ok so i just recently got an 04 400ex over the winter. well over the weekend was the first chance i got to really ride it. valves are adjusted perfectly. installed a manual tentioner to fix the wasted auto one for the timing chain that was on it and get rid of that noise. well we went riding saturday and sunday both. now when we left i was full on oil. day 2 i was riding with different people and they were telling me how bad the quad was smoking. so today we did a compression test which held at 120psi so its good. was’nt blowing past the intake or exhaust valves. but it felt like when i was riding that it just had a real lack of power and was bogging. well we pulled the valve cover and the seals looked good so we then pulled the head.

found the head gasket appeared blown in one spot the timing chain was stretched beyond belief. the one timing chain gide was wasted probly from the bad auto tentioner that it had when i bought it. and the cylinder has zero cross hatch left in it as well. the rings were worn out and almost all lined up on the end gaps as well.

so here is where my questions start. the one guy says just go to a 416 high comp which is .080 over bore at 87mm. should i just do that or go for the 85.5 or 86mm over bore options? 85.5 is .020 over, and 86mm is .040 over. and i didnt see an 86.5mm option when i was looking.

and what year crf 450 timing chain is the upgraded chain that will fit? because im going to get that as well. anything else i should really inspect or address closely? it all looked pretty good from what i could see otherwise.

i work in an engine machine shop so im taking the jug and head with me to work tomorrow to start cleaning up.

oh and yes the cam looked great to as well as the rod. looking to start ordering parts tomorrow asap in the day to get it back on the road asap so quick responses would be awesome.