Securing an Engine

[Andy J]

Acquired a vintage Flair model that appears to have its original engine secured using wood screws. Certainly have never used such a method before. If I do go down the route of using an identical method to fit a Laser 75 am I likely to regret it.

[Paul De Tourtoulon]

Never seen wood screws, I wouldn't, I would bolt the engine to a piece of aluminium, and then bolt it to the wood beams with a plate on the other side like a sandwich. You won't regret your Laser.

[Tom Gaskin 1]

If the engine is fitted to a nylon or glass fibre moulded mount, then self-tapping screws work very well. We used to use this method on the two-strokes in the military trainers/targets that I worked on, and I have used this method on up to 70 four-strokes with no failures until terminal arrivals! Otherwise use nuts & bolts.

 

Tom

Edited July 17, 2023 by Tom Gaskin 1
Added info

[Alan Gorham_]

I had for many years a Flair Cub which had the 40 four stroke secured by wood screws into Beech bearers. It was feasibly the only way the engine could have been secured due the design and build sequence of the cowl. Pilot drilling the holes into the bearers with the right size drill that left enough meat for the screws to cut their own thread was the key and it worked well for many years. 

[Andy J]

Engine mounts onto a pair of beach  bearers but there is no way that I can get a bolt given the underside of the front engine bay is fully sheeted. Only option would be to drill through the sheeting to allow a screw to be inserted from the underside but reluctant to do that.

[Paul De Tourtoulon]

Oh yes there is,,,

 

Drill your engine mounting holes for bolts slightly larger than needed, slip a length of wire through them and thread it through a blind nut and put a knot in the end,

then you can pull it into place, then get one of your engine mounting bolts with a big washer on it and screw it into the blind nut, as you tighten it, it will squash the wire that you threaded through, tighten it up, and it will dig into the wood, you can also use a bit of 30 minute epoxy to make sure that it stays in place. Repeat 3 more times et 'voila',,,

[Andy J]

Cant see that method working Paul, so given the beech engine bearers are a good 3/4 of an inch in depth tempted to go down the route of using a long pan head self tapper with a fall back plan of drilling through the engine bay underside skin to fit conventional M4 or M3 cap head bolts.

[Allan Bennett]

So long as the wood is good quality (not too soft) and the holes are not drilled too large, there's nothing wrong with using self-tappers or wood screws.  The OS61 2-stroke in my Flair Pulsar was mounted that way without any issues.

[Jon H]

15 hours ago, Andy J said:

If I do go down the route of using an identical method to fit a Laser 75 am I likely to regret it

 

Probably. Wooden mounts are a pain and self tappers...not recommended. I wouldnt even use self tappers on a 15fp. 

 

I know many will point out that this ancient model has apparently worked fine this way for many years, but we dont actually know that. Its previous owner may have tightened the screws between flights for all we know. 

 

If it were me i would cut them out and make whatever mods were needed to fit a glass/nylon mount. Long term its the better way to do it even if its more work now. 

[Engine Doctor]

John if you have never used Self Tapping screws, or have you , to fix an engine how can you comment ? We are talking about pan head ST screws and not tapered CS wood screws aren't we .  ST screws work perfectly well in nylon or glass reinforced plastic mounts and hard wood bearers if fitted properly. They were even supplied with some mounts . Perfectly fine for smaller engines up to a 40 but obviously not for bigger stuff. Many models on the past had solid or block balsa nose cones / cowlings sabded yo shape an the only acess was from the top. ST screws worked well for years and never came loose.

My only gripe about ST screws were that I'd often get an engine to fix that had been fixed using ST  screws and the owner didnt know how to fit them or drill correct pilot holes and they had chewed the crank case mounting holes !

This fixing became unpopular when cheap Chinese ST screws started being used and oftrn snapped or rounded off leaving the owner with a puzzle how to extract yhe broken screw.

[Jon H]

I use them in the past before i knew better. Engines coming loose, nylon mounts known to break due to stress caused by the sharp threads are just some of the problems. Its one of those cases where it might work most of the time but its not right and there are better ways to do it. You will always get better clamping force with bolts as self tappers rely on just the shear strength of the material. Soggy oil soaked beech is no match for a steel nut.

