What causes a piston to be “stiff”

Hey all, first post here. I have a set of used rods and pistons. for context, I purchased them used and already assembled. A few months back I had them balanced and cleaned at the machine shop.

Fast-forward after a couple months of them being in storage, I just noticed 1 out of the 4 is much stiffer than the rest of them. I can’t recall if it was like this previously. The others that are free will fall over with gravity and flop around with minimal effort. 1 of them however can we stood upside down and will not flop and requires a bit of effort to move it.

I feel like my machinist would have mentioned something when he was balancing them. Is this something I should be worried about? is there something I can do to fix this?

Yo man, welcome!

That’s definitely worth checking before installing. If one is way stiffer than the rest, it could be wrist pin fitment or just some debris from storage. Since your machinist worked on them, they likely would’ve caught anything major, but try this:

- Add some oil or assembly lube and see if it frees up.

- Swap the wrist pin with another piston to see if the issue follows.

- Wipe down the pin and bore—could be dirt or light corrosion.

- Lightly heat it with a heat gun to see if it loosens up.

If it’s still stiff, I’d have the machinist check it. Better safe than sorry! Let us know what you find.

Hey Leo! Thanks for taking the time to respond.

I started with applying some assembly lube. It certainly helped partially but can still tell there’s a little resistance. I’ll try disassembling and cleaning next and it if persists try switching them around. I’ll keep you posted!