Gudgeon Pin Fit To Piston & Small End.

[TeeJay]

Mmmm, not sure about this and whether it will later become an issue.

On a previous engine rebuild I had to warm the +0.040" Hepolite Powermax pistons to remove the gudgeon pin. The Con Rod small end was a sliding fit, such that when assembled the con rod could rotate on the pin without rotating the pin in the piston

I am now fitting +0.060" QED Standard Type (Omega) Pistons. The gudgeon pin on these pistons are a close sliding fit, no need to warm them. The pin fit in the Con Rod is also a close sliding fit but slightly closer, such that, when assembled to the Piston, the pin rotates in the piston rather that the Con Rod.

With engine heat, the Alum Piston will expand more that the steel con rod and bronze small end bush, resulting in a loser gudgeon pin fit to piston than small end.

All components are within specification and possibly OK, but I am not confident about this so thought I would ask the question.

Are these fits OK??
Thanks for any comments.

[rgh0]

Like you,in my experience, the "normal" assembly results in the gudgeon pin to piston fit being tighter than the gudgeon to rod small end fit so the pin moves on the rod not the piston when assembled and cold.

As you observe when hot and running the piston will expand and the pin will free up in the piston.

Assuming all the components are within their tolerance size range ( piston gudgeon bore, gudgeon pin, small end bearing) and you are just at the tight end of the gudgeon to small end fit and the loose end of the gudgeon to piston fit then all should be OK when hot and running. If concerned you could always get the small ends resized slightly so that while still within their tolerance range they are not so tight cold fit.

cheers
Rohan

[TeeJay]

Rohan, thanks for your comments.

I have had the components overnight in the home, rather than the cold garage.
The sliding fit to the small end is still slightly tighter than the piston.

On measuring each component It would appear that the small end is on the lower limit and the piston towards the top, very difficult when tolerances are in 0.0001". But the degree of sliding fits confirms this.

The specifications state that the small end to gudgeon pin should have a running clearance of 0.0003"-0.0005" and the pin to piston 0.0002".

So as you confirm:- "Like you, in my experience, the "normal" assembly results in the gudgeon pin to piston fit being tighter than the gudgeon to rod small end fit so the pin moves on the rod not the piston when assembled and cold."

I shall have the small end reworked to upper limits, as I would be more confident with the resulting assembly.

Thanks again.

[c42]

Honing the small end to size is certainly the way to go.

Regards
John