hey whats up. I have a 2001 400ex thinking about rebuilding the engine. Thing is, is that there is a noise sounds like the valves opening and closing but loud. When its idleing you can hear it. Is this normal? My friend that i bought it from ran it really hard.. so im wondering if i should rebuild it anyway? I would not make it into a 440, but is there like a upgraded piston i can use that will fit without doing anything else to the engine?

 

i do not know if the 300ex and the 400ex are the same in any way but my 300ex would do the same thing that your is doing and i thought that it was the valves but it was the cam chain was streched and i did not find out until it came off and did alot of damage so i would check that out first. and i do not know forsur but you might be able to get a higher compes piston with the stock bore.

 

it could be the cam chain or the valve rocker arm the adjuster screw al a little loose

 

yes the 300 ex that i got had that problem, i bought it broken and it was the timing chain, it had came off but did nothing to the motor. i found tha i had flattened rockers too. so it could be both of those and that is no reason to rebuild the top end unless you really want to since you need to take the hole top end off anyway. chain costs 100 bucks for a 300 and rockers are 36 bucks each. not sure on a 400.