Recycled Flyer

Hi all, I have an MP Jet 060 and the engine lugs look suspiciously thin to me and was wondering about their integrity in case of any impact.

So, has anyone tried nylon bolts as a sacrificial method of mounting?

Thanks.

ffkiwi

I suspect that while you could do it, residual and conducted engine heat would do the bolts no good at all. If you're concerned about lug integrity in a crash then use brass bolts for fixing [a suggestion made by some small engine manufacturers in any case]. A quick check on one of my own confirms that the lug holes will pass 2.5 mm, so there's plenty of choice in mounting bolts-2.5mm metric, 6 or 8 BA, 2-56 UNC

ChrisM
'ffkiwi'

Recycled Flyer

Brass, ok then.

Thanks Chris.

qazimoto

My Feedback: (1)

A fellow member of the the Ryde Model Flying Club in Sydney during the early 1960's was an Electrical Apprentice working for the Government Department responsible for building and maintaining the States Traffic Lights.

He supplied us all with free Brass 6 BA Cheese-head screws of all lengths, and these were used to hold in all our motors, including 6.0 and 6.5cc c/l combat glows, an application where there were lots of impressive prangs.

The brass screws almost never broke, which wasn't the case with otherwise similar Steel ones. The Brass cheeseheads did stretch appreciably however in a serious sudden arrival.

I later found out that while the Brass has a lower tensile strength than Steel, because of it's amazing ductility it required even more total energy to fracture it than the latter.

This property is called Toughness and it's almost the opposite of Brittleness.

So Brass screws are ideal for engine hold down bolts, at least in a prang anyway.

Regrettably their use seems to have gone out of fashion, probably because they're almost impossible to buy.

The B.A. sizes are getting very hard to find as well.

On the other hand Nylon screws, which have replaced brass ones in many simple non-critical electrical applications, have a mechanical strength just a little better than cheese.

ffkiwi

In my earlier reply I alluded to certain manufacturers specifically recommending brass bolts for mounting-those of you lucky enough (like me!) to own an original Doonside Mills 75 will find this recommendation in Ivor F's uniquely written instructions; as well Dave Banks, who initiated most of the VA miniatures, also makes the same recommendation in some of the instruction leaflets.

I suppose the ideal-in terms of a shearing 'weak link', would be aluminium bolts-which are available-but probably not in sizes useful to us-though I think you can get 4-40 in aluminium. Check out www.microfasteners.com

I think BA are about the only series you can reasonably readily (and I use the term with some reluctance!) get in brass.........but we are talking small engines by and large (though anyone with with any familiarity with ED Mk2 and Comp Specials will know about weak mounting lugs!)

ChrisM
'ffkiwi'

greggles47

Chris,

Use cable ties on it. As they stretch you can just move up another click!

Greg

qazimoto

My Feedback: (1)

Brilliant!

Ah Greg, always thinking outside the box.

ffkiwi

.....and no doubt the total airframe turbulation arising from the undamped vibration will be aerodynamically advantageous.............

ChrisM
'ffkiwi'

ffkiwi

[is it kosher to quote yourself?] ....anyway I should have checked my own suggested reference url above first.........and I'm pleased to report that Microfasteners list brass roundhead machine screws in sizes from 6-32x 3/4" down to 00-90 (though you'd probably only want the latter to mount a Valentine 0.05 or 0.1cc!) 4-40, 2-56 and 0-80 are pretty close to 6BA, 8BA and 10BA sizes ....which should cover most of our required useage. They also list 4-40 socket head in aluminium in 1/2" and 1" lengths..............
I can recommend Microfasteners as a good firm to deal with-I've been a customer for at least a decade and always had good service from them.

ChrisM
'ffkiwi'

Recycled Flyer

Hi Ray,
that explains why the old Peacemaker model that you gave me via Greg had brass bolts holding down the Thunder Tiger GP 15!

