Piston pin end clearance?

[Truckedup]

What do you say is a minimum and maximum pin to lock ring clearance ? Would it be the same for forged or cast? Would a larger bore need more or less clearance? Thanks

[Baprace]

What do you say is a minimum and maximum pin to lock ring clearance ? Would it be the same for forged or cast? Would a larger bore need more or less clearance? Thanks

I am happy with .005 pin to lock clearance, mayby .010 on the outside of my happy end play, .000 is a little too close for me but I have seen close end play work OK. If the pin will still rotate at .000 end play I think you are OK. JMO

[Truckedup]

So .025 would be too much?

[DCal]

It's not too much using a wire lock. I don't like .025 on a spirolocs but don't like .000 either. Thin flexible pistons and/or wristpins are the usual culprits.

[Walter R. Malik]

What do you say is a minimum and maximum pin to lock ring clearance ? Would it be the same for forged or cast? Would a larger bore need more or less clearance? Thanks

Minimum clearance is determined by the pin length and wall thickness as to how long it retains its heat longer than the piston itself.

With flat pin clips, .005" should be fine; more than .010" is probably to much.
Round clips can withstand a little more if the pin is not flat on the ends.

[Dan Timberlake]

According to the SAE paper, the 427 Ford GT40 program used standard production pistons even for the LeMans race. A few problems were found.
One was piston pin retainer "failure."
With those prehistoric pistons running in a LeMans cycle .001-.005" was "too tight."

piston pist lox .PNG

[modok]

IMO .005-.015 for most applications.
I see many replacement pistons run looser than that, and if that is OK or not......I don't know.
Nobody seems to be paying attention.
Also check the ends of the pin to see if they are cut square.

[turbo camino]

Also check the ends of the pin to see if they are cut square.

Well, that's a bit of a "how long is a piece of string?" conundrum, because I've repurposed various piston pins as miniature cylinder squares (I have a tiny surface plate), and every last one of them from SBC stuff to bigger diesel pins were all out of square as much as 3 or 4 tenths! I had to correct the ends on a 600 grit diamond plate to make them usable. You just can't find quality parts these days, I tell ya...

[modok]

Press-in pins don't need to be square on the ends, so you can't really blame them.
But I have found up to .0008" runout on big name racing parts too, that are specifically made to an exact length and for square clips.
and .001- .005" runout on chinese junk in a few rare cases.

As compared to, a lot of Japanese or German made pins, it just IS square, checking was wasting time, it really varies.
A German made cummins piston pin, I'd be surprised if they were over .0003" off, and that's 40mm or larger diameter, that's doin darn good. ALMOST good enough for use as a square

[Mark O'Neal]

For example, we cut the lock at the pin length, and a .990 double spiral lock groove at .092. I just measured the locks at .041 leaving a total of .016 for the pin to grow, or bounce around. Haven't lost one yet.....

This is kinda like when my brother asked an old cabinet installer why he didn't put a screw in the bottom rail of a base cabinet. The answer was "I haven't had one fall off the floor yet."

90% of engine building is opinion.

[hoodeng]

And grubby elbows.

[Truckedup]

The reason I asked the question is because of a failure on an 850 Norton, not my bike...You can see the eyelet left from the broken snap ring...The owner claims the snap ring was installed correctly, only 150 miles on new pistons..I asked about pin end clearance and he is supposed to check the other piston...These old Brit bikes almost always have wire lock rings rather than flat snap rings...The round rings with the correct pin are never a problem..

[Dan Timberlake]

https://i.postimg.cc/ZnMHxXYP/piston-circlip.jpg

Is the piston cracked up at 1:00 o'clock?

A rectangular section Truarc installed in a round section groove meant for a wire clip could be half unseated before the engine is even fired.
Unfortunately the evidence of the groove machining on THAT piston, on THAT groove is history.

https://www.rotorclip.com/images/wysiwy ... le8_10.pdf

The picture seems to show the finish and profile of end face of that wrist pin as rough and irregular. Hardly the optimum for severe service.
FWIW the remaining ear hints that it's rounded profile may be facing outward. In the old days some advocated TruArcs be installed sharp edges outward.

How were the wrist pin bushings fitted and finished?
Reaming, and even honing do not correct rod pin and big-end bore alignment.

I think a check of con rod straightness would be on my list too. The full skirted pistons' wear may or may not show modest un-straightness.

[modok]

Rod alignment should always be checked when the engine is apart. Speaking of that you might wonder if I am checking pin length and runout on every piston, well, not yet. But I do think it should be at least LOOKED at.
After installing the clips I will move the pin back and forth in the piston and look for the end play to be somewhere close to right, and of course 95% of the time it is fine. When it isn't, often it is because there is a burr or some dirt in there, or the clip isn't seated right.
Square grooves and the clips have sharp corners which can easily result in burrs hanging it up.
But Also this is how I have found pins that just fit wrong.

[Truckedup]

The pistons were bought new with pins and true arc clips...I don't know if it's cracked but I will see when I go over and check it out...These are made in Tiawan pistons, and are good quality for typical street use...I will also check out the retainer groves...maybe the clips were a mistake...Myself, I always use NOS British made Hepolite or forged made in USA...