stroke and bore of a yamaha chappy?

I've got a yamaha chappy that I want to fix up and get going good. The thing has just slightly over 100 on the compression tester, so I think the compression is kinda low. It's really easy to kick it over. The thing is really really fast when there are no hills, but it just turns into a slug when it hits the slightest incline.

I want to know the bore size and stroke for 2 reasons: so i can check to see how word the bore is and possibly get new rings, and, I noticed that the bolt pattern is the same as puch, so if the stroke was the same, i could put a puch top end on, maybe even a polini kit!

If anyone has a link to a manual online, it would be awsome

Re: stroke and bore of a yamaha chappy?

The original bore diameter doesn't matter. Measure what you have now and then search out new rings or a new piston to fit it.

The better way is to buy a piston / rings and then have the cylinder slightly bored so it's nice and straight fits that piston. Use the services of the bike mechanic or machinist to help measure everything and to pick the parts before you buy anything.. He can also examine the engine's bearings and stuff and maybe save you some grief later.

While you're in there, measure the stroke yourself. Remove the head and fasten the cylinder back down snug. Use the mark on the flywheel to bring the piston to TDC. Measure how far the piston fell after turning the flywheel 180 degrees to BDC. Take your time and you can get really close with just a straight thin measuring tool (or even with a cheap caliper).

Then find a kit built for the same or nearly the same stroke. Minor differences can be corrected with things like thicker/thinner base and head gaskets.

If that bolt pattern exactly matches Puch, it's a big clue as to where to look first..


Isolate the rings as the source of the low compression. Squirt some heavish motor oil around the piston crown and see if compression rises.

_it just turns into a slug when it hits the slightest incline_

This could be normal.. all vehicles are gonna go slower up a hill than on the flats. Gross weight matters when climbing hills.. A stock 2-stroke moped is expected to eventually slow to pedaling-speeds on anything like a long, genuine hill.

But if it's unusually bad, the problem could involve carburation or exhaust or other things. Give it a tune-up.

Re: stroke and bore of a yamaha chappy?

thanks for all that advice.

I do really need to know the original bore though, so i can measure the bore now and see how much it's worn. I found a site that has from standard up to 3rd overbore size pistons and rings, but it doesn't specify exact bore, only how much over standard bore it is, so I have to find out what the standard bore is.

Re: stroke and bore of a yamaha chappy?

i get ya.. A few other people around here have Chappys and afaik, the service manual hasn't turned up anywhere.

I've got a cylinder here with 8300 hard miles on it before the piston seized. The bore is scratched to hell. The piston and rings that came out were toasted. But the top of the bore is within 5 ten-thousandths (0.0005" or 0.0127mm) of brand new. This is well within manufacturer's tolerances and there might be absolutely zero-wear on this cylinder's top edge as far as i can tell.

So, unless someone worked the cylinder before, the very top edge of the bore, above the top ring, doesn't wear and is gonna be what the rest of the bore used to be. i'd feel confident using that top diameter as a baseline.

Compare that measurement to the rest of the cylinder and get a good idea if the cylinder is worn or not. If there's no sign of wear, even the 1st oversize piston might be too big.

As a last resort (if there's a return policy) buy a piston and measure it.. and calculate from that what the original cylinder bore must have been.

Re: stroke and bore of a yamaha chappy?

one other thought.. Honda has microfilm manuals and stuff that goes way back. Rings and piston i inquired about were not available but the specs existed.

Yamaha can't be much different.. i heard they even have a complete online parts database but dunno anything about that.. Anyway, drop by a dealer and ask a parts guy to look it up.

Re: stroke and bore of a yamaha chappy?

Here's something.. Google-translated from a page.. looks like a 40mm bore and 39.7mm stroke.

"THIS": is the Main page and THIS is where the stats below came from..

The pages are Google-translated and the original links are modified.. so i dont know if the above links will work.

If not, search the phrase "Bienvenue sur le site des passionn

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