into the bore that is a huge factor in how effective the ring seals combustion pressure in the combustion chamber, if the clearances are filled with carbon build up the rings loose a great deal of there ability to seal.













ideally the pressure above the piston gets behind the top compression ring and increases the force holding the ring face to the bore surface, noticeably;y more than the ring tension alone can do.


types of piston dome configurations

most used pistons need to be carefully inspected and cleaned, soaking them over night in a carbon solvent like a mix of carb cleaner and diesel fuel and using a stiff plastic brush will do a good job on mildly dirty pistons with some "elbow grease" considerable rubbing but you need to use the proper size groove cutting tool to remove the built up carbon from the rear area of the piston grooves at times and you don,t want to enlarge or damage the grooves surfaces because that potentially has a huge effect of ring seal.
rings must be able to retract fully flush with the pistons outer diam. without the ring ends butting or there being no room for high pressure gasses to get in behind the ring to help them seal on the bore.
having access to a decent parts washer HELPS

step one
make damn sure the rings you buy are designed for and correctly fit the piston groove design your working with, and you understand the clearance issues, those change with the intended heat range and engine combo, example hyper eutectic pistons require a good deal larger end gap than forged pistons, and use of nitrous will require a larger clearance still.

step two
make sure the rings are properly installed with the correct side facing up and the correct end gap has been measured and checked carefully,

step three
make sure the top ring gets placed in the top groove and the second ring gets placed in the second groove because in many cases THEY ARE NOT INTERCHANGEABLE

its just not that important in most guys list of things to do, but you might be very surprised at the difference in performance clean and properly installed rings in properly prepared ring grooves have on performance results.
now Im sure most guys say ,"why bother a broken piston ring can be used to scrape out the carbon build up where the old rings were after they are removed and soaking in diesel fuel for a few hours tends to loosen the baked on carbon fouling.
what many guys don,t understand is its not just the tension the piston rings exert on the bore walls that seats and seals the rings to the bore wall, but high pressure gases that force the ring to the floor of its groove and get behind the ring and press it out against the bore that do much of the sealing on the top piston ring, a job that can,t function if the correct clearances don,t exist or if those clearances are filled with carbon or oil sludge, or varnish from burnt oil residue,
so use of a cleaning tool , the correct back clearance, room for the high pressure gasses to get behind the top ring and proper oils with high detergents that burn with little ash residue are critical for proper long term function.
naturally the ring selected and its design will effect the proper ring seal, the cylinder wall hone and the bore condition will also need to be considered.
get the ring groove to large and inconsistent and the ring flutter/ or movement will hammer the ring groove, get it to small and there's a good chance the ring will bind in the groove, if there's not enough clearance behind the ring it tends to seal poorly, if the rings not end gaped correctly it may bind in the bore and yank the top of the piston off or at least crack the ring lands

soaking a piston you've removed from an engine in a 5 gallon bucket of 50%/50% mix of diesel fuel and carb cleaner for a few minutes to a 1/2 hour , before you start cleaning the crud off it usually helps a great deal to soften the crud built up on it
BTW rings generally have a mark or (DOT) on the side that faces up or a bevel that faces up on the inner upper rim

remember that when you go to re-install the compressed piston rings, and piston in the engine block,bores that dunking the piston in MARVEL MYSTERY OIL , just before, its slid into the ring compressor will coat the rings and bore contact areas enough to prevent many small problems that insufficient lube might case


viewtopic.php?f=53&t=247 ... index.html ... rc4032.pdf ... rc2618.pdf ... mance.aspx ... up_ID=1596

cheap but functional


ringgroovetool.jpg ... ProdID=237

viewtopic.php?f=53&t=2795 ... 140194.htm ... sPistRings


ball hones are fast and easy, to use but have the problem that they easily follow non-round cylinder surfaces, rather than tend to keep the cylinders surface flat and concentric like the bar hone design

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Staff member
Re: cleaning piston ring grooves

when your building a daily driver street engine combo, durability is the main concern,the best and most intelligent route is to select the pistons that match your application the best, getting the correct compression ratio and clearances and use the rings that the piston manufacturer suggests after the blocks honed with the correct deck plates that simulate the stress distortion the block has with the torqued head and main cap bolts.
obviously the piston ring groove clearances, depth and width and cylinder honed surface finish should effect your choices as should the conditions under which the engine will operate, ring gaps and the style of wings change with the expected heat and type of piston alloy and rpm levels ETC.
when your building a race engine youve got more options but durability, heat resistance,oil control and cost must remain high on the list

buy and use the correct piston groove cleaning tool, if your one of the guys that think scrapping out crud in a piston ring groove with a broken ring will give good results, I can assure you close detailed inspection will show minor scratches and ring groove damage that reduces the ring seal efficiency



checking groove clearance

checking ring end gap ... index.html

you have the option of ordering slightly over sized rings and carefully file fitting the end gaps,
no mater what size rings or end gap , you use the rings will normally take 5-20 minutes or so to lap into and perfectly seat and fit the bore walls
obviously the closer they match the application on start-up the more rapidly you could expect that to happen.
remember a properly honed bore using deck plated helps the process a great deal.
btw heres typical damage, resulting from high heat expanding the rings , with the gaps set to tight,until the ends bind and the piston ring grooves or rings at times are busted as a result of the rings expanding,and binding in the bores

if you find the rotating assembly is more difficult to rotate than you expected, you may want to verify some clearance issues that get over looked at times,
theres also some, other potential issues,
theres a slight potential for the piston wrist pins too not rotate effortlessly in the piston pin bores ,

that may add to the difficulty in rotating the assembly in the block.
the piston rings must have vertical and back clearance in the piston ring grooves



most piston compression rings have a dot on the upper surface to indicate the side designed to face the top of the piston

ideally the pressure above the piston gets behind the top compression ring and increases the force holding the ring face to the bore surface, noticeably more than the ring tension alone can do.






