I plan on using a dave brown mount for a 46 fx (or ax, haven't decided yet, what's your input on that?) on my four star 40. I am mounting the engine at about ten o'clock, then semi-enclosing it with a balsa cowl, however, everywhere I read I find that I am told to use nuts and bolts for mounting the engine, the dave brown mount comes with screws. if I used nuts and bolts I would have a very difficult time pulling/reinstalling the engine as it would be quite a reach for the nuts. I guess what I'm basicly asking is if it would be ok to just use the screws, or would there be some undesired effect

then a question for the engineers; on the wing mounting area, why cant one use t-nuts instead of tapping the blocks for the wing bolts? not that its a big deal, I was just curious.

For what it's worth....I am in-process on a Funtana. When I mounted the engine (Saito .72), I used the self-tapping socket-head screws that came with the model for mounting the engine. I've screwed them out and back in a few times while building and it still seems like a very sturdy mounting job.

Considering that your mounts came with screws and the difficulty you'd encounter going the bolt and nut option, I'd vote for just using the screws.

I'm sure others will weigh in on this.

Jim

The wood or sheetmetal type screws aren't as strong as the typical hobby store machine screws. The heads have been known to pop off the wood screws from vibration, and twist off from too much torque. For most applications they work great. I would give them a try, just keep an eye on them . Make sure that the mount beams are absolutely flat , so your high performance engine doesn't get tweaked when you clamp it down. Attaching a perfectly square mount to a warped firewall will leave the engine mounting beams out of whack. The act of pilot drilling, and running the screws in to the glass filled nylon raises little "volcanos, that you will need to knock back down before you clamp the engine down to the mount

Wingnut,

The screws will work fine in the motor mount.

T-nuts are used quite often instead of threading the wing hold down blocks. It's a personal preference thing. [8D]

Yes you can use "T" nuts if you wish. No problem there. I use Dave Brown mounts a lot. I drill and tap them for 4-40 cap screws. I drill all the way through the mount arms. I then use a tap and cut threads in the mount. I use hardened cap screws to hold the engine on. I have yet to eveen have a srew come loose. Do not use thread locker on the mount it will melt the plastic. The mount will hold the screw well without it. Good luck.

Mark Shuman

I do pretty much the exact same things as Phread59. I don't ever use nuts and bolts because as you have discovered they are a real pain. Well, the one case where I do use them is wood beam mounts, but I haven't built a plane with these for many years.

Make sure when you tap the mount, you use the propper drill size for the tap first. I have often seen people drill the mount with the wrong bit, then tap it and the screws don't stay in tight. It is something you will do a lot of over the years. Get a good bit and tap to match for doing mounts. Also the same goes for the wings. You can use blind nuts or T nuts if you want. 6 of one, half a dozen of the other. I usually just drill and tap the ply.

Good luck

when using the sheet metal type screws to secure the engine to the mount I,ve had two mounts break in flight where the screws penetrated the mount,no crash but that days flying was over[:-]

Oh yea Wing nut. I just tap the wood. But put a few drops of thin Ca into the holes. Let them sit for an hour or so then run the tap back through the holes. The CA will greatly increase the strength of the threads. And the toughness, will take less wear from the screws.

Mark Shuman

Here's a hint I learned from repairing HP printers. When putting a self tapping screw back into a hole it was in, turn it counter clockwise a bit, till you feel it drop back into it's original threads, then turn it in. This stops you from cutting a new set of threads each time you put the screw into the hole. - Joe

Joe - That's a good tip. I do that with every threaded fastener though - not just self-tapping. Prevents a lot of problems.

I have heard that the self tapping screws aren't as strong, but I have been using them since the plastic mounts came out and can count my screw failures on 1 hand. Many people drill the pilot hole too small and twist the head partway off putting the screw in. I still use them, although I usually replace the ones that come with the mount with hex-socket head type.

It's well worth the effort to toss the screws that came with the mount and get some quality socket head self-tapping screws from somewhere like microfasteners.com (where I get mine). I've dont nylon mounts both ways, drilling a pilot hole then running in a self-tapping screw, and by drilling a hole all the way though and using machine screws and nylon lock nuts. I actually prefer the machine screws, I can get the engine on and off a lot faster than with self-tappers.

How? Ah, there's the trick. I run the machine screws UP from under the engine. So the nut sits on the engine, and the screw head is easy to get to from under the mount. I don't even have to take the muffler off the engine to dismount it. I also like to use hex head screws and a ball driver to turn them. I chuck the ball driver in to my cordless drill. Now, just grab the nut with a wrench (or even pliers in a pinch) and spin out the screws. Using this kind of thing, I've preformed field engine changes between combat rounds in around 15min. Not quite a NASCAR quality pit stop, but not bad either.

I love the Dave Brown Mounts, I line up the engine with the proper amount of thrust and mark 1 hole. Drill and tap it and then continue until all 4 are tapped. When I install the engine I use socket head machine screws with blue loctite. to this day I have never had any problems with loose engines.

Dave Brown mounts are my favorite. I usually drill and tap the mounting holes in the beams like several others have said. What I do a bit differently is leave about 1 1/2 or 2 threads worth of hole at the bottom un-tapped to act like a locknut for the 4-40 screws.

Phil in MN[8D]

hi , my sig 4* arf came with glass filled motor mounts with no holes in them. do i need to use bolts and nuts or can i use self tapping screws with these mounts????
thanks
jon

jon,
Either way. I prefer to use machine bolts and nuts, but the self tapping should work fine. You'll need to pre-drill the holes though.
Dennis-