400 blew up

[400EXdude]

hey, my 400 took a crap on me and the piston is screwed...so i need to get a new one...i noticed that most of the pistons are the same price for stock or for like .040...what the difference...dumb question but i need to know lol..also im going with a 10:8:1 piston prolly JE or wiseco...will this give me much of a power improvement any help is appreciated thanks

[F-16Guy]

0.040" means that the piston is forty thousandths of an inch larger in diameter than the stock piston. That would bring the displacement up to about 406cc. What you need to do first is tear it down and find out why it blew up. If there are chunks of piston missing, they'll need to be found. If it seized due to heat and/or lack of lubrication, you may also need to replace some bearings. If it's only the piston, and you're not missing parts, then at the very least you need to take the cylinder to a machine shop and find out how much it needs to be bored to clean it up. Don't order a piston until you at least do that so you'll know what size piston to buy.

[400exredrider]

if you blew it your gonna need more than a piston ;)

[400EXdude]

well dad something about dirt gettin in through the intake...which i dont know how that happened cause i run it with the airbox lid closed most of the time...and i didnt mean i blew the whole thing i just need a new piston for it...and no there are not parts missin just severly scratched er whatever...but the kit i will prolly get comes with rings gaskets and all that stuff so thats good..(it also comes with a hot cam:) ) but anyways ill prolly go with 10:8:1 so i can run it on pump gas

also...how much do those nakisal coatings cost...thanks?

[Quadman250R]

.040=1mm over original bore
.080=2mm over original bore
or you can calculate itin thousandths of an inch like F-16guy said.

yeah a bigger piston will give you a power difference, if you go .040 over it wont be too dramatic but it will be noticeable.

[400exredrider]

thermal coated JE 416 piston 10.8:1 from CD is approx 175 shipped