"Dick" wrote in message
news:2c2qk.130$p72.75@trnddc05...
Dale,
If OP means original poster, that is me. For the record, my timing is 2
deg more advanced but still BTDC by 32 deg. After an A&P rebuilt and
installed my mags about 100 hours ago, I'm not sure if they were installed
per the manual spec 30 deg or the 32 deg I found by test recently. A
different A&P
doesn't have a problem with it either way.

I any case, I am just curious about any range established per "rule of
thumb", experience or paracticality..

Thanks for taking the time to give your opinion.


snip

I can't give you a rule of thumb. I can pass along an informed opinion
developed over the last thirty years as aircraft mechanic, with several
thousand hours of that spent working on small Continentals.

1. You shouldn't run your timing more advanced than specified. Two degrees
is too much excess advance for your engine; 30 degrees advance is a lot of
advance for an engine that turns about 2,000 rpm most of the time.
Continental specified 30 degrees to get good performance at a cost in
durability. When I set mag timing, I'm happy only when it's within 1/2 a
degree. But I would worry if it were 1/2 a degree too much advance on a
small Continental.

If the mechanic who put the mag on set it right to begin with, then your
points are wearing faster than the cam rubbing block, and the timing will
continue to advance as you fly. And the e-gap, or internal mag timing, is
drifting too. If the timing shifted 2 degrees in 100 hours, you have
something wearing too fast.

How, if you could not see the prop flange markings, did you establish TDC?
Are you fully confident that the advance is 30 and 32 degrees? By the way,
if you can see the flange markings from the top, but not the bottom, you
could time the mags using a cylinder other than #1. The second A&P may have
doubts about your methods. How did the first A&P set the timing; could he
see the prop flange markings, or did he use a timing wheel? The wheels can
be inaccurate if improperly set up or if the pointer friction is excessive.

2. How old are your cylinder heads? Not since overhaul, not since cylinder
barrel replacement, but since the heads were new? Have they ever been
welded? How old is your case? How many hours total time? Who did the
overhaul, and what was the condition of the mating surfaces and bearing
journals? Continental hasn't made a C85 in quite a number of years, and
advancing the timing a couple of degrees, especially from a fairly advanced
setting to begin with, will result in significantly higher cylinder
pressures. Are all your engine parts up to that?

The questions I asked above would be questions I would have in making the
decision to reset the timing. But, in the end, I would probably use the
time-rite or timing wheel and recheck the mag timing. If it had any excess
advance I would reset the timing. If it had two degrees excess advance, I
would pull off the point covers and look at the points.