I will need to mount a ST-90 to a Nylon engine mount.

My concern is that the holes in the engine mounting flanges are very close to the inside edge. I think I will have about 1/16" of "meat" left in the nylon mount arm after drilling out the holes. Instructions suggest #8 tap/bolt which will leave very little nylon on the inside edges. I could use #8 or #6 screws which would allow a slightly smaller drill hole but I still do not like it.

Now, this is not unique; I have always felt the engine mounting holes were too close to the engine side and the arms are difficult to get perfectly flush (there is a 1/32" bump on the side of the engine which prevents fully flush).

What is best practice here? I know there is lots of debate on screws versus bolts versus tapped and untapped but that is not what I want to debate. I am interested in how much nylon is enough and how to avoid losing most of the inside material on the mount while still having things firming secured.

Thanks.

I always get the arms flush against the case, that is the only way at times to have enough material between the mounting hole and the inside edge of the mount. If need be I'll cut/grind the mount to clear any part of the engine that prevents this. I don't think you can get an answer as to an exact measurement as the variables are to great, including exactly what the mount is made of.

The engine mount beams need to allow the engine to seat perfectly with no force applied. It is easy to distort the case enough to cause poor performance, early bearing failure, or seizure is the fit to the mount is less than perfect. File the inner edges of the mount beams until the fit with the engine is perfect. Have faith that a good set of deeply rooted screws will work OK in that mount. I don't like to use plastic mounts for these larger engines, but there's thousands of guys who do use them with success.

I will say the "bump" is likely 1/32" so the real issue is just the very small distance between the edge of the flange and where the holes where drilled. (I wish they just made a larger flange as I can see no reason why not).

Would you suggest #6 3/4" screws?
#8?

Drill small pilot and then screw in?

It was suggested to use #8 bolts but I do not see a way to drill that large a hole and still have any real material left of the nylon beam.

Use the largest sheet metal screw (not a wood screw) that fits through the hole in the engine's mounting flange.

A Phillips head instead of a Slotted head screw is a preferred choice for this application IMO(Along with a #2 Phillips screwdriver with a tip that's in good condition).

Bring both mounts up to, but not touching, the crankcase. ANY clearance is good, and better than none.

Hold the engine firmly to the mount, and mark the hole locations. A "scribe" can be made from bar stock or old piece of pushrod with a sharpened tip. Then, you can etch the hole location into the mount.

Using a Center Punch, mark the hole's center location on the mount. (This keeps the drill bit from roaming, and allows the hole to be drilled where you want it)

Drill a pilot hole. A pilot hole for the screw is created by using a drill size that is the same diameter(size) as the unthreaded shank of the mounting screw. You can begin by using a smaller drill bit like 1/16", then increase the size of the drill bit's until you reach the correct size needed for your pilot hole. In other words, the first time you drill, does not have to be the final pilot hole size, you can work up to it.

Clean out the holes of any drillings.

Lubricate the screws with light grease.

Assemble and tighten with confidence!

Take your time. The combination of drilling straight (90 degrees to the mount), plus, the correct size pilot hole...yields a satisfactory result, even though your apprehension about not much material to work with is absolutely correct IMO! Good Luck!

It's surprising that you appear to have mounting holes that close to edge of mount, never have had that situation. Anyhow a no.6 sheet metal screw will work fine. Do as was directed by Safetwire and you should be fine.

Wood Screws? you guys most be Carpenters. Real Mechanics use Allen headed bolts. I threw away my screw drivers 20 years ago, an Engine mounted as in the enclosed picture will stay tighter longer and require less maintenance I hope this will give you guys some insight on how to properly mount an engine. The fisrt thing I do before starting any build is to throw away all wood screws and machine bolts in the kit that requires the use of a screw driver and replace them with Allen Head bolts or screws. I do not like things coming loose. I have checked many planes for people complaining about their engines coming loose and find that every one of them had been mounted with the use of Philips headed sheet metal screws, so take it for what it is worth
jerrysu29 Call Sign Wild Man

If you fit a proper size machine screw, cap screw perfered to the stock holes in the engine lugs, your mount will not have to little "meat" for the screw to be threaded into it. All bets are off though if someone has re-drilled the holes in the engine beams.You wantscrew that fits snug in the hole in the engine beams. You want the hole square in both directions to the mount. My preference is to drill and tap the mounts. Use a drill press to do this to keep the holes straight. This gives as strong a mount as you can expect. The hole being square to the mount prevent any twisting on the beams which will result in a broken crank case in a crash.

