## differnent size piston ring

the rings for my riverside are really expensive (well, \$30.00) and it needs a 40.2mm. what if i use a 40.00mm ring instead. same height, same ends. or should i try to find one a little bigger and file the ends down a little.-jason

## Re: differnent size piston ring

too much ring gap beats the hell out of too little.. i'd go for it.

## Re: differnent size piston ring

Although \$30 for rings is more than i want to pay, this still bothered me a little so i did some calculation..

Circumfrence = pi x D

3.14159 x 40 = 125.6636mm

3.14159 x 40.2 = 126.2919mm

126.2919 - 125.6636 = .628mm. The circumfrence difference is 0.628mm.

My Honda ring gap Service Limit is 0.6 mm. (Standard specs are 0.15 to 0.35 mm.. the gap narrows in a hot engine. ) So, at best, you are probably gonna be at your service limit.

I would put the rings in the bore and measure the gap and check your bike's specs. Maybe you will see a lot less than 0.6mm gap, but maybe it will be wider.

Cut your potential losses by first making sure the ring grooves are immaculate so the rings seal the best they can.

Aside from compression losses, excess ring gap can cause cylinder wall and piston skirt problems. I would run the engine for a while and examine it for carbon in the ring groves which can prevent a ring from seating and sealing.

Carbon streaks in line with the ring gap on the cylinder walls and discoloration on the piston tell you there's a lot of blowby.

If you keep an eye on things while breaking in the rings, i still like the idea of trying the 40mm rings.

## Re: differnent size piston ring

Hi, I think you need to measure the diameter of the ring groove in the piston and then the inside diameter of the ring when it is compressed. That may be where you will have clearance problems when you try to fit it in the cylinder. Also when the piston heats up and expands it may put pressure on the smaller diameter ring and crack it in the cylinder wall. Just something to think about.

## Re: differnent size piston ring

AS Pepied said, for \$30 are you willing to take a chance on ruining your engine? IF It was my deceison, I would work overtime or do odd jobs like Offering to paint, mow grass , shovel snow, cut down trees. What ever is necessary to earn \$30. Good luck.

## Re: differnent size piston ring

its not that i dont _have_ \$30, its if i can buy a \$5 set of rings and make them work I'd rather do that. plus i dont like cosmotors, and thats the only place i can find them.

## Re: differnent size piston ring

We gotta assume that jason measured the rings and they are the same dimensions, just oversized to fit a reconditioned bore... it's understandable that oversized rings will cost more... but \$30? Maybe there's a cheaper source out there someplace.

Problems due to heat would come from the ring gap being too small, closing when hot, and the ring binds and doesn't seal in the groove .. or it expands into a port and breaks. But this would be extremely unlikely with undersized 40mm rings. Worst thing i can predict is excess blow-by. And unless the engine is high performance, even potentially serious problems due to blow-by is unlikely.

## Re: differnent size piston ring

okay, i measured the ring gap on the old piston (i have a new piston and cylinder im using, which is slightly larger bore) and it is about .95mm, pretty big. but based on the rest of the engine components (keep in mind it was made in the late 50s early 60s, i dont think they machined things with exact precision in mind). my point is i think ill be just fine. -jason

## Re: differnent size piston ring

the 40.oomm rings i were referring to are those from a different make ped. same height and everything, just for a different size bore.

## Re: differnent size piston ring

I've always had a sneaky suspicion that a whole lot of specialty engine parts like rings, psitons, con-rods, bearings, seals, etc in different brand bikes are made by the same companies and are virtually identical.. Bike manufacturers wouldn't want to pay for custom designed parts unless they had to.

## Re: differnent size piston ring

theyd have to be, its not like theres a huge demand for this stuff, enough to keep numerous companies profitable at least.

## Re: differnent size piston ring

"..i measured the ring gap on the old piston.."

Just to be clear about measuring ring gap, the ring would be slipped into the upper part of a (clean) bore and the gap measured with a feeler guage.. the ring wouldn't be on a piston, right?

## Re: differnent size piston ring

the rings were still on the piston, in the bore. im afraid to take them off because i have a tendancy to break rings. but i also found an old ring (same size) and measured the gap the correct way, and still got .95mm. where are of these rings coming from you ask? its a long story.-jason

## Re: differnent size piston ring

Hi again, just got home and reading the conversations. I agree that many parts are similar. Most designs are patented so they make just enough changes not to infringe on others concepts and have to pay royalties. At least that is what I think happens in American manufacturing. I do not know about the European side, where most of these peds were made back then.

As for the craftsmanship of manufacturing back in the 50's and 60's I would tend to believe it was much better. Back then, especially in Europe, people were given apprenticeships and really learned the trades. Today the machine is computerized and people just push the buttons and wouldn't have any idea on how to really machine the part.

But getting back to the ring and piston. if the piston is 40.2 MM then the ring groove is cut at a specific depth, lets say 2 MM. that means the diameter of the groove is 36.2 MM. On a 40MM piston the groove would be 36 MM. When you place the ring in the cylinder and measure the ring gap I think you also need to measure the diameter inside the ring while it is compressed. If it is less than 36.2 MM you may have a problem because it will be squeezing the piston. Now what will happen when the piston starts to expand from the heat of combustion? I just wanted you to check and make sure that the rings compressed diameter is at least the 36.2 plus a little clearance. I do not want to see you ruin the cylinder if the ring comes apart inside the cylinder.

Now there are allot of piston and ring manufacturers. You may want to check auto stores and see if Wiesco or Perfect Circle have a ring that is the correct dimension without costing an arm and a leg. Good luck.

## Re: differnent size piston ring

those are good points too. i went to perfect circle web site. do you get their stuff through napa autoparts and other such places? have you ever found pars for moped engines there? thanks for the tip.-jason

## Re: differnent size piston ring

Chec this link out. Polini makes a dirt bike 50cc that has a bore of 40.2MM. you may be able to get a ring. I also see where Polini makes a cylinder kit for a minarelli engine that uses a 40.2MM ring. Hope this helps.

## Re: differnent size piston ring

See details Polini - Cylinder Kit Minarelli

Minarelli LC 50cc Cylinder Kit 40.2mm SIMILAR to 166.0096/10 for Liquid Cooled Engines -Superior Performance and good replacement for broken cylinder. WEIGHS LESS THAN STEEL Version 166.0102 Liquid Cooled Version KIT YAMAHA-APRILIA SR H2O-MALAGUTI F12 L

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Here is the one description of the cylinder kit, they make two. I did a google search for polini 40.2mm piston and found it. I see where Polini is owned by Piaggio, the Vespa scooter company. They must sell replacement rings for these kits and there dirt bike engine. Just another avenue to travel upon to keep these classic peds swarmin!! Good Luck

## Re: differnent size piston ring

man thats great info thanks a lot!

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