Since the piston was at 1.25 oversize, I did a search on ebay and other vendors (eg CMSL) for a 1.50 o/s piston/rings set but this was not successful. I believe original Yamaha Pistons went up to 1.00 oversize but the aftermarket suppliers did go higher (reliability was not really an issue for those guys) which is fine to get the bike running but once you go too big you run into problems like the ports getting out of shape. This is because the ports (windows in the cylinder for the inlet and exhaust) are only a constant diameter for a very small distance after which they start to taper away. This is especially true for the inlet ports. The size and shape of the inlet ports are crucial to the running of a 2-stroke bike. As how Yamaha designed it, it was a compromise between power and economy. Change the shape and you alter the balance resulting in poorer performance. The guys who know what they're doing can get amazing results with big increase in power but is usually confined to a narrow range of revs which usually makes the bike unrideable on the street.
Anyway I posted on a local facebook page for pistons/rings/cylinder and got a response from Kenny at facebook page "Classic Japanese Motorcycle Parts Malaysia" who had an original NOS cylinder and (std bore) a piston (1st oversize).
A box.. unmarked except for the writing.
the contents.. a NOS cylinder ...
This is the paper stuffed inside the bore..it is some kind of anti-rust thing.
The whole lot.
The marking on the kit and inner boxes show the suffix -00 which I understand is for standard but the markings on the piston is 47.25. Probably some mix-up in packaging but I'll need to measure it up, especially the rings.
So now I have a nice new standard piston but only an o/s one to match. I could bore the cylinder to match but it seems a waste to do that so I went on the hunt for a standard piston and bore.
This is what I scored on ebay.
A standard size piston
And a set of standard rings
Currently on the way to me here in Malaysia!
In my hunt for a cylinder and piston here in Kuala Lumpur, my regular parts guy told me he had pistons up to 3.00 oversize! I brought my old parts to him and he said no problem. Cost MYR119 for a rebore to 1.50, pistons, rings, piston-pin and base gasket. This works out to around USD 30 at todays rates. For that price I told him to go for it but I think I'll go with the standard size NOS parts I bought for now. The potential problems as mentioned in the opening paragraphs above are not worth it on a bike which will not see much mileage! I should be getting the cylinder and pistons back in a few days and will post pics up here.
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