What parts usually need to be replaced in a rebuild?

These are the things I replace during my annual race motor rebuilds:

rings +0.5mm

new pistons +0.5mm

connecting rod Big end bearings, 4 halves

main bearings; 1 & 3 Need 4 halves, 2 Need 2 halves

(usually don't replace the balance shaft bearings 4 halves)

cam tensioner spring, 92081-1984

cam chain, 92057-1210

head gasket, 11004-1312

base gasket, 11009-1572

clutch cover gasket, 11009-1872

alternator cover gasket, 11009-1873

shift shaft seal, 92051-005

copper exhaust crush washers, 11009-1866 qty 2

There are three choices for Ninja 250 pistons:

1) Stock Kawasaki pistons

2) +0.5mm Kawasaki pistons

3) +1mm pistons (available through G-Force)

You can also use Cosworth +2mm pistons, but they are illegal for production racing and make the cylinder walls really thin, so if the wrist pin comes loose it can work its way through the cylinder wall and suck in a bunch of water and... I actually bought the bike with +2mm pistons in it back in '96. I didn't know at the time, and raced it that way for about 1 year before it blew up and I built it to production spec.