I know my 2 stroke buddies both have extra sets of big bore cylinders. One on a LT250r has a 330 big bore cylinder and my friend on a banshee has a trinity 420 big bore cylinders. Can you only buy a stock cylinder for the 400ex or is there such a thing as aftermarket big bore cylinders from companies like GT, CT, TC, LRD, or sparks?

 

yes, i think wiesco makes a 440 piston and sleeve (cylinder) kit. It comes with all of the stuff their piston kits come with but this kit also includes the sleeve.
Check out the lastest dirtwheels and look through some of the advertisements, you'll see it in there somewhere.

 

FST is now doing liquid cooled cylinder's for the 400EX. in a 500cc size. it looks pretty slick! check out the new issue of dirt wheels. I love that orange plastic too.