On Fri, 11 Jun 2004 05:02:26 +0000 (UTC), David Lesher
wrote:

Not only do (most?) engine manufacturers not allow the automotive
process you describe (to "save" a crank), there are some that don't
allow any grinding at all to go undersize on the journals.

Truth be known, with the "average" light aircraft/engine, you can go 2
or 3+ overhauls (properly operated / maintained) without buggering up
a crankshaft.

P-51 owners tend to rebuild the bottom end (spare engines on hand)
every so often precisely for that reason..... i.e., new/serviceable
cranks are impossible to find and/or obscenely expensive, and in
the long run, it's cheaper to re-bearing/rebuild the engine than it is
to run it until it starts making metal.

Bela P. Havasreti

Jim Weir writes:

When the engine starts making metal, you've got an expensive overhaul on your
hands. Normally, you can get two, sometimes three, major overhauls/repairs (see
another thread for which of these words you want to use) out of the bottom end
before you have to undersize the crank.

Do they resurface aircraft cranks? When marine/truck/pricey auto
cranks get worn undersize, they a

Built back up to the right diameter. A welder goes back and
forth across each bearing surface as the crank rotates;
building up a solid surface.

The crank is chucked into a "crankshaft lathe"... It's then
spun slowly while a toolpost grinding wheel grinds each surface
back to spec.

~Last step is to redrill the oil passages down the crank.

Now, it's almost straightforward for the main bearings -- they are
in line with the axis of the crank. But the *rod* bearings are a
different story. What's done there is the chucks at both ends of
the lathe have offsets - you shift them sidewise until the rod
bearing in question *is* centered. (The two chucks are kept in exact
sync by a driveshaft under the tool bed.)

The whole deal looks like f*$(^9& magic when you see it in progress.
The crank ends, mains and other throws are flailing around seemingly
randomly...but that one rod bearing is dead bang centered....

(The lathe I saw had a ~16' yes sixteen foot marine crank in it.)