A puzzle solved, thanks.

qazimoto

My Feedback: (1)

I can remember breaking only one lug off an engine and that was on an OS 15 Max 3 glow in the early 1960's.

However more recently I helped a mate sort out a bucket brim full of engine parts left over from the shed of a deceased club member.

We assembled many runable engines most of which were diesels. I got enough parts to eventually make two Oliver Tiger mk3's.

Apart from Enya 15 diesels, and many Taipans (which my mate Dave claimed) there were dozens of small British free flight sized diesels dating from the 1950's.

Almost all of them had broken lugs. Hans, (the deceased modeller) may have specialized in repairing them because a few had homemade brackets allowing them to be mounted.

I later discussed this rather odd coincidence with another modeller who had been an Engineer during WW2. His suggestion was that after the prolonged nastiness in the early 1940's

there was a vast proportion of the UK economic wealth tied up in scrap Aluminium alloy.

Some of this was used to cast model diesel crankcases of the cheaper engines right into the 1960's.

Regrettably the scrap alloy was contaminated with the often similar looking alloys of Magnesium and other metals which decreased it's strength and other properties.

Broken engine lugs on modern engines seem to be fairly rare. Perhaps this has something to do with the quality of the modern alloys?

Ray

AMB

One of the English engine makers bought up a load of scrapped Rolls /Royce Merlin piston castings after the war good quality aluminum melted down and used fot castings martin

ffkiwi

This seems a very reasonable theory Ray-I think your engineer mate is on the right track-certainly most current model engine aluminium casting alloys contain silicon which confers a lot more tensile strength than the older alloys which didn't have this element in the mix. And certainly a lot of early UK manufacturers started off using scavenged materials-Elfin, for example being co-located with a scrap metal merchant! Weight was often lighter then too-size for size-look at the Allbon Javelin and Amco 3.5PB-for examples of ultra lightweight engines......and also why we ended up with so many 'black' crankcase engines-because they were cast in magnesium alloys to save a few tenths of an ounce-and had to be chromated to stop them corroding-think ED Racer, Mills 75, 1.3 and 2.4 for common examples. So using weaker materials and often with the material thicknesses pared down to a minimum.
That-and the propensity for the average ham fisted schoolboy modeller to drill out mounting holes to a larger size if they were short on 8BA bolts or whatever-leaving even less metal to resist the inevitable.....

I don't think any UK engine firms had their own casting facilities either-and probably got their castings done in batches at the local foundry 'x' times a year depending on their production schedules-and said local foundry probably used a good proportion of scrap (possibly not even the same from batch to batch!) .....and I suspect that heat treatment and aging of aluminium alloys was in its infancy then.....the UK in particular was known for its steel expertise, not aluminium!.........which leads me to another minor point-when you read through a lot of the old engine tests in AM or MA, often the crankshafts-and sometimes the liners-were machined from EN 'xyz' steel-EN stood for 'Emergency Number' -the classification of steels suitable for war effort/economy production under the dire restrictions of the British WW2 economy. As most of us know-Britain emerged from WW2 victorious-but in a dire state economically-which continued right through until the late 1950s-so right through the heyday of UK domestic model engine manufacture.
Small wonder that manufacturers would have stuck with what was cheap and available-even if better but more expensive raw materials came along-which might have produced better engines-but would certainly have put the price up-and possibly required changes to production methods in terms of heat treatment, hardening tempering etc. We know to our sorrow that the Indian Mills people and CS couldn't get the latter right either, even 40 or 50 years later!

ChrisM
'ffkiwi'

Mr Cox

If you mount the engine on wood beams, I guess the wood will be the first to give. No need for overengineering...

qazimoto

My Feedback: (1)

Most of us go to considerable effort to make the timber engine bearers as strong and as stiff as possible. It makes the engine run more consistent.

Mr Cox

Yes, the whole beam does not have to give, it is enough if the screws and the threads in the wood give in.