Look at the bottom of the engine. With the mount almost touching it, you will have more than enough meat for a fully tapped screw hole, less for a screw and nut arangement.

Don

Who said anything about a (fine threaded machine bolt) most machine bolts come in NF or NC I do both, drill, tap also put nylocks on as in the picture. Most do not even know that metric bolts the same size say a 4mm come in diff. thread pitch's
why would any one want to re drill and use a motor mount that has already been drilled, all that does is weaken the motor mount.My goodness a new mount cost less than 10 bucks When properly drilled and tapped an Allen headed machine bolt is hard to beat. You want to use wood screws and a screwdriver by all means use them
jerrysu29 Call Sign Wild Man

Machine screw threads in plastic can crumble. I used to "through-bolt" plastic mounts, but on some planes the nylock nuts are not accessible. On some mounts with the angled beams, spot facing ledges for the nuts to bear against is more trouble than it is worth.
The screws that work every time, never back out, almost never strip out, are the sheetmetal screws. The threads are deeper and not so close together [compared to a fine thread machine screw], which is a stronger arrangement in plastic.

i agree....i consider using sheet metal or woord screws to be the most inferior way of mounting a motors regardless of size.......4-40 and a nice nylon lock nut is the way to go on 40 size...6-32 on 60-120, and 10-32 on small gassers, aqnd 1/4 20 for large gassers....its not a rule bit its is the safe way to go

The gap looks bigger in the picture due to the zoom but it is 1/16" from the crank case side to the starting edge of the hole.

"Through Mounting", where a bolt and nut are used, of course is the strongest/best method. (Duh)

I thought the original poster was going to use screws, therefore I suggested a type of screw and method of installation which has worked well.

If I were a carpenter, I would apply a heavy bead of industrial grade, outdoor caulk type adhesive to the mounts...press the engine down firmly until the caulking oozed up, out and all over the mounts...nailed it home, then, flipped the plane over and bent the extended portion of the nails over with a claw hammer for redundant holding power!

(Now that I think about it...somewhere, sometime...this just might have been tried!)

The reason that I proposed screws was that a #8 sized hole with through-bolt and lock nut was not going to leave more than 1/32" on the mounting arm.

I/32" is too slim of a margin. If you were working with an aluminum mount that hugs the engine better the tight clearance is easier to deal with.

A couple of points. First, I never use either sheet or wood screws on any engine bigger than .15 displacement. I have had them break. If you do use a screw, use a sheet metal screw because they are straight and have deeper threads than a wood screw which is tapered. Sheet metal screws are just not as strong as machine bolts. Second a 1/32 inch space between the side of the hole and the edge of the beam on the engine side will not be a problem. I have had that much space on a number of installations and never had a problem with the mount breaking.

I mount my engines to both plastic and metal mounts by using the largest allen head bolt that will fit the engines mounting holes and then drill and tap the engine mount for those bolts. I use a lock washer between the head of the bolt and the engines mounting holes. I have never in 40 years of doing this had an engine come loose or a mount break (except in a nose first crash). On some mounts I have used a bolt with lock nut but because of the taper of many mounts, this is not aways pratical.

Bruce

Hi!
Sheet metal screws work fine in plastic mounts(Craft-Hays). Have been using them on many Q-500 models (424).
But another way of doing it is to drill and tap the mounts.
Drilling and using nuts on the opposite side isn't suitable on many models...and those nuts are also hard to reach when you have to tighten the screws.
Using Ca glue to secure the engine to the mounts when drilling is one way of doing it.
But most of the time I just hold the engine firmly to the mount and drill just a shallow hole with my electric hand drill, then take the mount and using my bench drill, drill through the mount. Then I tap the hole (or screw in a metal screw if its a plastic mount)and use an Allen head screw to secure the mount to the engine.

