New here & question about mounting IC engines

[Jonathan M]

Hi

I returned to the hobby, after the regulation break of 35 years, about three years ago. My interests seem to be split three-ways:

RC fixed wing - Started on a RIOT which got me through my A Certificate but which I eventually rolled with a great thud into the turf (boredom was the culprit!). Moved on to a second-hand E-Flite Hurricane with retracts, which is easy fun but this doesn't much like our bumpy, molehilly, and sometimes turbulent patch. So, fed up with anonymous foam, and now determined to move to IC with enough mass to penetrate and ability to work up basic but proper aerobatics, I've just bought an Acro-Wot ARTF version with an ASP 70FS. (Later, I'll make the DB Sport & Scale 48" DH60 Cirrus Moth kit together with an unused OS FS-30 Surpass in my possession.)

RC soaring - Been having much fun on gentler days thermal hunting with an Elf 1m DLG, now assembling a 1.5m Blaster 3 for higher launching and more sustained hunting. (Also got a Middle Phase kit with aileron wing for general-purpose slope work, but this is on the back-burner for now.)

Indoor scale rubber - Going back to my earliest roots, I recently built a KK Chipmunk from a Vintage Model Company kit, then a KK Auster Arrow from a plan which I entered it in the BMFA Kit Scale category at the Indoor Nats. As if I wasn't already sufficiently time-poor, I'd like to enter something in Peanut Scale and also have a go at CO2 and/or electric... but the standard of making and flying in the Open Classes is quite intimidating!!

MY QUESTION relates to mounting IC engines, ie my ASP 70 FS, which appears to have 4mm dia holes: what is the normal method? M4 machine screws with normal nuts plus thread-lock, or nyloc nuts. Or is there some other non-metric category/diameter of screws/bolts one uses with IC engines?

Thanks

Jon

[Andy Meade]

Personally, I wouldn't use nylocs. Not only are they relatively poor at resisting vibrating loads to undo themselves, they are even worse when hot - which will no doubt be the case in the mounting around the crankcase.

I use a heat-proof thread-lock liquid along with a backup jam-nut on engines of that size. Anything bigger then I'd look at wiring the nut on, or using Nord-Lock washers which are excellent.

Welcome to the forum, and welcome back to the hobby

[Jon H]

I on the other hand would avoid thread lock like the plague and would use 4mm nylocs. I cant think of a time where i have ever had a nyloc come undone unless the nylon part was ruined like when i used one on an exhaust in my youth.

Our large 4 strokes run far hotter than your sc70 will and we supply nylocs with our mounting kits. If the engine is hot enough to damage the nylon then you have a far bigger issue on your hands!

[Andy Meade]

At work we have a group-wide ($900m+ p.a.) ban on using nylocs due to their heat inadequacy and very low off-torque, especially on gearbox mountings and also near heat sources.

Users are also swines for re-using nylocs time and time again, which again is a big no-no. Use once, chuck away, but we only really use them on machine guarding now as they are so rubbish.

But hey, use whatever works I guess...

Edited By Andy Meade on 07/09/2016 13:45:03

[Jonathan M]

Thanks for the welcome.

Useful to learn the limitations of nyloc nuts! What about using split washers with regular nuts (with/out thread-lock)?

Also, locking nuts with wire or other sophisticated technology only deals with one end of the fastening - surely one would need to lock the head as well?

[Andy Meade]

Locking the head is not required, typically, as the bolt tension created by the locking device should keep the entire locked assembly stationary. On larger machinery we sometimes specify a "shakeproof washer" (ones that look like they've been nibbled around the edge) under a bolt head, but they do get compressed easily.

Once the torque is set, and the fastener locked, you're all set.

Split washers are usually even worse, but I've never tried them with a locking compound too. Under testing, the ranking goes : split washer < nyloc < mechanical locknut (clevenut etc.) < loctite etc. < nord lock.  Wiring nuts is the best, but you may struggle at M4 to install it.

Edited By Andy Meade on 07/09/2016 14:05:09

[john stones 1 - Moderator]

Happy to use either method myself, nylocks work for me and so do's threadlock, prefer nylocks though coz i'm lazy n it's cheaper, whichever just check em out once in a while.

John

[Martin Harris - Moderator]

Having used Nylocs extensively for engine mounting and never having had one come loose, I'd be inclined to agree with Jon but I'd assume that Andy's company edict has been as a result of experience within the applications they have dealt with.

The point on re-use is interesting. I was told by a British Gliding Association inspector (and licenced aircraft engineer) that it was acceptable to re-use them as long as they resisted tightening by hand - and this was on primary flight control linkages! However, aircraft fastenings subject to heat normally seem to rely on visible methods such as wire locking.

