For mounting an engine (ST51) on a plastic mount, should I drill out and mount with 3/32x3/4 socket cap screws (bolts) with nylon lock nut? Or am I supposed to drill and tap?

Instructions say to tap but I do not see the need? With lock nut and thread lock, why do I need to tap?

First I hope you meant a 6-32 x 3/4 screw. #6 is the size, 32 is the thread pitch and 3/4 is the length. Using a nut on the back depends a little on the shape of the mounting beam, best to have a flat surface to bear against. Why thread lock if you have a lock nut?

Yes 6-32. Typo.

Not sure if thread lock is needed but my question was if tap was necessary. I suppose if there is not a flat bottom for the nut, then tap makes sense. Basically, I want to avoid buying a tap.

Taps are cheap, tap handle costs a bit more. Your choice. Necessary? I can't see the mount from here so can't tell.

If the bottom of the engine mount is flat then you can put a bolt completely through the mount and put a nut on it as you mentioned. If you do use a nut use a thread locking nut to prevent it coming off. Or you can tap the hole as well. Either way will be ok. Another alternative on smaller engines (60 sized and below) would be use sheet metal screws. Drill a small pilot hole and then screw the sheet metal screw in to cut it's own threads.

I had an incident with thread lock and engine mounts before. I used the thread lock and a tapped engine mount and it actually softened the threads to the extent that the mounting bolts pulled out. Thread lock should only be used on metal to metal contact, such as nuts on a bolt.

Hope this helps

Ken

Ken is correct in that thread lock can attack many types of plastics making them either soft or brittle. JR has a warning out against using it on their servo output arm mounting screws. If a tapped hole is done carelessly and the threads are a loose/sloppy fit then you will also need a lock-washer under the screw/bolt head.

The plastic engine mounts that I have seen have not been flat for the length of the beams and thus bolts and nuts were not an option. I've used #6 screws and drilled a hole just smaller than the screw, and like Ken said, used sheet metal screws into the plastic. I have had no problems with this installation.

I almost always use a 6x32 bolt with lock nuts. Never had a problem, but make sure you get them tight.

FWIW,

I tapped my mount and used lock nuts too. This is on
a GP mount with a 82 4s. I check them for tightness
at the end of each flying day. They have stayed tight
so far and will do the same on my next build. As mentioned
earlier, Taps are cheap.

Bob

For engines up to 46/50 #4X5/8 sheet metal screws are perfectly fine, Use a 3/32 tap drill size no need to tap the beam and please do not use locktight which is redundant anyway.

If you use a #6X3/4 sheet metal screw which is used on sixtys the proper tap drill size 7/64.

John

for certain applications tapping is so much more convenient, hell i learned to do it here on RCU. Buy the right size tap, the right size drill bit, and the correct bolts/screws and yur set. I have a variable speed drill and that what i attach the tap to instead of the handle. I used to be scared of tapping till i had no other choice, and ha to use it

I am surprised a sheet metal screw is sufficient. I figure a bolt and nut is not going to pull out.
What size drill and tap for a 6-32 bolt?

or, I could just use these (I think I read someone suggesting to heat the nut so it sets into the plastic?) [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&W=001603520&I=LXK048&P=K]bolt[/link]

Oh, and this is for a SSE which I believe has a flat bottom on the mount.

While we are at it, having read a billion SSE build posts, what is recommended for rear wheel? What Sig supplies? Or this [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&W=001603520&I=LXD851&P=K]Dubro[/link] or this [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&W=001603520&I=LXFV47&P=K]Sullivan[/link] ?? Seems lots of debate on this. I plan on using supplied main wheel bracket and likely a 11x6 prop (and at most 12.25x3.75)

and finally, I am amazed at pricing escalation. Someone posted SSE was $79 and raised to $89 earlier this year. it is now $109. Ouch.

Agine do not use a nutted bolt unless the bottom side of the beam is paralell to the top side of the beam. Using nuts on a slanted seat will cause the bolt to bend and fail everytime and if you go trying to grind flat seats into the beam then you are severely compromising the entire engine mount.

Using sheet metal screws is and has been a standard for years and works very well as long as you use the proper tap drill size for the screw. Much easier than tappping for a bolt as perfect alighnment is not as critical.

Also keep in mind in many installations where a hard mounted cowl ring and half cowl are used it will be imposible to access those nasty nuts.

Sheet metal screws work with the plastic mounts perfectly when properly installed (using the proper drill size for the indicated screw) easier than any other method and requiree no tap and allow easy engine removal or installation when half cowls with a cowl ring are used.

Dave Brown mounts (my favorites) because of more mounting versatility and options due to the round firewall portion and non square bolt pattern all includes the proper sheet metal screw size with each mount up to the 120 mounts which use #8 screws.

John

just go to your hardware store and buy the set , or if they arent sold together, buy the 6-32 tap, and it will tell you what size drill bit you need

If I read the suggestions correctly, for a ST51, I can use a #4x3/4 sheet metal screw after drilling a pilot hole with 3/32" drill bit. No need to tap.

Since I hijacked my own thread, I will post the SSE questions separately.

Chemie,
From your own suggestion above, I would go with the 6-32 or 4-40 screws with blind nuts IF your mounting rails are flat underneath. I am moding up a 4* and putting on a cowl and went with this route. I have been happy with the installation and feel that it will be very durable. I recommend not heating up the blind nuts to pull them into the mount, but instead drilling the mount first for the 4-40 screw (large enough for the screw to slide through, not thread through), then from the underside of the mount, drill through enough for the blind nuts to be pushed into the mount. The 4-40 hole will act like the pilot, and then you can press the blincd nuts in using a pair of pliers. I tried heating up my first blind nut to melt it into the mount, and I ended up heating it up so much, that the galvanizing coroded and now the screw feels like it is threading into a rusted nut. I also plan on using a light application of blue threadlocker to lock the screws to the blind nuts.
Let us know which method you choose.
Curtis
P.S. I also have used the sheet metal screws on an app with a Magnum .46 XLS, and so far after about 6 flights, it has remained solid.

The coarse deep threads of a sheetmetal screw really take a good "bite" in the nylon mounts. The friction between the screw and the mount acts to lock the screw so it should never loosen until you unscrew it.

I do prefer to drill and tap the mounts for 6-32 screws though. It really is just a preference.

$3.99 will get you a 6-32 tap, with the proper drill, from Sears http://www.sears.com/shc/s/p_10153_1...0P?vName=Tools
Home Depot, Lowe's, Ace, True Value, etc... should have the same thing (tap and drill skin-packed on a card).

sheet metal screw in nylon moints are selt locking they weill not loosen under vibration.the size you stated a few post ago of using a #4 should be a #6.KISS principle.

The Loctite type thread locker liquids harden in the absence of oxygen, thus they are called anarobic; in the absence of oxygen. It may be difficult to understand, but some plastics are porous to the migration of oxygen. The plastic bottle the liquid Loctite comes in is a prime example. That is another reason not to use Loctite with plastic type materials. I do believe I recently read of a thread locker that was compatible with plastic type materials. I love the statement "The only constant in life is change".