Motor mount query

[fly boy3]

Hi all, wondered what is the best practice for the following. I need to mount the mounting plate ( cross shape) to thin ply firewall after screwing same to motor. The holes are a small dia. about 2mm. 1, self tappers, 2' wood screws, or 3,bolts and nuts, nylock ? Motor is small about 200watts. Thanks ps motor weight 52gms.

[Old Geezer]

Best option: Bolts and T (captive) Nuts, 2nd best: Bolts and Nyloc Nuts (vital to also use washers as small size nuts will otherwise likely eventually pull through your thin ply). As an afterthought - you might slip another layer of thin/thinner ply behind the firewall to stiffen it up at very little weight penalty. If you're stuck for appropriate sized nuts, bolts, etc. - have a look at Model Fixings.

[fly boy3]

Hi OG, exellent idea re another ply wall. Why did I not think of that lol. (It's an age thing) I agree bolts and nylock nuts plus washers is the best plan. Just looked, all available at Model Fixings. Thank you very much. Cheers

[Geoff S]

If you can easily access the area behind the firewall then Nyloc nuts and bolts with washers is the best. T nuts are usually too deep to fit in anything under 6mm thick plywood. I'm guessing yours is 3mm. I wouldn't use anything under 3mm bolts.

Geoff

[Mike T]

Here's a 200W motor, fixed to a 1/8" ply bulkhead in my Twinstar II:

Self-tappers. Been there years...

[Nigel R]

For a small electric I would just use self tappers. Make them as large dia as is reasonable. When you first put the mount on and all screws are in, take it off again and soak the holes liberally with thin cyano. Give it a while, then reattach. The cyano does wonders to stiffen the wood around the thread.

Edit: if you can get to the back of the firewall, for peace of mind you could glue some small discs of ply behind the holes to give a bit more wood for the screw to bite on.

Edited By Nigel R on 26/10/2017 09:34:36

[Engine Doctor]

Hi there's plywood and the stuff used on models ? Agreed some,a few manufacturers use proper plywood that would be fine with self tapping screw with that power set up but,........... Most plywood these days is rubbish ply and won't hold a screw safely. As already suggested add another ply plate of decent ply . If you fit the motor then disassemble and apply some cyano to the screw holes ,allow to cure before refitting the screws and this will harden and toughen the wood and screws won't let go. Make sure prop etc is balanced as vibration can loosen mounting screws.

[fly boy3]

Thanks all, will digest and learn. Love the cyano tip, and extra ply wall. For Mike T, you must be psychic lol .the picture is near identical to my set up (even the motor) exept I have ply to balsa. Off thread question, what glue did you use for ply to foam. Cheers

Edited By fly boy3 on 28/10/2017 21:19:47

[onetenor]

FWIW Next time I have a ply to foam joint I will use Gorilla glue. Can't whack it

[fly boy3]

Cheers OT, I too will give it ago. Thanks

[onetenor]

Another way is to put ali or brass strip about 3 /4 mil thick epoxied behind the firewall . They have to be drilled and tapped to match your bolts .A plain washer and spring or other locking washer under the bolt head=This spreads the load across the firewall.if put in horizontally .

[Trevor]

Just be grateful that it is all so easy nowadays! With 'can' motors and gearboxes of all shapes and size, motor mounting was always a challenge - Link

Trevor

[Mike T]

Posted by fly boy3 on 28/10/2017 21:02:45:

Thanks all, will digest and learn. Love the cyano tip, and extra ply wall. For Mike T, you must be psychic lol .the picture is near identical to my set up (even the motor) exept I have ply to balsa. Off thread question, what glue did you use for ply to foam. Cheers

Edited By fly boy3 on 28/10/2017 21:19:47

It was a long time ago, but I think I must have used med or thick cyano (this is elapor remember, so cyanos work well). What I also did was drill some 1/8" holes around the edges of the fixed bulkhead, then drove in some bamboo barbecue skewers until they poked out of the wing. Pull them back a bit, then run thin cyano into the holes (both ends) and the bulk head ain't going anywhere.

You'll know that the nacelles on the TS are 'horseshoe' shaped and the space behind the 1/8" ply bulkhead is backed with 1/4" liteply, to act as an anchor for the PKs. As ED suggested, the screw holes were pilot-drilled, the screws installed, removed and the cut threads hardened with thin cyano. HTH

[fly boy3]

Thanks Mike, some good tips there for the future. Cheers