Scooterotica

Gp200. Crank is not shimmed. End of conrod is 20.5mm. New GOL piston has a gap of 22.65mm. I had a 20mm small end bearing in, but had rattle. It's actually 19.75mm.

I understand the gap on each side should be between 0.7-1.3mm. Is this correct? In the above case my SEB was smaller than the conrod end, and clearance would be just over 1mm. And I had a rattle.

20, 22, or 22.5 SEB will not give me the clearance I need.

A Polaris SEB #3080998 is 16x20x21.7 would give me 0.475 on each side.

What does one do? How narrow can you safely go and still have room for heat expansion.

Is it okay to try and file the gap larger? How do you do that and have it flat?




Tx in advance

David

Davidsquaredson wrote:Gp200. Crank is not shimmed. End of conrod is 20.5mm. New GOL piston has a gap of 22.65mm. I had a 20mm small end bearing in, but had rattle. It's actually 19.75mm.

I understand the gap on each side should be between 0.7-1.3mm. Is this correct? In the above case my SEB was smaller than the conrod end, and clearance would be just over 1mm. And I had a rattle.

20, 22, or 22.5 SEB will not give me the clearance I need.

A Polaris SEB #3080998 is 16x20x21.7 would give me 0.475 on each side.

What does one do? How narrow can you safely go and still have room for heat expansion.

Is it okay to try and file the gap larger? How do you do that and have it flat?




Tx in advance

David

Here is a pic of the piston, you can see some wear marks on the skirt from about 30 mins use.

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Have you checked the bore with the piston in both ends should be 3thou and check ring gap as well but on the small bearing a 22 should be fine

I dont think that has anything to do with your small end bearing,it looks like its nipped up.What is the clearance / piston to bore.

with the big end not shimmed you can shim the small bearing within the piston but you cant have both shimmed

Here is my Piston to bore:

Piston on its own at top is 65.56 mm; at bottom is 65.88 mm

Top of Barrel to piston (20 mm down from lip):
- Piston clearance is 0.3mm
- Ring gap: 0.406 mm

Bottom of Barrel to piston (20 mm up from bottom of skirt)
- Piston clearance is 0.15 mm
- Ring gap: 0.381 mm

0.3mm is 11.8 thou.If that is what it is,its no wonder its rattling.

yes its big 3thou is a about right

garry inglis wrote:yes its big 3thou is a about right

Found this

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Agreed. Based on the earlier comments I went to a nearby motorcycle shop and they measured using a proper bore guage. The widest part of my cylinder is 65.98mm. My piston varies 65.710 - 65.811, so my gap is between 0.28 - 0.178. Anyway you look at it, I need to get a rebore and go for an oversize piston.

Thanks for your comments, you helped me solve this.