I am using a washer between the bolt and engine when mounting to the engine mount. Should I also be using a washer between the lock nut and mount on the underside of the mount?

It definitely wouldn't hurt. You'll keep the locknut from digging into the mount.

Just make sure the mount allows the width of the washer. I had a Nylon mount fail when (it appeared) the washer pushed hard against the reenforcing web above the flat and deformed it, eventually causing it to fail. As I remember I also had a problem getting the engine to seat flush on the mount flats and probably put excessive torque on it when tightening the bolts. It was on a Stik style model and easy to spot (three bolts still held). Since then I file a flat on the washers with a Dremel so they have clearance and the engines seat as they should.

As Charlie P pointed out, the nut has to set flat. Most motor mounts have a surface that it parallel with the thrust line of the engine. Then the bottom surface of the lugs tapers, giving a thicker mount at the rear of the engine. The typical lugs in a 40 to 60 size engine are for a #6 bolt. The 91's have lug holes that accomidate a #8 bolt.

Personally, I prefer to drill and tap the motor mounts so I don't have to worry about this problem. It takes some accurate layout work and a smal drill press to get the holes exactly perpenducliar front to back and side to side, but it is worth it. You get a very stout mount by doing this. A lot of guys will drill a clearance hole and use #4 bolts and nuts. This provides for a sloppy fit, but the holes don't need to be as accurate. It also lets the engine move around a bit when mounting it and it is easy to get it off a couple degrees from the desired thrust line. In addition, with this type of mounting, a crash can result in a broken engine lug. If you decide to go to the bolt and nut setup, you need to file the bottom of the mount parallel to the top for a distance big enough for a nut and washer to fit.

I am rough on planes. I have had some very bad crashes, but only one of them damaged the motor. One crash was a 4*60 with a Magnum 91 four stroke engine. The motor mounts, the stock plastic ones with the kit were broken and the #8 stainless steel socket head cap screws I use were bent, but the motor lugs were not damaged. I crashed another 4*60 three weeks back and this time I had used the DU-bro aluminum isolation mounts and while the aluminum spinner back plate was sheared completely off by the thrust washer, the engine was still firmly mounted to the engine mounts. While the rest of the plane was a pile of balsa, light ply and broken plastic, the engine was fine. The engine I did damage was on the same Du-bro mouns. The rocker cover was the first contact with the ground and the engine was broken into about 10 pieces. The lugs were still firmly bolted to the the mounts which also survived.

If you can train yourself to drill and tap the motor mount and not drill out the motor lugs to accomidate off cut holes. you end up with a worry free engine mount. The clamp type, and a couple other mounts I've seen that use small screws in big holes always result in engine mounting issues later. Suddenly, the spinner is scrapping the cowl. The engine thrust line changes. Or, in one case I saw, the engine fell out in flight. Improperly mounted engines will cause all of these problems.

Years back, when I started my apprentiship, my Journeyman told me " a good mechanic is lazy, he doesn't want to do a job the second time, so he takes the time to do it right the first". Short cuts are not the best choice in the long run.

Don

Your local hardware store sells those drill-tipped self-starting hex-head bolts.. more like wood screws, but are not tapered, they are straight. But they have that chisel tip that looks somewhat like a drill bit. They will go into most of the mounts (not metal, obviously) and will hold like nothing I've ever seen.

I usually center the engine on the mount, put it exactly where I want/need it to be, then drill a small centering depression to mark the center of the hole. Do this using a drill that is exactly the same size as the hole in the engine mounting tabs. Do not actually drill through, just provide a tapered centering mark on the top of the mount. Then, make sure you set up so that you are exactly square with the mount and drill a small, say 1/16" hole through the mount. Do this for only one (the easiest) mounting hole. You now have a centered hole that is small but is exactly where you want the engine to be.

I have a nut-driver bit that is 1/4" and is on a shaft. The self-starting hex-head bolts have a 1/4" hex head. I mount the nut driver bit in a variable speed reversable drill. I then put the self taping bolt in the nut driver and allow it to self center, then screw it into the mount. It will do two things: it will drill its own hole, and tap it to fit the self taping screw.

Then put the engine mount back in the plane, and mount the engine on the mount. Screw the self-taping screw into the threaded hole you just created and snug it down enough to hold the engine in place so you can center up and mark the mount for the other three holes to drill. Remove the engine, the mount, and then using the above (drill a guide hole with the 1/16 drill followed by the self-taping screw.

Re mount the mount on the firewall and then your engine is ready for mounting and securing with those self-taping screws. And, they are tight. Real tight. They are not going anywhere. My Excelleron 90 with OS 1.20 AX, Venus II with OS 1.20 AX, Tiger 120 with OS 1.20 AX, and the two other 70 size planes both with OS .75 AX engines were all done that way. They have never moved nor gotten loose.

CGr.