[TR] Burned Exhaust Valve

[akgraves at cox.net]

I wrote a few weeks ago after replacing piston rings and rod bearings.  That
repair seems to be good - so far.  This past weekend I drove 100 miles to a
car show and it seemed to run okay but the fuel economy was poor.  Then I
discovered that my oil level was very low.  I topped up and checked it again
on my return (100 miles) and discovered that I had burned 2 quarts!  It did
not leak out.  A compression test showed 0 lbs on cylinder 6.  That's where
the oil went.  #6 plug was caked with oil and soot as well.  Thinking I had a
head gasket failure or new piston ring installation errors I removed the head.
The gasket looked good, nothing obvious, and the cylinder walls looked good.
Thinking that the head must not have been torqued properly, I began cleaning
my tools and shop for the next day.  Then I noticed a gaping hole in the #6
exhaust valve.  I removed it and there is a 5/16'' chunk missing!  My question
is what could have caused that?  During the previous work I removed the head
and inspected and cleaned all valves and lapped them into their seats.  They
looked good then.  I plan to drive to Triumphest and need to know what
happened so that it doesn't happen again.

Thanks,

Art Graves
Tulsa, OK
1976 Triumph TR6