Thanks to Brian at Power British and other listers who responded to my
panic a while back over the excessive ring gap I experienced when
fitting new +.040 pistons during the rebuild of CF38690UO's much
machined engine.
Understand that the fitting of oversize pistons and reworking of the
head was necessitated by shoddy machine shop work several years ago
during an initial rebuild and my naively not checking behind the
machinist, whom I thought was a friend giving me discount work. So I was
almost fanatical with the machine shop that did my recent work about
doing it right, all the while not trusting them much, especially after
they broke my custom grind cam. But that's another story.
Anyway, when checking the ring gap and finding it was too large, I
immediately set out trying to prove the machine shop had over bored the
cylinders. After some good advise from the list, and putting on my
thinking cap and putting away my emotions, I determined that my piston
to cylinder wall clearance is .002, so the problem must be with the
rings (pistons are not round Jim, and they taper). After several phone
calls to BPNW and Deves Piston Rings, I spoke at considerable length
with Maria at Deves. At first, she was unwilling to admit anything was
wrong, as the 2nd ring gap ( .018 ) was in what they call "maximum
tolerances". Then she conceded that if it were her engine, she'd insist
it be in "factory spec." and sold me a set of larger compression rings
at a discount. BPNW agreed to issue credit for the rings supplied with
the pistons if I returned them.
Moral: check and double check all tolerances when doing an engine
rebuild. Just because parts are supplied as a kit doesn't mean they're
correct. There is little doubt in my mind that if I had a local repair
shop or machine shop assemble this engine, the supplied rings would have
been installed. The process of filing the larger replacement rings was
tedious and time consuming, but I believe it will pay off in improved
compression, less oil consumption and longevity.
Again, thanks to all who helped me through my initial *panic* stage and
led me to the problem.
Jim Davis
Fortson, GA
CF38690UO
CF37325U
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