Nuts material question

Hi chaps,
I have a toy aeroplane engine (YS110) that's very prone to backfiring and then spitting the propellor. I want to use it with a spinner and the problem is the
spinner adaptors I have do not fit the prop nut very well and so the lock is not very good. If you don't know what I am talking about let me explain :
The prop is mounted on the engine crankshaft which is threaded M8x1.0 (metric fine). There is a drive plate and a spinner backplate behind the prop, then the prop, then a washer followed by the prop nut and a locking nut.
The prop nut has a forward facing projection which fits into a recess in the lock nut. The purpose is obviously to aid the locking both of the lock nut to the prop nut and the pair to the crankshaft. This projection is round in section, a few millimeters long, and slightly tapered with a radius where the section changes from round to hex. The lock nut has a matching indent at the appropriate end.
The spinner adaptor is threaded M8x1.0 at one end and M4 at the other with the spinner body being trapped between the head of an M4 screw and the spinner backplate. The adaptor ought ideally to go on the crankshaft after the prop nut and locking nut, unfortunately there is not enough thread on the end of the crankshaft after both nuts to get an adaptor onto it at the same time.
So, I want to make a spinner adaptor that will take the place of the locking nut as well as providing the M4 threaded hole for the spinner retaining screw. (As I said above, the ones you can buy don't fit the prop nut projection very well and consequently don't lock worth a damn.) My question is :
Is 303 stainless steel a suitable material to make this from ? The crankshaft is presumeably high tensile steel as are (again presumably) the existing porop and lock nuts. My reason for wanting to use 303 is I already have some in a suitable hex section.
Thanks,
--
Boo

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I got lost reading this but is removable loctite thread sealant an option? There are various increasing levels of permanence, some requiring the application of heat to release the bond.
Wes
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Erm yes, it was a bit longer than necessary, sorry about that :
The propellor is attached to the crankshaft by a nut which has a special shape that only mates with the supplied locknut. There's not enough thread to get a spinner adaptor and both nuts on the end of the crankshaft so I need to make a spinner adaptor with the same end form as the original locknut so as to do double duty. Is stainless 303 a suitable material for this ?
> but is removable loctite thread sealant an

Loctite is an idea but I've never heard of anyone using it in this application and I'd rather just use a locking nut/adaptor which is what other people do.
Thanks for the idea though,
--
Boo

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Unless you know the nut strength the job requires I'm not sure anyone can answer this for you. 303 has a middling ultimate tensile strength of about 40 tons/inch (not much more than mild steels) but very low yield strength. 316 is much better in this regard and then there are the really high strength stainlesses like 15-5 and 17-4 which at 70 tons/inch are as strong as normal high tensile nuts and bolts.
If normal nuts are coming undone anyway then personally I think 303 would be far too weak. Loctite sounds like a cheap and easy thing to try first IMO.
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Hi Dave,
Thanks for your reply. I don't know the nut strength required but have been told a suitable torque for doing them up is is about 30 ft lbs. The engines are very prone to backfiring and the prop inertia is high enough to loosen the nuts. This is excacerbated by the fact that the spinner retainer nut doesn't fit the prop nut. I don't think the supplied nuts are too weak, I think it's a form compatibility issue with the spinner retainer - prop nut which allows them to loosen in extremis.
I'm interested to know what material you would choose to make the nuts from ? Ideally something that's readily available in 12mm Hex section and that doesn't need post-machining heat treatment (note: I've no preference for stainless - the 303 just happens to be to hand).
Thanks,
--
Boo

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That's a horrendous amount of torque for an 8mm metric fine. It's right on the limit for a high tensile heat treated alloy steel fastener. 303 would strip like putty at that. You might as well use cheese.
The engines are

Dunno but something at least as strong as grade 10.9 or 12.9 HT capheads i.e 70 to 80 tons/inch tensile. The two precipitation hardening stainless steels I mentioned might just about do but heat treated alloy steel would be better. Someone went to a lot of trouble to do the calculations and design things strong enough to start with. You're playing with fire if you try and second guess them without the required mechanical knowledge.
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Have you tried reducing the glow current to prevent backfiring? Backfiring (preignition) can be caused by too hot a plug, I presume when the engine is running it does not backfire at all?
By the way these engines/aeroplanes are not "toy aeroplanes" in the "toy" sense. From a model engineering standpoint they have as many engineering issues to deal with as any land base model.
Regarding the lock nut I think you could make one from mild steel. Its purpose is not to hold the prop on but to prevent the propnut from coming off and the prop smartly following it, so in that sense it is not a true lock nut. The YS110 will turn a 14" -16" prop so a backfire will have quite a bit of inertia and can loosen the tightest of prop nuts. I have never tried "loctite" on any of my prop nuts but you can get different grades from nut locking, stud locking to "like it won't come off" strength. It's an interesting idea worthy of some experimenting.
If you have not already found it this web page may provide some help; http://www.geistware.com/rcmodeling/glowpower/ys_main.htm#Detonation :
Alan
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They can come off in the air also which is a bugger as you can lose the prop nut etc. in the long grass (depending on where you fly). Richening the mixture can help a bit.

On my OS engines, the lock nut has a slots on the rounded end which when tightened on to the prop nut would close on to the prop shaft as well as the locking action on to the prop nut. Because of this double action, I don't think that the lock nut needs to be tightened up too much and mild steel would be OK. Incidentally, this is one of the jobs I bought a lathe for three years ago and never got round to it.
I suspect also that the problem is worse when using plastic spinners as they can compress more.
Archie
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