if i get a z400 and then put the yoshi 440 kit in it, your boreing that over right, 40 over? is that ok to do with a new but droken in engine, boring over and will it decrease the life of the engine?

 

Yes that would be boreing your engine. I dont see any problem about the motor if its already broke in and everything. And any time you mod your quad it can effect its reliabilty then again it may not really. You never know

 

you are boring it 40cc's over not the common term of .040 over.

 

Generally boring a motor will decrease its realiability. I don't know why everyone is so hung up on big bore kits. They aren't that great. When weighing out the options like cost vs power gained vs reliability concerns usually they are way better mods. I'd do the big bore last. I'd get a pipe, carb, dual cams, heavy duty valve and a slighly high compression ratio piston(On the Z I believe you can simply use the drz base gasket for a small bump.)