Re: 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?
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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