 

58 minutes ago, Engine Doctor said:

We are talking about pan head ST screws and not tapered CS wood screws aren't we

 

 i have seen countersunk wood screws in use in the past so i dont know. I would like to think not, but even if they were pan head jobs it does nothing to change the fact that steel bolts are better. 

 

The model in question is the flair f2b if i remember rightly and there are several ways the it could be modified. None of them are especially complicated. 

[Nigel R]

what about threaded inserts?

 

if the beech bearers are already in, then that might make a reasonable way to get a larger thread into the wood, and something harder for a machine screw to go in to.

[Andy J]

Well starting to be convinced that I need to drill the underside skin to allow access for 4 suitable screws fitted using an appropriate hex driver with self locking nuts on the engine side.

 

Will drill the bearers first using the engine as a template and then use a heated length of piano wire to burn through the contoured skin under before opening up to clear the cap bolt head.

Edited July 18, 2023 by Andy J

[Nigel R]

3 hours ago, Engine Doctor said:

 Perfectly fine for smaller engines up to a 40 but obviously not for bigger stuff.

 

That's me on the naughty step, I use self tappers in nylon mounts for 60/90 size stuff...

 

[extra slim]

I would screw it if easier, (but a proper job as described) and then check it periodically, and if it becomes loose, upgrade to a permanent method as described by Jon. You might never need it, you might do, who knows, if it does work loose you know what to do. If you have the time, inclination, going to live for years and years, fly the backside off the plane, then skip to the belt n braces. Fairly straightforward conundrum IMO. 

Incidentally, another method,  I have also used a BOLT as opposed to a screw in a nylon mount, used to "self tap itself" into an ever so slightly smaller hole. Due to a diagonal bit on the bottom of the mount I was unable to get a nut on the end.. Been good for over 10 years.. Squeaked a bit driving it in, but it aint going anywhere!. 

[Paul De Tourtoulon]

36 minutes ago, Nigel R said:

 

That's me on the naughty step, I use self tappers in nylon mounts for 60/90 size stuff...

 

I was waiting for someone to admit that first, 🤐my old OS .25 fsr's were held in with Multiplex servo screws, and my Rossi .40's with 3mm self tappers, 4mm for my Rossi .60 and never had one fail or even get loose, all screwed in to plastic engine mounts; still used on my 4 strokes up to 10cc.

[Engine Doctor]

2 hours ago, Nigel R said:

 

That's me on the naughty step, I use self tappers in nylon mounts for 60/90 size stuff...

 

No thats fine if it works for you. I dont use ST screws very often these days but would never rule them out as an option as it depends on the model structure. J7st m6 personal preference not to use them on engine over the 40 size ,

The thing is its a hobby and everyone can experiment what works for them. Providing an engine  of any size is secure then all is good. 

These days of ARTF dominance ist the airframe thats likely to fail and let an engine go. Im currently repairing a large extra that the engibe mount let go when taking off. The structure boast composite reinfored ply . Its the definition of the ply wood thats the problem. Its soft wood rubbish probably rejected from a far eastern pallet factory ! And theres little evidence of any glue holding the plys together. 5he compisite is so thin it adds little strenght but dies stiffen the rubbish ply , that is until the plys seperate. 

Edited July 18, 2023 by Engine Doctor

[Andy J]

Well the M4 clearance holes are now drilled, just now need 35mm M4's from MF which will be fitted from the underside.

[Geoff S]

6 hours ago, Nigel R said:

 

That's me on the naughty step, I use self tappers in nylon mounts for 60/90 size stuff...

 

IIRC SLEC supply self tappers with their nylon mounts (at least they did?).  They're very hard to screw in even with a correct pilot hole. I can't see them failing to hold

[jeff2wings]

Surprised no one suggested T-nuts ? Looks like there is sufficient access to position them

[Paul De Tourtoulon]

6 hours ago, jeff2wings said:

Surprised no one suggested T-nuts ? Looks like there is sufficient access to position them

That is what I suggested, I called them blind nuts, so maybe my English is really getting rusty.

[jeff2wings]

LO! That's both of us then,also known as captive nuts nuts or blind nuts 

[Allan Bennett]

12 hours ago, Geoff S said:

IIRC SLEC supply self tappers with their nylon mounts (at least they did?).  They're very hard to screw in even with a correct pilot hole. I can't see them failing to hold

They still do with one of their mount ranges, up to 0.90 size.