I just purchased a basketcase 2003 400EX. The original piston had blown and the engine was disassembled when I bought it. It came with a Wiseco 440 big bore kit with 12 1/2 to one compression piston. Does anyone have any advise on installing? It looks pretty strait foward but I'm concerned with that high of compression that there are other thing which may need to be changed. Valve springs? Head studs? Rejetted? Will high octane fule be necessary? Aside from the top end and jug, should the lower end be disassembled for cleaning and inspection?

 

It would not hurt to inspect the bottom end. The 440 12.5/1 pistons really like pulling head studs and causing the head gasket to blow. There is a few companies that sell heavy duty studs. I think Curtis Sparks is one of them. You will probably need to run a 50/50 mix on fuel also. The valve springs will be fine, until you buy a "regrind" cam, from somewhere like Web.

 

Thanks for the advise. By 50/50 mix, do you mean 50% high octane and 50% low?

 

yeah with 12.5 : 1 compression you will need to run probably 93 pump with some octane booster. You do NOT want detonation, or your back to where you started from.

 

he means a 50/50 mix of RACE fuel and premium pump gas.

 

with the 440, i would run 100% race gas. the 400ex is air cooled and the 440 kit you had to resleave it. which means it gets hotter. race gas burns cooler because it doesnt take as much of a spark to light it. plus gas today is not like it used to be. even the 93.

 

Thanks for the tips folks! Anyone out there want to trade a 440 Wiseco 12 1/2 : 1 piston for a lower compression piston. Not sure I need quite that much bang for the buck.