1mm overbore high-comp pistons?

[DannoXYZ]

So I'm rebuilding engine over winter for next season and going to bump up compression to restore some of lost-power compared to pre-gens. Rules also allow up to a 1mm overbore. But all the aftermarket pistons are 2mm over.

Anyone have source for 1mm larger pistons?

[jrshooter]

Originally Posted by JacRyann

So I'm rebuilding engine over winter for next season and going to bump up compression to restore some of lost-power compared to pre-gens. Rules also allow up to a 1mm overbore. But all the aftermarket pistons are 2mm over.

Anyone have source for 1mm larger pistons?

check with spears racing, they also have thinner head gasket.

[Racer x]

I use JE pistons. Super quality forged pistons and ring. Makes a 265 cc engine with 2mm over bore. 12.5 to one.
I don’t recommend cutting the head because the valves are very close.but I took .010 th off the block top to add a little compression and change the cam timing for better top end

[DannoXYZ]

I was pitted next to Spears last weekend @ Buttonwillow! They had some neat bikes. There was a compact 2-stroke of some sort.




Unfortunately, they only use off-the-shelf pistons from JE, so increased compression in stock or 2mm over only. For 1mm, it'll have to be custom order. In which case, i'll get from my usual suppliers, Wossner, Arias or Arrow most likely.

I'll slot the cam gear, gives me finer control over timing.

What fuel are you running for 12.5:1? Stock ignition timing?

[NinjaBraap]

Lucky! We don't get any engine mods here in Oregon with OMRRA for the Ninja 250 class.

[DannoXYZ]

Seems AFM's going in pure-stock direction for production classes. Outlawed non-OEM oil-filters, for good reason! Years ago, they outlawed newgen 17" wheels upgrade for pregen 16" bikes. That effectively killed off 250 Prod class as that made 60-75% of bikes non-competitive as most were pregens. Newgen 250s never caught on due to cost of buying new bike just for racing. For that amount of money, most people would just get a 300.

Somehow the 1mm overbore and increased-compression upgrade has survived. JE can make custom 1mm over high-comp pistons but I'd have to buy set of 4 along with set-up charges. I may just go with Wiseco 12.5:1 pistons in stock bore for now.

[NinjaBraap]

It's still going pretty well here, but I'm hoping it goes in the direction of our sister organization WMRRA up in Washington. Even though it's still called Ninja 250 cup they now run it more like Ultralight SS and allow any 200-300cc two and single 4-stroke, and just throw extra stipulations on anything over 250cc to keep everything as close as possible. Almost no one I know uses the pre-gen 250s, our entire class is made up of 08-12 250s, almost bone stock and little to no excess weight. Really comes down to skill for us.