Frank, Zoran and Gary,

Thanks for the replies.

Heli-Coils appear to be made by Stanley Engineered Fastening, and
Time-serts by Time Fastener Company.  I searched timesert.com for any
mention of "heli" or "Stanley" and found only a user comment about
switching away from Heli-Coils.  Of course, there could be a holding
company that owns both.

Thanks for the info that the insert threads are sealed.  That addresses
my earlier question.

There really is no evidence that the Heli-Coil solution isn't working
as it should.  There were no leaks for 20 years.  My experiment with a
drain plug with a large shoulder and matching soft washer moved the
sealing surface well beyond the area affected by the thread insert.  It
still leaked.  So, while the Time-sert may be a better choice than the
Heli-Coil, it really does not address the evidence.

Gary sent a message to me off list that suggested that I close off the
drain orifice completely with epoxy and go forward by using a pump to
remove the oil through the dip stick tube.  I like that idea since it
most probably will solve the leak, or if it is a case crack, that
solution should provide better evidence of that.  What I'm uncertain of
is how that approach will affect rebuilding the engine in the future,
and what the downside is of not quite removing all of the engine oil.

I remain puzzled, but thankful for your advice,
Mike