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Nylon engine-mounts: self-tappers or through-bolts?


Jonathan M
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For normal glow 2 or 4 stroke, say from .30 to .80 sized engines, what is the norm?

 

I can see that for installations with little or no access to the underside of the mount then self-tappers are convenient.

 

But I've always so used bolts with spring-washers or similar simply on the basis that (a) it feels more secure, and (b) if I strip a thread by over-tightening then its easier to replace a bolt etc than the whole engine-mount.

 

 

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I don't use plastic engine mounts of the SLEC type or similar these days, but back in the day I drilled and tapped the appropriate thread into the mount, secured the engine with cap head screws and added a self locking nut to the remaining exposed thread on the underside for belt and braces. Never ever had a fixing loosen off. Just nip up the screws securely and resist the temptation to lean on them.

Edited by Cuban8
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For many years I used these Socket Cap self tappers from Modelfixings with 100% security and no issues whatsoever on 09's through to 120's. I drilled the appropriate pilot hole & used a spring washer 'just in case' but I doubt the latter was necessary. Not built an IC for many years now but I still have a couple of packets of the screws in stock.

 

https://www.modelfixings.co.uk/cap_head_woodscrews.htm

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John, that was exactly the Modelfixings page I was looking at, which prompted me to ask the question.

 

I have an 0S35 to fit into a 48" aerobatic with a fixed balsa cowling, so won't have easy access for nuts etc on the underside of the mount.  Sounds like I'll be fine with 3.5mm self-tappers then.

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I have lost count of the number of Irvine 53's held in with self-tapers into plastic mounts, some getting on for 10yrs of regular use. Pilot hole then check periodically and nip up if required. Same applies for all my 15-46's, in fact it's my preference for small 2*.  4* that's another matter with all bolted as I feel the vibration may put undue strain on the thread and loosen quicker, although I have never actually mounted a 4* with self-tappers.

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38 minutes ago, Ace said:

I have lost count of the number of Irvine 53's held in with self-tapers into plastic mounts, some getting on for 10yrs of regular use. Pilot hole then check periodically and nip up if required. Same applies for all my 15-46's, in fact it's my preference for small 2*.  4* that's another matter with all bolted as I feel the vibration may put undue strain on the thread and loosen quicker, although I have never actually mounted a 4* with self-tappers.

And me, 2.2mm multiplex servos screws on .25's, and 3mm self tappers on my Rossi 40's and OS 46 fx.

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I would not recommend self tappers for mounting engines in anything to be honest. You get much better clamping force with bolts and nyloc nuts and if they come loose they have to work much harder to actually fall out completely. If access one side is limited fit the bolt head under the engine and put the nuts on top. You should be able to get a suitably modified allen key on the back and then do up the nut with a socket and easy access to the top. I have done this a few times and it works very nicely. 

 

The idea of using servo screws to hold engines in gives me chills. You might have got away with it using MPX screws, but i have some cheapo generic servo screws and i would not be trusting them to hold an engine in place. 

 

Just get some nice nuts and bolts to suit the holes in your crankcase (M3, 3.5 and 4 are common sizes) with the right size washers. Tack the engine to the mount with cyano and spot all the mounting holes with a long series drill bit. Break the engine loose, drill though a fraction oversize (3.2mm for 3mm etc) and put it all together. Its strong, reliable, and it also looks like a proper job which is quite satisfying in itself. 

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I now use Nordlock washers (link) under the heads of the bolt.  Do not use another washer in between the bolt and the Nordlock.  You need more torque to undo such an arrangement than when you tighten the bolt.  Never had one come loose on 120 four strokes.

 

If you mount the bolts from the side without access, you could use a metal plate to which to silver solder the bolt heads and glue the plate the bearer.  Alternatively, for smaller sizes use a bolt with a slotted head and solder a piece of piano wire through both bolt heads to stop them rotating.  That was how it used to be done in my early control line days! 

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35 two stroke, self tappers, likely 3mm or 3.5mm. Any thing else is starting to feel like overkill. Sleeve with brass tube if engine lugs dictate. I'd be happy with m3 hardware on the firewall too.

 

I use M4 on 10cc sizes. 

Edited by Nigel R
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Some plastic mounts come with self-tappers and instructions to use them (in case of confusion with fixing the mount to the firewall). 
 

While my preference is to use plain shanked bolts for “precious” engines, I’ve elected for self-tappers for .15 - .25 two strokes in 1/12 scale combat models for ease of field repair and never had one show the slightest sign of loosening in hard service. 

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9 hours ago, Cuban8 said:

I don't use plastic engine mounts of the SLEC type or similar these days, but back in the day I drilled and tapped the appropriate thread into the mount, secured the engine with cap head screws and added a self locking nut to the remaining exposed thread on the underside for belt and braces. Never ever had a fixing loosen off. Just nip up the screws securely and resist the temptation to lean on them.

I'm with Cuban8 on this. Tap the plastic mount; use cap head socket screws and add a locknut. Makes for a proper job.

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Helpful discussion here. Thanks for the idea about sleeving the mounting lug holes Nigel R. Hadn't occurred to me even though I do that with Dubro wheels which have 8swg (4mm) holes when you are using 10swg wire on a light electric model. I have an ASP 46 and the mounting holes are 4.2mm. I have been reluctant to use it, as a screw to fit the hole meant a substantial pilot drilling into a nylon mount. Seemed like it would weaken the mount.

 

I have always used self tappers as the American mounts like the Dave Browns that we got in the shops down here in the colonies (NZ) had a tapered underside so didn't lend themselves to having a washer and nut to secure a bolt passed right through. SLEC ones look like the beams are parallel.

Nigel

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The beaut thing about nylon filled plastic engine mounts is that in a sudden 'arrival' they (hopefully) give way before the engine lugs.

 

Many vintage models used a Paxolin plate for the same reasons (and the ease of thrust-line adjustment and swapping engines) and indeed, my small model diesel engined models also use brass bolts to secure the engine as the bolts are cheaper to replace than the crankcase.

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I've always used self-tappers in the moulded engine mounts for two strokes, up to 61's with no bother at all - metal mounts and nuts & bolts for larger and any four stroke.

 

Back in the mists of time I used to work for a company making r/c target aircraft, and their 61 powered small-arms target/trainer used Super Tigre 61's, self- Tapper'd onto SLEC moulded mounts, using the supplied screws. No problems with working loose in a fairly high-intensity environment (if squaddies had bullets left, we kept flying the target)

 

Tom

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29 minutes ago, Paul De Tourtoulon said:

Here we are a rubbish HK 2.2mm screw halfway screwed into a piece of plastic, 21 kilos and still holding, so x 4 screws =84 kilos for an OS 25 fsr, prop 9 x 6 @ 12.000rpm = 1.8 kilos static thrust,,,

 

 Still worried that it won't hold ,.😅

 

It isn't a question of not holding (even small rubbish screws have a significant area of thread), it's about one or more screws not working even slightly lose under vibration, and also about the risk of stripping the plastic thread if over-tightening or misaligning on repeat screwing.

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