They were not identical. With a different diameter the shape (volume) on the top is also different. It can also be the height of the piston from the top ring up.

Furthermore, the top ring does not go all the way to the surface between the head & cylinder. There is some small volume here that counts towards the compressed volume. This volume is increased for the larger bore, so the compressed volume is also larger by the bore which helps to maintain the correct compression ratio as bore increases. This is a very small volume but since this is compressed volume it is worth 11x that volume (for an 11:1 compression ratio) for uncompressed volume (displacement).

Re valves:

Kawasaki thinks the 300R & 250S will run at different temps as evidenced by the fact they felt the 250S needs a fan & the 300R does not. They may have also thought the 250S could be subjected to WOT on the interstate for hours where the 300R would not. They may have applied a different heat treatment, material, coating, etc. Who knows.

I would order the valves & if they spec the same use them.

KLXer's final thought: 300R cylinders & pistons. Dont out think yourselves. Kawasaki sells the 300R cylinder & piston as an official, factory warranty approved accessory in non US markets. The kit has even been included as a promotional item as incentive from Kawasaki. The engineers are OK with you running a 300R cylinder & piston on your 250--they did it themselves for the upgrade from the 1996 KLX250R to the 1997 KLX300R.

HERE IT IS: straight from the official Kawasaki Austrailia page

http://www.kawasaki.com.au/index.php...=MC&SubCode=OR