Nylon Engine Mounts - What sort of screws do you use to secure the engine?

[Robin Colbourne]

Having had the Leo 37 engine in my Harry Gilkes 'Cat 500' stop when coming out of a spin yesterday, the resulting arrival snapped the Radio-Active nylon engine mount.  It occurred to me that the through holes for the machine screws holding the engine considerably weakened the mount as this is where it snapped on one side. 

 

SLEC engine mount instructions recommend using self tapping screws to hold the engine, however I've had a self tapper snap in the past and the broken part chew the bottom of the engine's mounting lug.  I also don't like having the threaded portion of a self tapping screw in the lug.

 

It occurred to me that what I need is a self tapping screw that has a plain portion under the head and is softer than a normal self tapper, so it could be drilled out if broken.  The closest thing to this would be an old style dome headed wood screw.  These are tapered though, so if they start coming loose, they come out quickly.  Soldering a wire across the screw slots should prevent this, but is extra flaff.

What screws do you use on nylon mounts?

[J D 8 - Moderator]

 I drill them through for fitting with bolts, only engines I have ever held in place with screws are little Cox engines.

  It was likely good the engine mount broke and not the engine.

[John Lee]

Whilst I no longer fly IC for the 5 decades that I did I almost always used self tappers in nylon mounts without any problems whatsoever. 

 

I've still got left in my parts draw a few of my favourite fitting - Quality steel No 6 Socket Cap Woodscrews.  

 

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

 

The screws have no taper after the initial point and never budge if you use the correct pilot drill as suggested in the Modelfixings table.

 

ps the above applies up to & including .50 size engines. Above that I preferred a more 'engineering' solution, generally involving drilling & tapping metal.

Edited May 28 by John Lee

[Jon H]

Frankly, i would use bolts and nylocs on everything. There is simply no reason not to

[GrumpyGnome]

Nuts and bolts every time....

[Jim Hearnden 1]

In nylon mounts always self tappers, in wood bearers etc. then bolts & nylocs

[Robert Welford]

I also use bolts and Nylocs. 

[Allan Bennett]

I used self tappers without any issues when I was flying an OS61FSR.  Bolts and nuts are only good if the underside of the mount is parallel to the top, or you have a tapered washer the correct angle to avoid any bending stress on the bolt.

[Simon Lumsdon 1]

Cap head bolts and nyloc nuts all easily obtained from ebay, cap head bolts very easy to tighten and access using allen keys. 

[Robin Colbourne]

Thanks everyone for all your replies.  Clearly all approaches are used. 
John Lee, thank you for the link to the hex head wood screws.  I'll investigate them further.

[Paul De Tourtoulon]

I have a few fibre glass fuselage planes like the Super Knatty, and it's a shame to hack it out for a few nuts, so it's self tappers for me, and they don't loosen.

[Robin Colbourne]

14 hours ago, J D 8 said:

   It was likely good the engine mount broke and not the engine.

JD8, you are absolutely right.  Replacing a £2 engine mount is a much easier repair than either a broken engine or re-attaching 'F1', the plywood bulkhead, to the front of the fuselage.   That wasn't lost on me when I reached the 'arrival' site.   The picture below is back in the pits.

[Robin Colbourne]

One more thing I noticed.  Despite having blue Loctite on all four screws attaching the engine to the captive nuts in the front bulkhead, all were free to rotate, so had loosened a turn or two since installation.  Better cleaning and fresher Loctite required!

Here's the broken mount.

[Nigel R]

I've used self tappers exclusively in nylon mounts without any issue, right up to 15cc. Holes will always weaken the mount. It will still be plenty strong enough to withstand any flight loads.

 

[J D 8 - Moderator]

  It is likely the case that over time the wood of the bulkhead compressed some making the bolts work loose in the nuts.

[Tom Gaskin 1]

When I was into I.C. I always used self tappers into composite engine mounts, up to an OS 70 surpass, with no operation issues at all. Larger engines or metal mounts would be nut & bolt territory.

 

Tom

[Robin Colbourne]

Latest installment in the nylon engine mount saga...

The head of one of the self tappers sheared off in the new mount as I was winding it in. 😒  The projecting thread on it then broke up when trying to unscrew it with pliers.  Eventually by holding a hot soldering iron on the broken screw for 30 seconds or so, to soften the surrounding plastic, I managed to unwind the screw a bit before the pliers took all the heat away.  It took two goes with the soldering iron, but thankfully the broken screw is out now.  The drilled pilot holes were through holes, so I wonder if the screw was just particularly brittle?

The replacement screw has gone in ok, so time to get it back on the model.  I've straightened the main undercarriage.  It was not only bent in the arrival, but also had some toe-out before which had caused 'interesting' take off runs.  It has toe-in now, which should tame things a bit.
 

