Absolute Tightest Valve Clearance before Failure - Honda Motorcycles - FireBlades.org
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post #1 of 6 Old 11-01-2009, 1:58 AM Thread Starter
 
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Absolute Tightest Valve Clearance before Failure

Has anyone measured or have a calculated guess at how tight you can let the clearance get on a 929 before failure occurs? I know what the tolerances in the service manual are (IN- .16mm EX- .27mm both +-.03mm), but I'm interested in what the absolute tightest you can let an exhaust and intake valve get before major issues happen. I know there are probably a thousand different variables.

I just guessing that on the Intake you can let it get pretty close to no clearance before issues and on the Exhaust I'm not too sure with the extra heat expansion, but I'm guessing here again it might be the difference between Intake and Exhaust which it .11mm.

Anyone have any thoughts, ideas or have actually measured this out somehow?

-Richie-
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post #2 of 6 Old 11-01-2009, 1:30 AM
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Re: Absolute Tightest Valve Clearance before Failure

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Originally Posted by nomad13RR View Post
Has anyone measured or have a calculated guess at how tight you can let the clearance get on a 929 before failure occurs? I know what the tolerances in the service manual are (IN- .16mm EX- .27mm both +-.03mm), but I'm interested in what the absolute tightest you can let an exhaust and intake valve get before major issues happen. I know there are probably a thousand different variables.

I just guessing that on the Intake you can let it get pretty close to no clearance before issues and on the Exhaust I'm not too sure with the extra heat expansion, but I'm guessing here again it might be the difference between Intake and Exhaust which it .11mm.

Anyone have any thoughts, ideas or have actually measured this out somehow?
The EX has larger clearances to allow for expansion already as it runs hotter than the IN valve.
The minimum spec is about as tight as you want it to be as any spike in temperature can push it over to where it holds the valve open during combustion and burns the valve face - and then you need to pull the head off to replace the valve.
One advantage to rocker systems is that you can set the clearances at minimum spec to maximise cam duration knowing that they'll get loose rather than tight. With shims they invariably get tight though as the valve "stretches" so you can't do this unless you're happy to go in and check them very regularly - like you would on a race engine.
The valve doesn't actually stretch very much but "valve stretch" is the usual way of refering to the way the valve clearance reduces with wear to the valve face, seat and collet groove.

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post #3 of 6 Old 11-01-2009, 1:34 AM
 
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Re: Absolute Tightest Valve Clearance before Failure


Larry is awesome
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post #4 of 6 Old 11-01-2009, 3:10 PM
 
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Re: Absolute Tightest Valve Clearance before Failure

As Bladeracer said...

As the valves get tighter than spec, the valve will begin to remain slightly open under operating temps. At first, you will notice a loss of power. The longer you allow it to be out of spec, the more damage will inevitably occur leading to failure of the valve train like a burnt valve. Keep the valves within spec if you want it to live a long healthy life..
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post #5 of 6 Old 11-01-2009, 5:15 PM
 
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Re: Absolute Tightest Valve Clearance before Failure

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Originally Posted by bladeracer View Post
The EX has larger clearances to allow for expansion already as it runs hotter than the IN valve.
The minimum spec is about as tight as you want it to be as any spike in temperature can push it over to where it holds the valve open during combustion and burns the valve face - and then you need to pull the head off to replace the valve.
One advantage to rocker systems is that you can set the clearances at minimum spec to maximise cam duration knowing that they'll get loose rather than tight. With shims they invariably get tight though as the valve "stretches" so you can't do this unless you're happy to go in and check them very regularly - like you would on a race engine.
The valve doesn't actually stretch very much but "valve stretch" is the usual way of refering to the way the valve clearance reduces with wear to the valve face, seat and collet groove.
Um, not all rocker systems get loose. Ninja 250s are well known for burning exhaust valves if you don't adjust them on time.
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post #6 of 6 Old 11-01-2009, 9:44 PM
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Re: Absolute Tightest Valve Clearance before Failure

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Originally Posted by Junkie View Post
Um, not all rocker systems get loose.
True, I should have said most rocker systems do :-)
Between cars and bikes that I've worked on (including one EX250R) I have never seen a rocker system clearances decrease. I'm sure there must be some that do though. Generally, the numerous moving parts and the clearances for each all add up over time to wear faster than the valve "stretches".

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