Just thought I'd pass on a short story about the #4 cylinder in the IO-550-C
(left engine) of my Baron.

The engine has about 1000 hrs (factory rebuilt) and compression hadn't been that
great. During the last inspection I got 30/80 on the compression check and with
my tester 58/80 is passing. I borescoped and found a spot on the exhaust valve I
didn't like much. It had that faint telltale green hue in one spot. I pulled the
jug, inspected the valve, found lots of wobble, and sent it off to Mattituck. I
told them I thought the bore looked good and just do the valve and guide. They
insisted that it would probably need to be bored. After inspection they found
the bore just fine, changed the valve and guide then repaired the seat.

During reassembly I looked everything over and was checking the rocker arms. The
exhaust rocker has a .040" hole that runs from the oil galley in the bore (where
the rocker shaft fits) to the surface of the arm. While blowing it clean I
discovered that there was very little air passing through that hole. That hole
is there to squirt oil on the valve stem.

Soooooo .... rocker shaft oil squirt hole is partially blocked, valve stem gets
insufficient oil, valve stem and guide wears, valve doesn't seat properly, valve
face doesn't cool, valve and seat burn a bit, low compression ....... and all
because not quite enough oil through that tiny hole because of some blockage
which I'm pretty sure was due to either incomplete machining or a metal chip in
the hole.

Ted Stanley