Scooterotica

How much play is acceptable on the side to side off the Small end bearing. I fitted one of those cheap186 kits to my brothers scooter and the piston seems to me to have too much play from side to side on the grudgeon pin. Is the answer to fit piston shims or a wider small end bearing? Thanks graeme

The main thing is that you don't want the conrod eye running off the end of the small end rollers. I saw one where the gap in the piston for the bearing was 24.5mm. Even with a 22mm bearing I'd fit 1mm piston shims.

How big is the gap either side of your small end bearing when in the piston?

What is the width of your small end bearing?

Stripped the top end back down and shimmed the piston. Now got around 1mm play. I am a bit happier with this unless anyone knows any different.

Is the crank shimmed?

What crank is fitted?

Ideally you don't want both ends shimmed, it should be one end or the other.

Sort of agree, but it depends on your bearing width. you really don't want to restrict movement at both ends of the rod. Shimming both ends is generally a no no but would definitely be the right thing to do if you had 25.5mm play and a sloppy big end. ooer.

The crank is a sil redone with a jap big end bearing. It is shimmed.the reason I asked the question is because as hendry said I too thought the spacing between the piston and the small end was around 25mm and when I have the scooter started there's a knocking noise which I think sounds like piston slap.it still has in my opinion enough play in it for the piston to move about freely so don't see a problem with what I have done. Anyway after I come back from work I will start the scooter and see if the noise is away. If it's still there Might just strip it again and take the shims out again and rethink my problem.

I'm interested to know were you got the shims from,I was under the impression that no-one made piston shims any longer(unless someone has started re-producing them) and you had to use a wider small end bearing.If your using anything but proper piston shims,there is a possibility that they could wear prematurely and eventually drop out and wreck your engine.

engine running i doubt youd here side play in a small end; shims top n bottom is a nono the rod needs float one end or tother

Muppet wrote:; shims top n bottom is a nono the rod needs float one end or tother

I get what you're saying, but surely if you have a 22mm bearing the rod is free to float on the small end bearing. The shims are being used to centralise the bearing, not the rod. I'm basing my thoughts on the assumption that the inside diameter of the shim is matched to the outside diameter of the gudgeon pin (16mm) rather than the outside diameter of the bearing (20mm).

Started the scoot today and the knocking noise was noticeably worse or took the shims out and back to the quite knocking noise. More investigation required. I too can see the logic in what your saying but what do I know.