pistons and rings

I soft seized my 65cc metra and bought a replacement piston. I think when I cleaned up the cylinder, I actually made it a little bit bigger. I have the 43.5 mm piston now and saw from the metra kit site they have 45.7 and 45.9 etc..., So I have a couple of questions.

1.) How do you know what size piston you need?

2.) I see the parts on the site, but where do i get them?

Thanks you guys are always helpful!

Re: pistons and rings

How did you "clean it up". Ideally a micrometer and snap gauge will tell you what the clearance is between the two. A .0015-.002 inch fit is optimum in my opinion but there are peds running around with much higher clearances

Re: pistons and rings

I used 220 grit sandpaper and scratched up all over the inside. I was pretty careful about staying uniform, and it looks to be uniform throughout, the piston slides easily but the rings look like they are about to pop out of the piston, but there still is good compression.

Re: pistons and rings

Hmm, unless you spent an hour and a lot of pressure behind that 220 you probably didn"t increase the bore size appreciably. The piston should slide easily but with no slop feel sideways. You can detect as little as .005 inch this way. Tell me how you are looking at the asssembly when you see the rings about ready to pop out. Pistons are designrd to be smaller diameter at the top to account for expansion from more heat towards the top but a I don't know if that is something you could visualize. If you are really going to get into engine building you can get a set of micrometers or their less accurate cousins, calipers, cheap these days. If somehow you actually sanded the bore down too much you can ask a dirt bike shop where you can have it bored to the next piston size or two sizes up. You can run it and just see what happens. Oh, a cheap measure is to try to slip various feeler gauges past the piston in the bore w/o the rings on

Re: pistons and rings

I have the cylinder off, and as I look in from the head side, the rings seem about ready to pop out, I'll try to get a good pic later today or tomorrow.

Re: pistons and rings

ok, sorry for the delay, but here is the pic, see how the ring is clearly visible around the piston? my 50cc stock looked tighter than that. Also, I noticed a slight scratch in the piston where the rings end, almost as if the ends of the front most ring (closest to head) is scraping the cylinder as the piston moves. Is this all ok?

1219961407_piston_too_small.jpg

Re: pistons and rings

I see what you mean. From that photo it almost appears as if you have L or dykes ring. That is a used piston it looks. Except for size did it look like your old piston. Does the piston skirt fit without slack in the bore?. Was this a used cyl also? So many questions. I still can't imagine you sanding the cylinder that large. If you persist with this cylinder you can resolve the issue with a good read out caliper or better yet micrometers and snap gauges. Scratches from the ring ends into the cyl are normal within reason because rings are actually not round. They are cut from larger diameter tubes to give them that spring tension

Re: pistons and rings

The rings should def not be out that far. When you slide that piston up and down by hand you should not see the rings. Def mic it up as flagrant said.

Re: pistons and rings

ok, so back to my original question #2 where do i get a bigger piston?

Re: pistons and rings

It was a new cylinder, that soft seized and messed up the original piston pretty good. I sanded the cylinder to clean up the smears create a "rough" surface, which has since been smoothed out by the rings. Both pistons were labeled with a 43.45 on the crown. The first piston and rings are totally fried, but this piton is new and has maybe 100 miles on it.

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account