Piston Ring Groove Clearance
Pistons are grooved to fit rings that seal the cylinder’s compression and allow for lubrication of the cylinder walls. Piston rings come in a set. There are two compression rings. The top ring is affected by the most cylinder compression pressures. The second compression ring reinforces the top ring. The third ring down is the oil ring. It controls lubrication between the piston and cylinder bore.


Place the new ring into the top piston groove, and then place a feeler gauge into the gap between the new ring and the upper land. Move around the pistons groove and obtain a few measurements. Compare this reading to specifications. If this reading is too much and the gap is too large, the piston must be replaced. The top ring takes the most compression. This causes the ring to slap against and wear the lands in the piston groove.



and of course the pistons must have the correct piston too bore clearance. and connecting rod can only be installed facing one direction






bearingoffset2.jpg ... talog.aspx ... mance.aspx ... ctions.pdf ... _Types.pdf

viewtopic.php?f=53&t=2795&p=8966#p8966 ... 0Rings.htm ... CBMQ9QEwAw ... ion_18.htm
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Active Member
Grumpy I am having trouble figuring the correct ring gap. Pistons are 4.030 speed pro 12cc dished 9.5 compression.They recommend .016-.018 and .020-.022 for top and second rings . And .020-.022 and .022-.024 if its modified street, What would determine modified street? The rings are perfect circle they recommend .016-.018 top and .020-.022 second , but step it up also for modified street. The 383 motor is iron 906 vortec heads,Lt4 hot cam,1.6 full roller rockers, 750 eldebrock and gmpp intake #12486570. Thanks Dusty


Staff member
its always a very good idea to call both the piston and piston ring manufacturers for their advice, and ask questions if they are not giving clear answers.
rings expand with heat and the higher heat generated during high loads and high rpms tends to reduce the clearance gap more than in a common passenger car application used for daily transportation,
but Ive generally found its better to error on the larger end of the gap size tolerance range than to get the gap to tight, I normally .005 x the bore size in inches,on the top compression ring since it sees higher heat levels and .0045 on the second compression ring, on non nitrous builds, for example, for a 4.03 bore sbc 383 thats a .020 top ring gap, and .018 for the second ring, testing has shown now real power loss, or lack of compression until the gaps exceed about .050, so its not a super critical dimension as long as its large enough to prevent the ring ends from butting and locking the piston in the bore which under extreme heat will tend to pull the top ring land off the piston, which almost instantly results in a catastrophic engine failures






measure carefully as the piston groove depth and back clearance must match the rings you use or youll have major problems
I'd also point out that the clearance in the piston ring grooves and back-spacing has a huge effect on the way the rings seal.if the oil is not frequently changed sludge build-up behind the rings greatly reduces the ring seal efficiency.





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Staff member
91TransAmGta said:
Got a 350 short block here with eagle crank cut 10/10 new bearings for main and rods, new rods, forged domed pistons. with 64cc 2.02 1.95 camel heads. compression is at 10:1 but i have a question regarding my current pistons are they still safe to continue using for like another 6 months? I was putting a 150 shot and i took the system out i wont be using nos anymore. so is it safe to use this motor for another 6 months to a year? as a daily driver and maybe it will see the track every other weekend. has a comp cam magnum 280H cam with comp cam lifters and 1.6 roller rockers. double timing chain. its a flat tappet cam.

if you didn,t damage them already theres no real reason they won,t work fine for several years, detonation and extreme heat damages pistons not necessarily the result of nitrous , yes nitrous can cause problems if not properly used but its not always going to damage pistons, in fact Ive used nitrous for years with zero issues on my vette , pictures would be really helpful?

91TransAmGta said:
heres pictures .




if your pistons have obvious visual damage you probably have more you can,t easily see also, those are obviously heat and detonation damaged , Id suggest replacement of at least the two that are obviously significantly damaged, rings can be damaged as can piston grooves


when pistons get hot like the two, showing the most damage posted above, pictured ,there can be damage you can,t easily see without further dis assembly
pistons can take a great deal of abuse and yes theres a decent chance you could get away with driving the engine for a few weeks if you first carefully inspect further, and find no more extensive damage , but I,d suggest limiting the rpms and stress and babying the engine until you get it rebuilt, because that damaged surface makes further damage much more likely. in fact your obviously pushing things and yes theres a limited chance it could get rapidly worse.
yes I knpow you want to install new head gaskets, the intake ETC, and drive it so you have some transportation, its a risk,
look most of us are working on a very limited budget, and I know exactly what prompted the question, but those damaged pistons will not work over the long term

91TransAmGta said:
heres pictures
Great news my uncle gave me a new set Keith Black flat top pistons. .30 over so i need to get it bored out now. and does a performer rpm intake along with a 750 vacuum secondary sound good here? compression will be at 9:1 i think with flat top and 64cc heads

sounds like you have a good plan that should work, and yes that intake and carb should work ok.