[Denis Watkins]

After 40 years using nyloc nuts, they will not come off, due to vibration, if they are fitted new for that motor. Repeatedly used nylocs can come loose. Correctly fitted nylock nuts on 4mm 25mm long socket capheads will still be on your motor as your family disposes of your property when you pass away

Edited By Denis Watkins on 07/09/2016 15:09:46

Edited By Denis Watkins on 07/09/2016 15:10:35

Edited By Denis Watkins on 07/09/2016 15:12:02

[Jon H]

Andy if your company bans them for your specific use case then that's fine, but it's far from conclusive that they are no good for all things. Most nylocs are safe to about 120c so no problem there as I would not expect to see more than 80c on an engine crankcase. As for vibration they were originally designed for vibration resistance so I don't see that as an issue. But as I said before, if your use case is beyond their capability then obviously they aren't suitable for that specific task

[..]

Absolutely nothing wrong with nylocks to secure a model engine ? Unless the company in question fits model engines to airframes I don't see the connection in them banning them ?

[cymaz]

I like two methods

• Cyno in the threads
• Solder a strip of wire between the nuts, they can't undo then.

[Bob Cotsford]

I'm another who has used nylock nuts for decades and I've never had one come loose. If I don't have a suitable size nylock then I just use a second lock nut. Again I've never needed threadlock, but I do use a flat washer and a split washer under the nut. On two strokes up to 60 size I've got away with just tapping the nylon mount with a taper tap and relying on the friction of that, again no failures to date. In the past I've used this method on old school aerobatic models where the engine is inserted through a hole in the side of the cowl, eg my Tornado.

[Peter Miller]

I notice that everyone talks about Nylock nuts.There is another type with a split friction grip on top. I forget the correct name.

I can also remember some nylock nuts which incorprated the nylon into a cap which covered the entire thread. This prevented the thread from rusting. Not sure where they were used on aircraft but popular on motorcycles and cars on the squadron.

Nylock nuts are used on full size aircraft. I can remember using them on Javelins to mention just one type.

[Andy Meade]

Pete - I think the type you're thinking of is the nord lock - have a Google. They are the most secure additive fastener I know of.

[Martin Harris - Moderator]

or more likely Aerotite? 

I remember that Rolls-Royce used a different form of stiff nut on their small gas turbines - a friend who worked at Leavesden showed me an example which was basically a nut which had been deformed across its diameter designed to flex back within its elastic limit - they were supposed to be able to be reused many times...

Edited By Martin Harris on 07/09/2016 19:56:36

[Andy Meade]

The deformed nuts are what i mentioned earlier - a mechanical locknut like a "cleveloc"

The top shoulder is ever so slightly squished. Manufacturer's recommend they can be re-used, but the off-torque is greatly reduced after the first removal.

[iqon]

Nylock or double nut

[bert baker]

Whats wrong with self tappers into a Nylon mount,

[Jonathan M]

Learnt lots already! Thanks.

[Martin McIntosh]

To be quite honest I use self tappers into glass filled nylon mounts up to at least 110 size. Some poorer quality ones can have the screw heads break off if they are not square to the threaded part but this is rare.

The problem with fitting nylock or T nuts is that most mounts are tapered underneath. I always used to use metal mounts which were drilled and tapped but these are few and far between these days.

[Denis Watkins]

I know there is a limit to how far we go in preparation, but if the mount is beefy, I machine a small flat in the slope of the engine mount. If it is a small motor mount, I have sadly, machined sloping washers, I just think it is worth it

[Steve Hargreaves - Moderator]

+1 for nyloc nuts for me too. I use cap head bolts through a plain washer; through the engine mounting lug; through the mount then secure with a plain washer & a nyloc nut. I don't think I've ever had an engine come loose. Whilst I can see the logic of heat softening the plastic insert I can't think that a nyloc nut securing a model glow engine will see much of a temperature rise. The crankcase doesn't get that hot & the heat transfer via the bolt would be minimal I believe.

I have to admit to using self tappers into nylon mounts too.....the engineering side of me looked on in disgust as I secured the engine to the mount (a 91 2 stroke) but the engine has shown absolutely no sign of coming loose after many hours running. If using self tappers it's very important to use the right size drill. The drill used should be the same size as the "core" of the self tapping screw to ensure the threads get a good deep grip in the mount.

Whilst in the Confessional Box I will admit to tapping an M3 machine thread into a (glass filled) nylon engine mount too although this was only for a 0.30cu. in. engine. The built up cowl prevented the fitment of backing nuts. Again the engine showed no sign of coming loose.....

Right I'm off to say a few Hail Marys now....

PS The more Vintage Modellers amongst us might remember holding small diesels into models using rubber bands. This used to work quite well until the fuel rotted the rubber band you had a free flight engine & a model with a serious CoG problem....

[Jon H]

Personally I would not use self tappers in glass/nylon. The stresses in the material from the screw can cause a failure especially in cold weather. I wouldn't use self tapping screws to hold any engine over about 3cc

[Rich too]

I'm with Jon! Self tappers? Tbh I've almost always just gone with whatever the manufacturer recommends and then you can't go wrong?