I have a GP "glass filled nylon" engine mount for my OS .91 FX. The instructions for the plane say to tap the engine mount. Would it be better to do as it says or get some nuts and washers for the other end?

It seems to me that the threads in a tapped mount would eventually wear. Nuts may also allow for a plate to be built where weight could be added.

Thanks for your ideas,

Herb

Hi herb, Probably the best thing to do is tap the mount and then add a locknut to the bottom. If you use an aircracft type locknut it wont be neccessary to add locktite to the threads. On smaller engines no locknut would be neccessary. Good Luck, Widgeon

Thanks, Widgeon. I think that is what I may do. Why didn't I think of doing both?!

Herb

Herb, either way works.

If you don't tap it, just use a locknut on the buttom, no loctite.

If you do tap it, you wont need a locknut on the bottom, no loctite.

Daz...

The only downside to tapping a plastic mount is the increased likelyhood of the mount breaking. A clean drilled hole through the mount beam is stronger than the tapped hole with the same major diameter.

I've had a couple of my engines bolted to the GP glass mounts with socket head sheet metal screws and after two or three years they still cannot be tightened any. The bolts that hold the mount to the plane get slightly loose occasionally but the ones holding the engine never do. Some of these mounts have two or three sets of holes in them so I can change engines easily.

Done well, tapped mounting will work just fine. The main reason I don't like tapping is that if you ever break a bolt inside a tapped hole, you're up poo-poo creek. You'll have a tough time drilling the stub out because the surrounding plastic or aluminum is way softer than the steel bolt and the drill bit will wander. It's the same reason I prefer thru-bolt mounting for mufflers.

If you are going to tap a nylon composite mount, however, be sure not to use a really fine thread like 2-56. The plastic don't have a whole lot of shear strength with that little "bite".

Thanks for all of your input!

Herb

One more comment...not all glass-filled nylon mounts are created equal. I've never had a problem with GP, but I had a SIG mount fail when the tapped threads stripped out and the engine vibrated loose.

Now I only tap thick aluminum mounts (> 1/4" like Dubro's) and use through-holes and lock nuts on plastic and thin metal mounts. Lock nuts are nearly foolproof and add minimal weight...

I always drill through the mount and install a nylon insert nut. I also make sure that the surface the nut is seated on (the bottom of the engine mount) is parallel to the top surface. This sometimes requires filing. You cannot tighten a nut against a sloped surface or the stress will shear the bolt. I never have to re-tighten these nuts.

Another option I did on my kaos. drill the hole slightly larger than the thread. On the underneath side partially drill a wider hole to accomodate a blind nut of the appropriate size (6-32 etc). drill only deep enough to get the blind nut to set in completely. Press the blind nut into the fiber/plastic mount with a vise. A tight easy solution and no back side nuts to have to mess with in a tight cowl

I've been tapping fiberglass mounts for years. Make sure you lubricate the tap, the molded material behaves a lot more like aluminum than you'd casually assume. Broken taps in holes are a real nuisance ( as in buy another mount).

BUT, my own opinion at work here, a .91 on a molded mount? Huh? Time for a cast-metal mount. That is one big lump of shivering metal to go sticking on a composite motor mount, a two-piece one at that.

I've read a good trick on the newsgroups - For built-in locknuts on composite mounts, don't tap all the way through. Start tapping from the top. When the end of the tap appears at the bottom, stop tapping. Then you'll have a built-in locknut. The taper in the tap cutters will leave shallow threads that grip the bolt firmly. I've used it on a few and it works great.

Thats a cool idea bgi, I will definetely try that one. Daz...