In my aviation days, engines given a "normal" life span, were rated for
so many hours of operation and when that limit was reached the engine
was shipped to a certified remanufacturing facility torn down completely
and rebuilt back to original factory specs, in which case the engine
would come back with a "yellow tag, indicating factory new specs,
therefore one could expect factory new performance and lifetime. There
were other types of rebuilding with less than factory spec that would
still extend life, but that is not for this discussion.

Along the way from "zero" operating hours to life limit, there would be
a time to do a "Top Overhaul" to extend to the full lifetime of the
engine. Depending on the FAA rules involved in the operation where the
engine was being used, one could do this top overhaul at the specified
numbers of hours of operating time, or as indicated by oil consumption
rate and compression tests. The top overhaul consisted essentially of
rebuilding the top end,ie, the case was not opened. The cylinders,
pistons, rod ends, rings, valves, guides, seats, rockers and shafts, all
mike and components adjusted by replacement or repair to bring back to
within wear limits. At the time of a top overhaul, a crankshaft runt
test would be made with a dial indicator to determine the amount of
wobble in the crankshaft. If that check was in spec, then the case was
left intact. If not, then it was time to crack the case for overhaul,
and that would include the case and all internal components.

But back to the matter of a top overhaul. On the WBX, is it an advantage
to consider the half life of the engine, and do a top overhaul to get
the full life that the crank and crank bearings would/could  provide if
the cylinders are kept up to snuff, or would it be best to just run it
to the point the cylinders give up and then do a major rebuild including
tearing down the case.

I am curious about this. I have often heard it said "Don't re-ring the
thing or do a valve job, as the new compression will wear the mains and
crank throw bearing out faster then you will have to rebuild it anyway.
I'm not so sure about that, given its a common practice to top overhaul
opposed cylinder aircraft engines.

I seek input of experiences from the list membership.

TIA.

John Rodgers
88 GL Driver