Thread Chaser for New Block???


Staff member
With all the expenses of building a new engine and starting with a Dart SHP block, will I need to purchase dies to chase the threads in the block??? It's new, but does that exempt me from any of the normal procedures associated with building a new engine.

Comments or thoughts???


Staff member
people and company's occasionally make mistakes, your always better off using a thread chaser on all the threaded holes and blowing out the debris with high pressure air so YOUR sure the holes are correctly prepped.
just remember that a thread chaser tap is not a thread CUTTING TAP ... 8/10002/-1

if you use taps youll need several sizes of tap holder t-handles


Lisle 70500 Tap Socket Set (this square drive socket set, socket set allows you to use swivels and ratchets on thread taps)
if youve ever chased threads in a block thats still in a car you have very likely found the typical taps T-handle can have clearance issues - 4 - Search - SKUs&msclkid=51e6168ce34c1039d01d4b9e7013b0f7

drilltap.png ... tId=751008

these are thread chasers designed to clean and straiten old threads without cutting new threads or removing metal
Bolthelicoilrp.jpg ... index.html ... 021201tr/1


DON,T FORGET TO CHASE THE BOLT THREADS and VERIFY the BOLT LENGTH and THE threaded hole DEPTH exceeds the bolt LENGTH to prevent binding and stripping threads ... 050701br/2 ... hreads.pdf



harbor freight sells a good air nozzle for clearing thread holes ... 42939.html


One neighbor of mine ,he found two rather extensive sets,SAE/METRIC TAP & DIE SETS MADE IN EUROPE NOT CHINA, at a yard sale,that were for sale, where a widow was moving after selling her home,
the garage was being cleared out, and it was rather obvious she had no clue what the stuff was,
that the tools had a bit of surface rust on only a few pieces and one or two missing pieces but he only paid a negotiated $130 for both sets.
I hope I can slip him a few bucks extra profit and get the stuff as he has as much use for it as a pelican has for snow shoes as he can,t and won,t,
even attempt to do an oil change, he simply bought the stuff because hes seen similar stuff around my shop and realizes its not cheap, and he got it at a bargain price.

die_sets.jpg ... 000P?mv=rr ... 712&ucst=t

viewtopic.php?f=27&t=1262&p=6142&hilit=dies+taps#p6142 ... Bdie%2Bset

yes you'll eventually find these sets almost in-dispensable, you won,t use them daily but cleaning out threads before assembly and the use of anti-seize on threads will eventually become almost a habit


"there is a difference between a thread chaser and a tap. There are actually three types of thread cutting taps for each size threads. A taper tap is for starting the threads, the plug tap is for making the threads go deeper/closer to the end of a blind hole, and a bottoming tap is just that, for getting closer to the bottom of a blind hole. A thread chaser is made different from a tap in that is does not have the sharp cutting edges, the flutes ground into it are not as deep. A thread chaser is for straightening out or cleaning out threads in either an open hole or blind hole. The body of a thread chaser has more metal in contact with the threads already cut into a hole therefore reducing the chance of cross threading"

After lots of issues with an old Mark IV I decided to sell it and bought a new Dart Big M BBC.

Had it completely machined and when picked up asked if everything went fine. Guy told me "we ran a tap on 3 main holes as the bolts were hard to turn giving incorrect torque readings.." I cringed... No chaser, a regular tap.

Finally took the block home after arguing and getting nowhere.. Whats done it's done I guess. Left it there for a while.

Last week bolted all mains up with the stock Dart bolts. Torqued up to 100 using darts lubricant. All OK, nothing felt funny and held up just fine.

Had a set of new ARP studs and installed those. Again torqued up in 3 steps to 110 as per ARPs instructions. All OK. Also clearances did not change at all with the studs, so will use these for piece of mind.

Returned to the shop, much to my regret, and got the taps dia measured. They are 0.012" larger than the studs dia.

Also measured the "side to side play" of the studs (without the main saddles on of course). Screwed them up all the way and then backed 1 turn. Measured as close to the main surface as possible. All of them move 0.0015" to 0.0025". The ones these ppl redid (the outer on the 3rd cap and the inners on 4th and 5th cap), have 0.003 to 0.004" side play (they of course felt "looser" just by feeling while screwing them on...

Now my question is: Should I leave as it is or fix it? I know the studs are clamping as they should, but once you spin a 75 lb crank to 6.000 or 7.000 rpm there will be a lot of punding down there, isn't it? So clamping force is one thing but I suppose the mains have to endure the extra forces a running engine will have... For (crossed fngers) a long time... This is not a race engine that will get torn down every weekend to check on things.

I have access to a nice large mill. Was thinking of drilling these threads out and inserting 1.25" helicoils tightly in place.. Am I overthinking this? So afraid to break up something I will never be able to afford again that I'm getting paranoid.

Sorry for the long post.. any opinions appreciated. If there's another way to check these holes please let me know.


well my first thought was why not call and talk to



as it is their block and their studs.
but I've used several DART big M blocks and dozens of sets of ARP main cap studs , and honestly, I don,t think your running any risk. big m block


youll need tap and die handle sets

btw youll need 3-4 different sizev tap handles (HF is barely functional but dirt cheap)
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Staff member
people and company's occasionally make mistakes, your always better off using a thread chaser on all the threaded holes and blowing out the debris with high pressure air so YOUR sure the holes are correctly prepped.
just remember that a thread chaser tap is not a thread CUTTING TAP
If you can screw the bolt all the way in by hand, is not really a good indication of the thread conditions.....right???

