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piston question

Hi all,
recently I bought new pistons for my Triumph.
I noticed that the gudgeon pins move freely in the pistons. Normally that's a rather tight fit.
They turn not that freely in the small end bushes.

This means that when the engine will be running, the pins will probably be moving in the piston instead of in the small end.
Is this OK or not ?
Should I use some lock tite to secure the pins in the piston ..?

Any ideas ?

Re: piston question

(You do not specify what bike you have, this answer is in general terms with regards to engines that have aluminum pistons and small ends with bronze bush.)

Not ok.

The small end bushes should/could be reamed to correct size. But the fact that the pins move freely in the pistons i unacceptable. It won’t take long until they have worn the aluminum extensively and things will go bad. They should be a snug fit that may even need a bit of heat to be fitted.

Locktite will not help since it will dissolve when the piston gets warm.

Best regards,


Re: piston question

If the piston pin is retained in piston by circlip either side then it is fully floating. Normally the pin should be a neat firm fit in the bush by hand without any detectable free play and require the piston to be warm to be same fit, as piston will expand more. When at operating temperature the pin should move freely without any detectable free play in piston. A loose fit in a cold piston would be too loose.

email (option):

Re: piston question

If the pins move when the pistons are cold, they were made wrong. They should not allow the pin to move until they are hot.

I would demand my money back from the seller.

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