Bent Valves

I am just curious about possible causes for bent valves. The cam chain has some serious stiff sections and needs replacment. No contact between valves and the piston were made. Would the chain bend the valves or are the two problem unrelated?


The valves most likekly did contact the piston, and you just can not see it. Exhaust valves are the ones bent by chain problems. Be sure to inspect the valve guides very close. They break when the valves are bent. Also if you have a 98, replace the connecting rod while your in there. It is a lot of labor, but a necessary maintenance item on the 98. They become very brittle over time, and break the top pin hole (info from stroker after my 98 broke the top end of the rod). Yamaha redesigned the YZ rod and crank for the 426.


Pretend it's flat and give it the gas.

The bike is a 99 and thank God not mine!!! The valves were bent so slightly that we couldn't tell until we spun them on a valve grinder. Being a 99 does this mean that the connecting rod should be fine or no? Thanks for the info. on the guides I will pass it along.


99 YZ400F, 86 XR250

76 RD400, 71 Z50

My understanding from stroker was the rod was redesigned for the 426. All 400's had the small piston pin hole and were subject to breakage. Several people I have spoken with say overreving the engine (missed shifts to the rev limiter) will float the valves and could easilly bend the valves. I also have found that grinding some of the compression release actuator where it contacts the valve shim bucket will help keep the valve from contacting the piston if the compression release is accidentally hit.

Hope this helps, it is getting pretty painful rebuilding my blown up 98.

YZ400Court (Soon to be KTM520EXC Court)


Pretend it's flat and give it the gas.

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