I do have a set of tap and dies up to 3/4" from Sears that I bought 30 years ago and you are right, I do need these more often then one would think.

If I wasn't spending several thousand dollars already I would just buy the thread chaser without question. I just know from experience there will be many things that cost $20-$30 at the end of this project, that I hadn't even considered . These expenses usually add-up to several hundred dollars before it's all said and done.


The Grumpy Grease Monkey mechanical engineer.
Staff member stands to reason that,the stronger the tapped hole.

one of my engineering instructors used to say
that if a decent quality steel bolt
is threaded into a cast iron or steel surface,
TO A DEPTH OF THREAD at least 1.75 times its thread diameter,
its not going to pull out before the bolt snaps under tension loads,
in aluminum, that thread engagement must be increased to about 2.5 times,
its thread diameter its not going to pull out before the bolt snaps under tension loads

Tapping Torque Vs Thread Strength
Suggested Percentage Of Full Threads In Tapped Holes

It stands to reason that it takes more power to tap to a full depth of thread than it does to tap to a partial depth of thread. The higher the metal removal rate, the more torque required to produce the cut.

It would also stand to reason that the greater the depth of thread, the stronger the tapped hole. This is true, but only to a point. Beyond that point (usually about 75% of full thread) the strength of the hole does not increase, yet the torque required to tap the hole rises exponentially. Also, it becomes more difficult to hold size, and the likelihood of tap breakage increases. With this in mind, it does not make good tapping sense to generate threads deeper than the required strength of the thread dictates.

As a general rule, the tougher the material, the less the percentage of thread is needed to create a hole strong enough to do the job for which it was intended. In some harder materials such as stainless steel, Monel, and some heat-treated alloys, it is possible to tap to as little as 50% of full thread without sacrificing the usefulness of the tapped hole.
Deep Hole
Thin Sheet
Stock Or
Hard or Tough

Cast Steel
Drop Forgings
Monel Metal
Nickel Steel
Stainless Steel 55% - 65% 60% - 70% -
Cast Iron
Mild Steel
Tools Steel 60% - 70% 65% - 75% 75% - 85%

% Of Thread Calculation for Cutting Taps

On page 1924 of my Machinery's Handbook (27th ed.), under the heading "Diameter of Tap Drill", they make the statement:
"Tests have shown that any increase in the percentage of full thread over 60 per cent does not significantly increase the strength of the thread. Often, a 55 to 60 per cent thread is satisfactory, although 75 per cent threads are commonly used to provide an extra margin of safety."
However, the table that immediately follows, Recommended Hole Size Limits Before Tapping Unified Threads, lists min and max hole sizes for various lengths of thread engagements. For 1/4-20 threads less than 1/3 diameter long (< .083"), the table calls for 87 to 100% thread depth. For thread lengths from 1½ to 3 diameters long (.375 to .750"), they call for 75 to 85%. I haven't checked any other sizes, but they don't seem to be following their own 60% rule.

For 1/4-20, my handy-dandy Starrett reference card calls for a #7 (0.201") tap drill, which is 91% thread depth. A #2 (0.221”) tap drill gives a 54% thread depth, with a lot less effort required to tap and less chance of tap breakage.

At present, I'm dealing with 1/4-20 by 0.50" thread depth hole in 6061-T6. How big can I make the tap drill without sacrificing strength, assuming 0.50" thread engagement? Anyone know of any good references or on-line calculators to answer this sort of question? - Screw Threads Design.pdf


Thread engagement has two factors, length of thread engagement and percentage of thread engagement.

Length of thread engagement is measured by the length of interaction between the fastener and nut member (i.e. nut or mating material for screw.) For example a standard thread forming screw applied in
10mm of material will have more length of thread engagement than the same thread forming screw in 8mm of material.

Percent of thread engagement is based size of the mating hole compared to the fastener and how much the height of threads of the screw are engaged. For example a #10-24 thread forming screw will have around 80% thread engagement in a .1683” hole versus around 70% thread engagement in a .1710” hole.

Thread engagement is important because it directly relates to the integrity of the joint. With too little thread engagement (combination of length and percentage) the nut member may have a tendency to strip out at lower than desirable torques, in effect lowering the strip/drive torque ratio. On the other hand with too much thread engagement the torque needed to drive the screw will tend to be high, which can lead to inadequate clamp load or even breaking fasteners during assembly

Percent and length engagement can also be traded for one another; i.e. if the nut member material isn’t optimally thick, a smaller hole can be used to get a more optimal amount of thread engagement and in effect a better joint.

Simply put, more thread engagement can result in higher tensile strength a joint. Tensile strength is the force required to pulls something until it breaks or the capacity of that material to withstand that load. If a bolt is longer than needed to develop full tensile strength in a nut member, that excess material is wasted. On the flip side, if there is not enough bolt length engaged in a nut member, the bolt has a higher probability of stripping out before full tensile strength capability is achieved. Generally speaking, you need at least 1-1.5 bolt diameter engaged in the nut member to achieve optimum joint strength with a thread forming fastener, depending on the strength of the nut material. There are different factors that affect this such as the nut material; steel is a close to a 1-1.5 relationship, while softer material, like plastic, will need more thread length engagement to achieve optimum joint strength.
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