6-32 machine screws to hold an engine down? 4-40?

I'm trying to progress with this CL design and use what I already have. I have plenty of 6-32 screws from RC engines.

The MVVS .49, the holes are to small for 6-32?

4-40? Is that enough machine screw. Especially in a glass engine mount. Thin and fine thread?

Do I drill out the MVVS? To except the 6-32?

Thanks in advance?

CB

I'd say 4-40 will be ok.

If you use high quality screws, you will be fine. Try socket head cap screws. The screws on this engine didn't break.

[photo not recovered: 40122.jpg]

Attachment #1, (jpg file)

4-40's with a flat washer under the head. You can drill and tap a nylon/glass mount for them, and the mounting lugs or bolts will break in a crash. Steve

I generally use allen machine screws.

I don't know, 4-40, in a glass filled mount. If it was aluminum I wouldn't be concerned.

Well, if I used extra long screws that would protrude from the bottom of the mount. I could use lock nuts for added relaxation.

CB

I use grade-8 4-40s on combat planes, which have more powerful engines than stunt planes, but I used 6-32s on stunters because there are lots of engine removals during the life of a stunt plane. Also, a high-falutin' stress engineer who has flown some stunt recommended 6-32s. You may wish to use the smaller screws if the bigger holes for the 6-32 would weaken excessively the structure to which the engine is mounted.

Howard Rush
Bellevue, WA

Split the difference. Use 5-40 bolts.

>Split the difference. Use 5-40 bolts.

Or go M3 grade 12.9, a bit fatter than 4-40 and stronger than grade 8.

Why use such strong screws? Aluminum engine CC arn't THAT strong. It seams to me like using a really strong glue (like epoxy) on balsa when Elmer's/Comet/Ambroid/Duco/etc. would be just fine. And for all the smart people out ther; No I don't glue in my engines!

GA

I don't know anyone who would use Epoxy to glue balsa to balsa.

A snap to a wingover does put stress on the screws that hold the engine to the mount.

4-40 that 40 threads pr inch. Fine stuff for securing an engine to a glass filled mount.

I'm comfortable with the 6-32, but, as I said, I'd have to drill the holes a bit on the engine.

CB

>I don't know anyone who would use Epoxy to glue balsa to
>balsa.
>
>A snap to a wingover does put stress on the screws that hold
>the engine to the mount.
>
>4-40 that 40 threads pr inch. Fine stuff for securing an
>engine to a glass filled mount.
>
>I'm comfortable with the 6-32, but, as I said, I'd have to
>drill the holes a bit on the engine.
>
>CB

And if you drill out the mounting lugs you have made them weaker,
Less material, less strength. Stick with a Grade 8, 4-40 socket head.
I have been using them on .60-.65 motors for years with no problems.

I would add that I ALWAYS use a new splitlock washer. Anytime you break the screws loose or remove them, toss the split locks out and use new ones.

Larry Buttafucco Stunt Team

Larry,

Well, guess what I dis 1/2 hour before I read your reply.

Correct, I drilled the MVVS with a 9/64 bit to allow for a 6-32 screw to fit neatly in place.

I marked off and tapped the glass filled mount to except 6-32 screws.

Done, and I have to live with it.

Good, bad or evil.

CB

The motor lugs will be just fine with the bigger holes. I'd be worried about the engine bolts pulling out of plastic. It's better that you used 6-32 in that regard but, is there any way you can get a nut on the other side of the mount?


MM

Yes, I can put a locknut on the other side.

I will just use longer screws. Won't look pretty, but it cannot be seen. It will be inside a removable cowling.

The removable cowling is something I have to consider now.

CB

>Larry,
>
>Well, guess what I dis 1/2 hour before I read your reply.
>
>Correct, I drilled the MVVS with a 9/64 bit to allow for a
>6-32 screw to fit neatly in place.
>
>I marked off and tapped the glass filled mount to except 6-32
>screws.
>
>Done, and I have to live with it.
>
>Good, bad or evil.
>
>CB

I'm sure it will be fine

Larry, Buttafucco Stunt Team

I drill and tap the glass filled mounts just like metal mounts. No backup nut and no problems ever with 4-40 or 6-32 up to 60 size rc ships. Been doing it for decades...but the last dozen or so I just drilled through and used sheet metal screws. They don't pull out or loosen either. Wood mounts are a totally different deal though.

I think the engine mfgs give a clue on what size screw is correct by using whatever size fits without enlarging the engine mount.

bp

BP,

I've seen guys use sheet metal screws with glass filled mounts.

It didn't take much to open the holes on this MVVS .49.

The 6-32 screws almost went in on their own.

I now have this aluminum mount I just found.

>BP,
>
>I've seen guys use sheet metal screws with glass filled
>mounts.
>
>It didn't take much to open the holes on this MVVS .49.
>
>The 6-32 screws almost went in on their own.
>
>I now have this aluminum mount I just found.
>
>


Ron B.
F4Fguy

If you can't hold down even the worlds meanest .60 with #4-40 socket head cap screws in an aluminum or glass filled mount, you've got more problems than just the screw size. Properly torqued, the screw itself will give ca 300lbs. of force. Even with a huge safety factor, four #4-40s are far more than enough. As to the mount material, if properly tapped, a 75% thread will develop the strength of the screw at 3-4 thread lengths. Using a tap drill sized to give closer to 100%* engagement will give better female thread strength yet, and in the glass filled mount a degree of self locking.The suggestion made re 2.5mm is not a good one, as the standard metric thread is closer to a unified standard fine than the #4-40 which is a coarse thread. In aluminum or other softer materials (like glass filled plastic) always use coarse threads.

* If you try this, be very careful, it's easy to break a tap.

Ron B.

Ron,

I'm grinding down an aluminum mount to fit the engine. I will tap the four holes with a 6-32 tap.

I should be fine with that, correct?

CB

>Ron,
>
>I'm grinding down an aluminum mount to fit the engine. I will
>tap the four holes with a 6-32 tap.
>
>I should be fine with that, correct?
>
>CB


Ron B.
F4Fguy

You'll be fine with that, but it's gross overkill. #4-40 is more than adequate. I've used #4-40s on a MOKI 61 on a pipe and never had a fastener failure. I've had the mount fail between the rail and the firewall, but never the screws or mount threads.

Ron B.

I don't have a 4-40 tap.

Besides, now the engine holes are drilled to accept a 6-32.

I may have drilled them prematurely.

CB