Brass Insert Engine Mounts

Following up on the aluminum engine mounting plate idea, I've been using 4/40 threaded brass inserts for engine mounting. I got the tip from Bob Hunt. He said he'd been doing it for several years. It's pretty easy to do, spreads the vibration load of the engine directly to the wood, and has been holding up really well on aluminum piston engines. I haven't tried it on an iron piston stunt 35, but I will.

Space the bearers snugly to the engine so the mounting holes are centered on the beam(you may have to mark them directly on the wood, because the engine probably won't fit at this point). Drill the holes a few thousandths oversize. The minor diameter of the inserts I have is .168 in., so a numbered drill is needed. Put a 4-40 nut on a short Allen head screw and then a washer. Thread on the insert, and thread it into the wood. It should go in snugly, but it the wood starts to pucker up around the hole, it should be redrilled a size larger. The insert should end up .005-.015 in. below the surface of the wood. The washer will keep it from going in too deep. Hold the nut with a pliers, back the screw out a turn, and then back the nut off to release from the insert. Put a drop of thin CyA on the OUTSIDE of the insert to glue it permanently in place. Finally, carve out the inside of the mounts with a Dremel sander to allow the motor to drop smoothly into place.

Voila' , no more screws coming loose. This setup seems to work about as well as all-metal motor mounts.

Well stated. I have been using threaded inserts for a couple of years now. I use the ones from Micro Fasteners. For 4-40 I use an 11/64 drill bit. First, I use centering drill. I believe it is from Du-Bro. It has a long 1/16" drill bit through brass tube with a centering cone on the lower end. It works really well and get the initial hole dead on center every time.

I use a longer socket head bolt with a jam nut against the threaded insert. That way you can set the threaded insert almost dead even with the maple mount.

What I don't understand is why you need to use a dremel sander to undercut the engine mounting lug? Did I read that right?

Bill Francis
My real name

For most engines the mounting holes are less than 3/16 in. from the case. If the mount holes are centered in a 3/8 in. wide bearer the bearers will have to be relieved after inserting the brass pieces so the engine case will fit in between the engine bearers. Usually just takes a couple minutes with a Dremel sanding drum.

I've been using inserts for donkey's years and love 'em. Always had trouble getting that darned bolt out again after inserting the insert but I didn't know about the nut and washer trick. Thanks for that one boys.