text news wrote:

Second option is to use Norvic but go for their "Millenium" rebuild at
£11491 in which "new cylinders are the
Millenium investment cast type and the crankshaft and connecting rods are
rebalanced to a finer tolerance than standard Lycoming limits" whatever that
really means.

Most companies use a technique called "sand casting" to make the cylinder blanks.
Millenium uses "lost wax" casting, AKA "investment" casting. The lost wax technique
captures finer details and will allow greater precision. You will be able to see
this in the cooling fins. There are also claims that investment casting produces
a better quality metal with less tendency to have embedded stress points. It is
certainly possible to accomplish this, since one can use higher pressures when
forcing the metal into the mold. "Rebalancing to a finer tolerance" simply means
that they try to make the counterweights on the crankshaft match the pistons. If
they do a better job of this than Lycoming does, the engine will vibrate less.


I understand that with "re-built" you get a new log book. This sounds good
but in practice it is because the engine has been re-built from a collection
of untraceable parts, hence the new log book, so the advantage seems
emotional rather than mechanical!

You've been lied to. When Lycoming creates a remanufactured engine, they use mainly
new parts. Whether new or used, all parts used meet the spcifications for new parts.
That's the logic behind the new logbook.

When Lycoming overhauls an engine, they also use used parts "from a collection of
untraceable parts", but those parts only have to meet the specifications for return
to service. Either way, they install new pistons and cylinders.

One big question......
As far as we know, there is nothing fundamentally wrong with our engine and
it has a "known" history, no prop strikes or anything nasty. What is the
chance of finding a cracked crankshaft or something equally nasty?

There's an excellent chance that the crankshaft will be pitted, possibly badly
enough to require replacement with a solid crank. It's highly unlikely that it
would be cracked. The camshaft is more likely to require replacement.

If we have our own engine overhauled (zero timed) is that better than
swapping it for a Lycoming overhauled engine whose history we have no
knowledge of?

The main thing that gives problems is the cylinders. In a field overhaul, your
old cylinders are usually bored and re-used. If necessary, the bore may be
plated back up to size. Other things may be done to recondition them, but, any
way you look at it, they've put up with a lot of abuse. Few cylinders will reach
TBO three times; some won't make it twice. Lycoming replaces the cylinders with
new.

I have heard that engines can only be re-worked about three times before
they are out of spec. If we get a Lycoming exchange engine how do we know
that it hasn't already been re-worked two or three times and would be
rejected at the next overhaul?

As far as the "bottom end" is concerned, you don't.

George Patterson
A diplomat is a person who can tell you to go to hell in such a way that
you look forward to the trip.