LCGB Forums

Hi all.

I have recently installed a new 175 cc 'race-type' crank. The cylinder is mildly ported and fully run-in.

I did not fit gudgeon pin shims when fitting the small end bearing. At low revs, I am noticing a fairly audible 'knocking' noise as the engine ticks over. I had a look at the piston (60 mm) which, when it the cylinder is within tolerances, and noticed a small area of slight wear on one side at the top of the piston skirt and crown interface.

I am thinking there is too much play in the piston's sideways movement at the top of the cylinder.

My questions are:
1. Do I need to fit gudgeon pin shims OR fit a wider Japanese-type small end bearing to take up some of the play in the piston's movement?
2. If the latter, can you suggest a good supplier of these bearings, please?

Hope you can help.
Many thanks. H.

Shimming of the conrod is designed to ensure it runs true at one end or the other.

If the crank is shimmed you should not fit piston shims.

If it isn't then you probably should.

Where are you measuring the clearance on the piston / bore?

Hi.
Thanks.
The crank is shimmed. The piston-cylinder clearance was measured approx. 5 cm from the top of the cylinder, with the piston at the top and via feeler gauges.

Check it at the skirt with the piston down where the rings don't run.

OK will do.
Thanks again.