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Best practice

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fly boy330/12/2018 16:51:09
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3421 forum posts
17 photos

Hi all, wondered what the best practice is for mounting a 40size engine mount to ply firewall. Captive nuts, or Allan bolts, washers and nuts ? Cheers

Martin Harris30/12/2018 16:56:40
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8181 forum posts
206 photos

Tee nuts for me.

...and a dab of threadlock.

Don Fry30/12/2018 16:58:12
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3144 forum posts
38 photos

If you have no access after build, glued in captive nuts, if paranoid, with sone wood glued over them. They won't move. If you have access after build, please yourself.

Cuban830/12/2018 17:10:07
2300 forum posts
6 photos

Tee nuts for me as well. Just make sure that the firewall wood is plenty strong and won't crush when you wind in the fixings fully. I use a suitable cap head bolt with a large steel washer under its head. Never had one come loose. Beware hi-strength loctite on the threads, might give problems if you have to get the mount off. I've known screws to shear with HS stuff if you're unaware it's been used or you've forgotten about it and don't get the threads hot to soften it. Medium strength is fine.

cymaz30/12/2018 17:14:38
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8206 forum posts
1137 photos

Tee nuts and a dab of epoxy before getting them into the hole. Just don't get the epoxy onto the threadsteeth

Don Fry30/12/2018 17:33:56
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3144 forum posts
38 photos

If you do, a tap will clear the thread.

Geoff Sleath30/12/2018 17:44:07
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3146 forum posts
247 photos

A bit of Vaeseline on the thread is a good idea to stop epoxy sticking. I always drill the holes and fit the T nuts before fitting the firewall to the fuselage.

Geoff

Allan Bennett30/12/2018 19:02:03
1433 forum posts
37 photos

I usually use spike nuts (tee nuts?) as shown in the photos above. Apart from making it simple to attach the mount when access behind the firewall is tricky, they spread the load wider than the washer that would normally be used on a bolt of the same size.

The only disadvantage I've found is that you then have to get the correct length bolts to attach your mount, so that they don't protrude so far through the firewall that they can damage the fuel tank or battery behind it.

cymaz30/12/2018 19:04:47
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8206 forum posts
1137 photos
Posted by Allan Bennett on 30/12/2018 19:02:03:

The only disadvantage I've found is that you then have to get the correct length bolts to attach your mount, so that they don't protrude so far through the firewall that they can damage the fuel tank or battery behind it.

  • Cut to length
  • Put an order in with Modelfixings
Steve Balaam30/12/2018 19:21:34
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76 forum posts

I've gone over to using threaded inserts available from Modelfixings starting from 4mm thread size.

I wouldn't use anything else now

fly boy331/12/2018 12:29:13
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3421 forum posts
17 photos

Cheers lads for all your help. Main reason for asking was that my firewall is long and narrow, and I was concerned that the tee nuts were so close to the edges that they might split the ply. Thanks

Geoff Sleath31/12/2018 15:23:50
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3146 forum posts
247 photos
Posted by Steve Balaam on 30/12/2018 19:21:34:

I've gone over to using threaded inserts available from Modelfixings starting from 4mm thread size.

I wouldn't use anything else now

I like them, too but it's a shame 4mm is the smallest size. I asked Ian at Modelfixings some time ago if he could get 3mm ones but he tried and failed. They'd be useful for things like cowl fixings.

Geoff

PatMc31/12/2018 16:32:47
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4004 forum posts
501 photos
Posted by fly boy3 on 31/12/2018 12:29:13:

Cheers lads for all your help. Main reason for asking was that my firewall is long and narrow, and I was concerned that the tee nuts were so close to the edges that they might split the ply. Thanks

A piece of PCB with nuts soldered on the copper side then gf side cyanoed to the firewall. Spreads the load & no danger of damaging the ply edges.

Edited By PatMc on 31/12/2018 16:34:07

RC Plane Flyer31/12/2018 18:56:51
556 forum posts
21 photos

Flat Head Metric Threaded Blind Rivet Nut Insert Rivnut Nutsert Screw Stainless Steel M3/M4/M5/M6/M8(M3)

Sorry for picture size these units available in M3

Allan Bennett31/12/2018 21:05:01
1433 forum posts
37 photos
Posted by cymaz on 30/12/2018 19:04:47:
Posted by Allan Bennett on 30/12/2018 19:02:03:

The only disadvantage I've found is that you then have to get the correct length bolts to attach your mount, so that they don't protrude so far through the firewall that they can damage the fuel tank or battery behind it.

  • Cut to length
  • Put an order in with Modelfixings

I do cut to length, using my Dremel with a cutting disc. Should have mentioned that. I'm also a fan of Modelfixings.

Geoff Sleath31/12/2018 22:42:45
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3146 forum posts
247 photos
Posted by RC Plane Flyer on 31/12/2018 18:56:51:

Flat Head Metric Threaded Blind Rivet Nut Insert Rivnut Nutsert Screw Stainless Steel M3/M4/M5/M6/M8(M3)

Sorry for picture size these units available in M3

Where please? yes

Geoff

Denis Watkins31/12/2018 22:48:50
3392 forum posts
151 photos

They are called threaded rivets Geoff

Get them from engineer suppliers ebay

RC Plane Flyer31/12/2018 23:20:04
556 forum posts
21 photos

Hi type M3 Sodial inserts. Amazon

Happy New year

Ron Gray01/01/2019 08:02:22
1259 forum posts
333 photos

Those are rivnuts and work the same way as rivets but provide a threaded fixing. I don’t think they would be suitable for ‘front fixing’ the same way that the threaded inserts do as there is nothing to prevent them pulling out (epoxy maybe but would you risk it?). Of course youn could rivet them to an alloy plate then screw that to a bulkhead.

Ron Gray01/01/2019 11:16:21
1259 forum posts
333 photos

Look here for a range of fixings **LINK**

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