[TR] Burned Exhaust Valve

[Jim Muller]

On 15 Sep 2009 at 11:43, akgraves at cox.net wrote:

> Then I noticed a gaping hole in the #6 exhaust valve...
>  My question is what could have caused that? 

I'm sure one of our mashinist experts will answer, and maybe 
contradict me (in which case I'll learn something).

Several thoughts come to mind.  A burned exhaust valve is typically 
caused by it being set too tight.  Of course, with a pushrod engine 
when you remove and replace the head the valves need total re-
adjustment.  Then when (if) you re-torque it that tightens them up by 
pulling the head down onto the block, so you need to to adjust them 
again.  Even so, 100 miles is a short distance to burn a valve unless 
it was damaged to begin with.  Also too early ignition timing and too 
low fool octane will make the engine internals run hotter and 
exaggerate the burning.

Even so, that would not necessarily cause the loss of so much oil in 
only 100 miles unless it was being pulled up past the rings and not 
being blown or scavenged back down.  A more likely cause would be a 
badly worn valve guide, which could similarly have contributed to the 
valve not closing against it seat properly.  As I recall, a typical 
sign of worn valve guides is that after you sit at idle for a few 
minutes such as for a stoplight and then pull away, the exhaust blows 
a lot of oil smoke (which is blue-ish), though it does not otherwise 
seem to burn oil excessively.  While you have the head off and are 
dealing with the valve, check (or have a good machine shop) check the 
guide and seat too.  Might as well check all of them!

-- 
Jim Muller
jimmuller at rcn.com
'80 Spitfire, '70 GT6+