Wrist Pin questions

[Aikmanson]

I have probably a stupid question, but I am not finding an answer readily available to help me. I am building a 2.8l stroker M20 engine, upon assembly of my pistons with the new short rods, the wrist pins were much too tight on the new rods, so I took them to the machine shop. Here is my question:

What is the proper setup of the wrist pin to piston clearance, and wrist pin to rod clearance. Should the wrist pin rotate in the piston, and rotate in the rod?. I ask this because the guy at the shop honed the hole on the piston out so that the wrist pin now floats freely in the piston, and honed the hole on the rod so it floats freely in the connecting rod. I was under the assumption that the wrist pin was an interference fit on the pistons, as they were when I removed them from my old m20. He was very adamant that it is supposed to be a free floating design on both the rod, and piston.

Can somebody shed some light on this situation for me? Do I need new pistons now..? If so, anybody got some good condition late style M20 pistons?

[nando]

I don't think it's interference fit on either end. the wrist pins are held in by clips, not friction.

[SkiFree]

I don't think it's interference fit on either end. the wrist pins are held in by clips, not friction.

I concur.

[digger]

It's a full floating pin setup. Id guess 0.001" rod to pin perhaps a little less between piston and pin 0.0008" but that's what ive seen recommended for performance engines

[Aikmanson]

Thanks for the help guys, sounds like the machinist knows more than I gave him credit for. I am glad I didn't give him a hard time about it, and only second guessed him when I left the shop. Digger, I am the engine builder (pre edited version of your post!) and trust me, I do not know everything that I am doing, hence asking for help.

Hopefully this helps answer someones question in the future.

[digger]

the original comment i edited out weren't aimed at you, i originally interpreted the machinist asked you what the clearance should be which is why i suggested you find a different one that knew. but then i re-read what you wrote properly and yes he apparently knows what he is doing

[Aikmanson]

the original comment i edited out weren't aimed at you, i originally interpreted the machinist asked you what the clearance should be which is why i suggested you find a different one that knew. but then i re-read what you wrote properly and yes he apparently knows what he is doing

I know, no worries. Anyways thanks for the help. While we are at it, is it worthwhile to replace the circlips that hold the wrist pin in place, or are they generally not a failure/wear point?

[LJ851]

While we are at it, is it worthwhile to replace the circlips that hold the wrist pin in place, or are they generally not a failure/wear point?

That depends on who removed them. If they still look new as in perfectly round, it is generally safe to reuse them. If they are shaped like footballs (american) then get new ones. New ones are certainly a safer choice if that is an option.