To be honest this all sounds like a bit of a mess.

There are a number of issues here ... however it is difficult to comment in
any detail until I know what sort of epoxy is specified and without further
details of the surface preparation and details of the design.

1. Most common epoxies have a Glass Transaition Temperature (Tg) of approx
90 deg C. If the epoxy is taken above that temperature then two things
happen ... first it softens and the stiffness and strength reduce
dramatically. Second irreveraible damage is done to the epoxy and it will
never be the same again - even when cooled. Unless you are using one of a
small number of epoxies that are designed for high temperature operation
(some of these have Tg of approx 400 F, 200 C I think without reaching for a
calculator) then it is certain I think that if used on an engine the 90 deg
C limit will be exceeded.

2. Next there is the issue of surface preparation. If the plans say prepare
by running with 80 grid paper then it is fairly clear that the guy who wrote
the plans knows little about what he is doing ! Epoxy metal bonding is
reliable if the surfaces are prepared properly but from what you have said I
doubt that is the case.

3. Bolted and bonded joints should be avoided because it is difficult to
predict the load transfer etc ... I won't go into detail on this - perhaps
later.

4. What are the differences in the thermal expansion coefficients of the
aluminium and epoxy ... from MIL-HDBK-5J alumnium is approx 12.5e-6 in/in/F
and Magnesium is 14.0e-6 ... not a lot of difference perhaps this is not an
issue.

5. Galvanic corrosion .. magnesium is at -1.6 V and alumnium at -0.75 V on
the galvanic table. That is a big different. You definitely need something
to separate the two or the magnesium is going to get gobbled up !!

6. I think galling of the metal is a possibility but if appropriate
tolerances are used for the bolts and holes than this would be less of a
problem. Hard to say without seeing the drawings etc. Sounds to me like if
the holes are flogging out then the design has some fundamental problems and
that one should not be relying on epoxy that probably cannot withstand the
temperatures to fix it.

My gut feel is that you need something between the alumnium and the
magnesium for corrosion protection and possibly the help the galling issue.
I would assume that this is sufficiently ductile an rubbery that it will not
pick up any load and that the fasteners will transfer all the load. I would
use a rudder like sealant compound that can take the temperature ... not
epoxy. When you use sealant of this type in a joint with fasteners extra
largers of safety should be allowed because of the extra flexibility that
this produces in the joint (typically an extra factor of 1.5).



"Ryan Young" wrote in message
om...
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