+1.5 250 RZ pistons & rings?

[jools]

At 1mm oversize the liner is looking a little on the thin side, 1.5mm ? I don't know. I would have thought someone else has done this... Anyone gone 1.5mm oversize on a 250 ypvs ?

[James P]

Jools; Only you can judge how "bad" the damage is. However...provided there are no cracks, I would run a smooth file over the scuffed area on the piston, then send it away for coating with Molybdenum Disulphide (I use HPC in Leongatha, but there may be other providers).
The coating is normally about 0.025mm thick, but a half-thickness or double-thickness coating can be applied on request. You can then hone the cylinder to re-establish the correct clearance and fit new rings (if required). From my own experience, the coating makes the piston practically seize-proof...as long as there is some oil in there!
If it was me, I'd get the other piston done too, just so both sides are the same!

Regards,
James

[jools]

That's a tip James, good idea, I'll give them a call to check their price for the moly coating.
I'll hone the cylinder soon, and that'll give me an idea how deep the scoring is. I don't think it's too deep, we'll see, and I think the piston will 'dress' with some judicious work with the file.
Cheers.

[jools]

Your opinion required. See above the photo of the 2 pistons side by side. The one on the left is an original Yamaha RZ250 +0.5mm piston, and on the right a Yambits Mitaka replacement piston which is from a pre-powervalve model (RZ250 ypvs +1mm pistons are unavailable). Note the different piston port shapes/sizes. Should I open up the piston ports on the replacement to match the larger Yamaha ones?
The bike did run fine on the smaller ports, but the area is MUCH smaller eh?

[hybrid]

Another one of those discussions that many can't agree on.
Personally, I don't think I would bother making them bigger. With the amount of time it would be covering the intake port, I can't see it making all that much difference.

Then there's the other side of the argument - if it wouldn't make any difference, why would Yamaha bother making them bigger in the first place?

I would be concerned about compromising the strength more than worrying about gaining that 1HP.