[Paul De Tourtoulon]

I have had a couple of screw heads break off like you, but lucky enough to grind in a slot and unscrew them with a flat screwdriver, that's today's rubbish metal, I like your soldering iron technic to heat them up to get them out.

[Nigel R]

Screw might well have been defective. That said, a screw forming its own thread (even with a pilot hole) into glass/nylon is having to fight against a fair amount of pressure/grip/resistance (not sure what the technical term is here). 

 

 

If I remember, I will cut a couple of slots into a screw with a dremel, then use that to "cut" a thread in the nylon mount. I don't use that screw for actually mounting the engine in anger though.

[Mike T]

If you use sheet metal (P-K) screws, which are 'thread-formers' then they may well bind and shear.  If they start to go stiff on you, quit while you're ahead and use the soldering iron trick to heat the nylon and free them off.  Then replace with a new screw. 

 

The only way to avoid binding is to drill (or ream) the pilot hole for the screw's shank slightly over size, or preferably use thread cutting screws, which have a spiral flute to clear swarf.  Also beware removing and replacing them too often, else they can go slack and you then need to drill out for the next size up!

 

Re screws vs through bolts, I tend to err on the cautious side and use bolts on .60's up.  But the fact that the screw heads sheared is testament to the grip of the thread in the nylon...

 

 

[Robin Colbourne]

1 hour ago, Nigel R said:

Screw might well have been defective. That said, a screw forming its own thread (even with a pilot hole) into glass/nylon is having to fight against a fair amount of pressure/grip/resistance (not sure what the technical term is here). 

 

 

If I remember, I will cut a couple of slots into a screw with a dremel, then use that to "cut" a thread in the nylon mount. I don't use that screw for actually mounting the engine in anger though.

Nigel R, given how brittle the screw appeared to be, I don't think grinding a slot would have worked.   The gripped length in the mount was about 8mm and the diameter of the screw inside of the thread about 2 to 2.5mm.

Edited May 30 by Robin Colbourne

[Engine Doctor]

Ive often used self tapping screw on nylon mounts in the past . Only ever had a couple of failures when the rubbish screws snapped . Generally though provided a decent pilot hole is drilled they go in and hold very well. I now mostly use cap head machine screws with nuts as self tapping screw quality seemed to get very poor when most are made in the far east, Even some cap machine screws seem to be made of butter steel . Our LMS Rapid RC keeps an excellent range of screws of all types of reasonable quality in handy sized packs that dont break the bank .

If a nylon mount fails through the screw in an "arrival" its possibly saved the engine or model from more serious damage ; whereas an ally mount either breaks the engine or pushes the forces into the airframe . 😶

[Phil Green]

Drill, then bolts & nylock nuts.  My reasoning is that the area around a correctly inserted self-tapper is under constant stress, thats how all screws work. Over a large area that stress has the surrounding material to work against. In an engine mount it can only work against fresh air - the bearer will very likely split if subjected to any impact, shock or further stress.  I've even seen them break on flick-starting.

 

 

 

 

[Robin Colbourne]

I cant remember whether it was on a free flight or control line model, but I have had an unhardened steel machine screw snap in a crash, whilst the cast light alloy lugs on the Davis Charlton engine it was securing survived unscathed.

Given how hard and brittle self tappers are, if the pilot hole isn't absolutely perpendicular to the top face of the engine mounting lug, the side load on the head of the screw is setting it up to break.

Hopefully I'll see how my new mount gets on at the weekend, although given the forecast wind direction and strength it may be better for slope soaring.

[Martin Harris - Moderator]

A slightly off the wall alternative...

 

A couple of days ago, I decided to resurrect one of my old 12th scale combat models - a Hellcat with FAA D-Day markings - to commemorate next week's 80th anniversary. The engine mount had been damaged on its last outing several years ago so needed replacing.  I was contemplating re-using the self tappers that I'd used for fast field changes during competition so drilled pilot holes in the new mounts (with an unremembered drill with the correct looking core diameter for the screws) but then decided that perhaps capheads would be better.  The M3s that I had to hand were slightly larger than the pilot holes but screwed in steadily so just for fun, I tightened them into the mount.  They tightened well with no sign of the threads relaxing even with considerable torque, so I left the two rear ones as they were and added nyloc nuts to the front ones, which were through the thinner parallel section of the arms, for safety.  No sign of loosening after test flying yesterday...

 

I suspect the formed thread might be stronger/more vibration resistant than the alternative of a properly tapped hole but if you chose to use this idea, you'd need to satisfy yourself of its long term efficacy and find a suitable "tapping" size for the capheads chosen.  My feeling is that the finer parallel threads formed would be less weakening than the coarse sharp edged thread cut by self-tapping screws.