Hi Steve,
The reason I use music wire is because it is stronger than both the wood and the epoxy joint against the up and down shear forces created by a running engine.
I had these shear forces break the glue joint in my first crutch but since  I have been using the wire no failure and more importantly no more shaking or vibration problems because a solid crutch seems to reduce all that nonsense.

The wire is also locked in by the ply doubler so it isn't going anywhere and I have never had one come out. I doubt I could hammer one out without damaging the crutch.  I really rough up the wire with a file to give the glue something to grab onto and get a good bond.

Anyway, this setup has worked sucessfully for 30 years without any problems.
I had a friend who saw my crutch and decided to use wood screws and it worked for him. I tried it but the screws weighed more than the wire and didn't add any more strength so I went back to wire.
It is interesting you mentioned threaded rod I once got the idea to use threaded rod and to glue & bolt the crutch together. It was heavier and if anything weaker because I had to remove wood to countersink for the nuts. I had all but forgotten about that one. We tend to forget our mistakes.

Steve, as long as your dowels can hold up to the ongoing shear forces I don't see a problem. The mounts made out of a glue lam of 1/16" ply should also work fine.  Plywood will compress when you tighten engine bolts so you can  drill out holes where the engine bolts go and then drill engine mount holes through the dowels or drill out and glue in aluminum or brass tubing so the engine mounting bolts goes through the tube. These are other peoples good ideas that both work and I have used both over the years.
 
Steve, best of luck with your project, I hope it turns out the way yo want.
                                                                 Pat Robinson

Hi Guys,
I am in the process of creating an engine crutch for a classic plane using 1/2" by 3/8" maple but the problem is trying to find straight, parallel and square wood to make it out of. I have a fair sized stock on hand and I have even raided local hobby shops to check their stocks but no joy.
I have cut down oversize stock till it was straight and true but within a day
tension in the wood grain got released by my trimming it down and I ended up with a correctly sized but bowed piece.
My question, has anyone tried to create "engineered wood". My idea is to cut
1/8" thick lengthwise strips for the top and bottom and fill the middle with vertical grain maple, use a structural epoxy like t-88 and clamp that sucker tight. When it is dry sand it true and straight. All the different grain directions should cancel out any tendency for the piece to bow.
Some questions I have are will it be strong enough? or will the vertical grain
create some unexpected harmonics or vibrations in the airframe?
So guys have any of you tried anything like this before ? What were your results?
Without feedback I am reluctant to try this out on this airplane but I may try it on a "test-mule" in the future. 
I don't remember having this much trouble finding straight stock before. Perhaps, it is just my local problem or it could be the maple is now dried differently or whatever reason, it is still frustrating  .
Thanks for listening guys,
                                            Pat Robinson

Hi Pat,

Straight maple mount are hard to find sometimes.  I’ve used the following method for building engine crutches for years on different models.  Take a typical 3/8” X ½” maple engine barer and use the ½” side maple barer against the engine.   Determine the required spacing of the engine mounts make up a 3/8” medium balsa cross-grain block that fits between the maple mounts.  Glue the maple mounts and the cross-grain balsa together with slow epoxy glue and clamp together until dry.  Then trim off the outside of the barer on a belt sander to get a very clean and straight outside edge.  Make a “C” shaped engine pad from 1/8” aluminum or carbon fiber sheet and bolt or glue to the maple mount.  This also allows for additional room in the tank compartment for tank height adjustments.  Also, you can contour the front of the crutch to match the curvature of the fuselage sides from the typical F-1 fuselage former down to the plywood nose ring for the spinner.  This makes a really solid mounting platform for the engine. 

Later,
Mikey

Maybe we should ask "Smiling Bob"! 

You know, I've heard that after taking that stuff awhile, just whilsting a couple of bars
of Smiling Bob's song will automatically cause the wife/girlfriend to head for the bedroom..

(Sorry, like the Vonage song, I'd like to inflict a slow and painful death on the composer
of Smiling Bob's song..)

Please, please, please. Do not invoke any more of these earwigs, Dick! ;->

L.

"Some mornings, it's just not worth chewing through the leather
straps." -Emo Phillips

I don't know if anyone has tried this but I have made and sold motor mounts in the past. I buy my wood from a local vendor that stores it where moisture can get to it - not direct rain but moist air. When I cut blanks for motor mounts thay almost always form a slight curve so I just cut them just a bit larger than the finished dimension. I store them in a low humidity area for a few days and let them dry out. Then I come back and true them up to finished dimensions. It doesn't always work but most of them I am satisfied with. I do more wood working than model airplanes so I have learned how to get around problems.

I use a laser cut or pin routed 3/8" birch plywood for the Mo'Best ARC/ARF profile
fuselage. It is a one piece engine mount crutch. Nose doublers are 3/32"
aircraft birch plywood over 1/64" plywood fuselage skins. The outboard (motor)
side has an additional 3/32" tripler that the engine sits on.

Not counting the inboard 1/2" balsa nose tripler, the nose thickness for the engine is
3/32" + 3/32" + 1/64" + 3/8" +1/64" + 3/32" = 25/32" > 3/4".

L.

"The first rule of intelligent tinkering is to save all the parts." - Paul Ehrlich

Thanks,

I originally used 4 laminates of laser cut 3/32 birch plywood for the motor crutch
assembly. Although this worked, there were issues of alignment and thickness of
the glue. Ultimately I realized that it should simply be cut out of quality marine or
aircraft birch plywood.

Mo'Bests have always featured a very rigid nose, which allows the "stunt run",
and that is also helped by the recessed fuel tank, which locates the tank more
centrally behind the engine (it also decreases the weight of the nose assembly).

Here are some more illustrations which might give you a better feel for the nose
construction. The 1/2" balsa inboard tripler is useful in absorbing those vibrations
people are concerned about. The ARC/ARF version uses a pair of Brodak aluminum
engine clamps, which allows a number of engines to be used without nose modifications.

L.

"The reason so little is done, is generally because so little is attempted." -Samuel Smiles

Hi Keith,

WOW! Psychedelic motor mounts! - who would of thunk it?
Keith, let us know what kind of wood and how it works out- interesting.
                                                         Pat Robinson

Sorry, I did not see this thread earlier. I use phenolic plate to make motor mounts. The only trouble is proper orientation, it consists of layers and they must be perpendicular to screw holes or other machining, otherwise it separates.

The advantage is clear, it very well bonds to epoxy or other glues, it is strong, it does not soak oil, it will last far stronger compression under temperature (from engine screws) and it even allows to make threads directly in the material

Disadvantages are also clear, the worst is the weight, so I use relatively small pieces (in crossection) and I use lightening on back side of motor mount. If I remember well, the weight of both pieces was almost 2oz. It needs machining as it is relatively hard and I cannot imagine to cut it out manually.

I do not see any ill effects of its strength like aplilifying vibrations or so. It is closed in balsa sides, ribs.

The construction is visible on some pictures here:
http://www.rcmodely.sk/F2B/

Channels are milled before cutting in larger plain piece of material.

Those channels on picture are later filled by small plywood piece glued in to seal the compartment under tank floor. On another model I just filled that opening by epoxy with microbaloons.

igor