I'm working on a Hummel Aviation Ultracruiser Plus. The way the
engine mounts is quite different from the mounts on most VW powered
homebuilts. The Conventional Wisdom bolts the thing to the firewall
using the clucth end bellhousing, often with an accessory case in
between.

The Ultracruiser Plus is different. Two aluminum angles are bolted to
the sides of the magnesium case, in the sump area, and, suitablely
reinforced, are used to bolt Berry mounts to "bed" type engine
bearers built up out of aluminum, that extend from the forward
fuselage.

These angles are bolted and epoxied to the side of the magnesium case.
My point: what good is the epoxy?

It's probably not carrying any loads. A basic tenet of structural
design is that the stiffest load path carries the load, and the bolts
through the angle and into the case (secured with nuts and washers
inside the sump, before the engine is assembled) seem a bunch stiffer
that the epoxy.

It's not a sure stop against leaks. Epoxy is a wonderful material, but
it doesn't bond particularly well or reliably to metals. Plus, it's
mechanical properties, from it's modulus of expansion, to it's
ductility, are far different that the aluminum, steel, and magnesium
sandwich is it the Mayonnaise of. My concern is the epoxy will
eventually crack.

I lost the reply from Scott Casler of Hummel Engines, I'll paraphrase:

"The epoxy is to keep the angles from working and hogging out the
holes. The epoxy I use is a real good sealer, you've got to grind it
off."

My thoughts are this: LAP the angles to the side of the case (instead
of sanding with 80 grit), but use Permatex or Curil T to seal things.
Use close tolerance bolts in reamed holes in the side of the case and
the appropriate Loctite product to seal the bolts. And I'm inclined
to put the bolt heads INSIDE the engine.

Comments?

To see what this installation looks like:
http://flyhummel.com/forums/album_pic.php?pic_id=170

Ultracruiser (with 1/2 VW) is the same deal