Then I take a long drill and using my electric hand drill I drill shallow holes having the engine lugs as a guide.
I then remove the one bolt and put the mount in the bench drill, securing it in a wise, and drill the remaining 3 holes.

Sheet metal screws in nylon mounts are self locking

Yea Bruce.

I think you covered most of the reasons for drilling and tapping and the ones not to use wood screws, sheet metal screws etc. Those screws have a much smaller body diameter in the thread area. They are also brittle. The threads will eat into the aluminum of the engine. Take a look at the second photo of Chemie's last phost and notice that the engine lug has been drilled, and the bottom of the hole is oblong from that.

Guys, I've crashed more planes than I like to admit. i"ve yet to break a motor mount lug because I always drill and tap, using a drill press to make sure it is straight and square, and on target. Imounted a Mag 91 on the stock plastic mounts for a 4*60 and used Stainless steel #8 socket head cap scrws. Full tthrottle lawn dart into the ground, well dried adobe clay. The motor mount broke, the stainless steel cap screws bent, but the engine wasn't damaged. I've seen lugs broken off when using thru drilled holes, stove bolts, a size or two to small because the holes were drilled off pattern, well you name it. The motor mount is probably the most stressed part of your plane.Not mounting your engine firmly, and accuratly is asking for trouble.

This photo shows how a properly mounted engine can survive a crash, well at least the lugs didn't break and the mount is usable in another plane. The rocker cover was first contact with the ground. It used to be an O.S 52 four stroke.

Don

If the engine is held with adequate clamping force, the threads will not be able to worm out the holes in the mounting lugs. There are limits to various methods for what works, real experience is the best teacher. Especially in competition where there are others with more experience who can show you what is needed to work and what is unnecessary over-kill.
The worst kind of "teacher" is the type who thinks he knows it all, especially when it comes to things that he has never tried.
Someone said earlier to use #4 hardware for .40 sized engines. I don't, I use #6 for .32 engines up to .50s. I've had the heads of high grade #4 machine screws pop off too many times, so learning from that experience, #6 hardware has rarely failed me on this size range of engine.
It helps to have a numbered drill index to custom match the pilot hole size to a sheetmetal screw. Some wax or soap on the screw threads helps them cut into the plastic or wood.
The Du-Bro or Great Planes socket head sheetmetal screws [SHSMS] work great if applied correctly.

Campgems...That's what I call a real "Yard Sale"!

Here is a photo of sheetmetal screws holding a .61 in a Tiger 60. The screw length should have been such that three threads are showing after final assembly and tightening. Note the clearance between the engine mount and crankcase. No problems to date.

Drilling and tapping works well also.

"Through Bolting" is the most secure method.

I use a small flat washer between the head of any chosen fastener, and the engine mounting lug.

No matter which type of fastener you choose, I recommend after a few flights, to check the fasteners for tightness, even though this might require removal of the spinner/prop/muffler/cowl, etc.

Furthermore, every now and then before flying (and before starting the engine) I think it's good practice to grab hold of the engine and give it a test for mounting tightness/security. You might get surprised one day[8D]

Not having access to a drill press is a bit of an issue too...

Their not that expensive, just do a bunch of extra Honey doos for the little woman, then go out and buy one. keep in mined that it is easier to ask for forgiveness then it is to ask for permission LOL!!! Once you get it you will wonder why you did not get one years ago.
jerrysu29 Call Sign Wild Man

Can anyone guide me through the offset to mounting a Nitro engine? I'm scratch building a 60" top wing plane. I want to mount a OS .46

Most kits build the correct offset into the firewall. If you are scratch building and not from plans, you may want to keep the firewall at 0,0 offset. I would think a couple degrees down and maybe one right would be a good start point if you want to build it into the fire wall. You can use flat washers behind the motor mount. Just make sure to add them in pairs. IE two top screws to lower the engine. Now 2 top and bottom on the left side to dial in a little more right thrust. What you want to avoid is getting the motor mount setting on an uneven base. That will put some twist into the mount and may cause early failure. The washers alone, even used in pairs is going to put a bit of twist on the back lugs.

Don