Streached valves????

[Flyin-Low]

My buddy just bought a 2004 CRF250 and there was a slight ticking in the top end so he took it to a mechanic to have it checked out. When he got it back the guy said that the valves are "streached" and if they have been like that for a long time that he will need a new top end soon.


So im just wondering what do they mean when a valve is "streached" and why would you need a new top end?!

Ive never heard this stuff before.. And also people are telling him to replace the valves on a dirtbike every year...

-Martin

[speedfreaksguy]

I would find a new mechanic. It is common knowledge that the stock CRF intake valves deform, not stretch. The valve face is beaten against a much harder valve seat causing the valve to deform and sink deeper into the head. The stock parts are inexpensive to replace. Your friend may want to consider replacing the intake valves with stainless steel valves. Whatever he does, make shure to have the valve seats recut before installing the new parts or they won't last long.

[250X_project]

I agree.

The valves don't get longer, they just come up further by sinking into the seats, as speedfreaksguy said.

Personally I wouldn't worry about it until it really does need a top end rebuild. Until then I would just keep the valves adjusted. No big deal.

Although, the ticking would be an excess of clearance rather than the the valves deepening into the head. If still ticking after the adjustment I would check the cam chain and tensioner.

my 2cents;)

[speedfreaksguy]

The CRF250R is prone to eating stock intake valves. Sometimes after only a hanful of hours. He will know when they tighten up because it will be hard to start. The stock valves can only be shimmed so many times before they need to be replaced.

If the mechanic that looked at it shimmed it, I say ride it until they tighten up again.

I had to replace valves in 06 model year Honda's already, so it wouldn't be a stretch to say those may be shot.

[250X_project]

Originally posted by speedfreaksguy
I had to replace valves in 06 model year Honda's already, so it wouldn't be a stretch to say those may be shot.

That's terrible. What are the OEM valves made of?

[speedfreaksguy]

Stock intakes on the CRF250 are titanium.

[400exrider707]

Originally posted by 250X_project
That's terrible. What are the OEM valves made of?

titanium valves wear out really fast. But they are light in weight and help to prevent valve floating especially on those 250 thumpers with high rev limits.

[250X_project]

Oh. I don't know why they don't just use steel valves with stronger springs.

[bwamos]

Originally posted by 250X_project
Oh. I don't know why they don't just use steel valves with stronger springs.

Because it robs some horsepower.

Pushing more weight around. Combine that with heavier springs, and the exponential relationship of acceleration.. it take a LOT more force to move those springs at the high rate of stop and go.