Panic! Damaged prop shaft threads - can't fully unscrew retaining nut. Help please!

[Nick Stock 2]

Hi Everyone, happy New Year and hope you are weathering the covid storm safely.

The prop retaining nut is already screwed on the shaft. It can be tightened up to the prop retaining washer (with the prop behind it) and tightened up no problem. However there was an adapter nut in front of that nut in which the long spinner socket cap screw screws in to to attach the spinner to it all. That nut was only screwed on by the end 2 ish threads and seems as if the previous owner has damaged the end threads of the prop shaft, meaning that the prop retaining nut locks up on it when I try to unscrew it.

I also cannot remove the cowl (if I were wanting to remove the engine from the mount) as the spinner backplate and prop cannot be removed to allow this.

When the nut locks up when trying to unscrew, I cannot apply any significant force other than that of the engine compression which is not enough to provide enough 'locking force' to push against and the engine simply turns over.

I've tried WD-40 fast release penetrant and heating the nut up with a (very small) butane blowtorch. Not powerful enough to get red hot or anything like that but either way, neither have helped much.

Would anyone be able to suggest a solution to the above?

I've looked in to removing the spark plug and 'dropping cord' in above the piston so that it presses up against the cord before it reaches tdc, providing a locking force to act against .. but I'd hesitate to do something like this without assurance that no harm can come to the engine and which specific cord to use!

Other than that, I dread taking a dremel to the prop retaining nut to try and create some 'relief' cuts and I really am concerned that I'll risk damaging the threads of the shaft further, whilst at the moment, there could still be a chance that the damaged threat portion can be cleaned up.

Finally, if anyone else is familiar with the type of spinner nut adapter I mentioned earlier (the spinner is an aluminium one that comes included in the arf kit in this case a 33cc H9 spitfire) - is that adapter (same internal and same outside hex size as the engine retaining nut) enough on it's own to replace the prop nut that comes with the engine? I say this for two reasons - 1/ if I've had to wreck the prop retaining nut in the process of getting it off and 2/ so that it grips the shaft with 10+mm of thread instead of 2 threads (2-3mm).

Any help incredibly appreciated.

Best

Nick S.

[Dwain Dibley.]

You need to buy the appropriate piston locking tool for your engine.

The other thing is, can you thread a new nut on, to recut the threads on the shaft.

Thirdly, post a pic as that will help us a lot.

D.D.

Edited By Dwain Dibley. on 11/01/2021 22:52:25

[Martin Harris - Moderator]

I would use a split die to clean up the thread but not everyone will have this capability to hand so if you have a small triangular needle file it should be possible to clean the thread sufficiently to get the nut off. As you won't be affecting the thread where the nut tightens against the prop, there's no real safety issue.

If you need to get some working clearance and the prop isn't too important to you, you could weaken its hub with a Dremel and split it off.

Thinking some more, a second nut would be easier to screw on than to remove the original as you can hold the prop against rotation.  Some cutting clearance slots filed in to it might help if the thread is very damaged.

Edited By Martin Harris - Moderator on 11/01/2021 23:16:35

[Peter Jenkins]

I would recommend buying a die and "rearranging" the thread rather than using a second nut as the die will win and the nut might not! Just tighten the prop nut and use the prop to hold the shaft steady as you run the die up the shaft.

[Martin Harris - Moderator]

The main problem I can foresee is that dies have a bit of a lead and he only has a couple of threads showing beyond the prop nut so the damaged part may not get cleaned up. That's why I suggested that it might help if he removes the prop with a little butchery of its hub. A thin piece of aluminium bar with a transverse slot could be used in place of the split off prop to hold the engine against rotation with the "captive" nut used to hold it.

Alternately, the die may be more effective in this case if used back to front. I suspect a die nut will have a lead on both faces so Nick will also need to get hold of a die stock - not so straightforward to borrow one from a clubmate in these pandemic days...

I'm not a fan of piston locking tools but possibly in this case you won't actually need to put massive strain on the piston and con rod to persuade the nut over the damaged thread - but if you decide to try it, proceed with caution.

Edited By Martin Harris - Moderator on 12/01/2021 01:07:31

[Jon H]

I would not recommend a piston locking tool, especially if the engine has a cast piston. It would be very easy to punch a hole though it.

I would probably try and tidy up the threads enough with a sharp file and then wind the nut off.

If you cant quite get the purchase to undo the nut try and shuffle the cowl back a little way and grab the prop driver of the engine with some of those adjustable pliers. Make sure you put something between the jaws and the driver to prevent damage and i would also remove the spark sensor as you just know its going to get busted. Once you have a really firm grip of the prop driver just undo the nut with a spanner.

It sounds a bit brutal, but i have had to use this method on my ASP radial as the crank is really soft and gets damaged easily.

[Peter Jones 21]

From Nicks description I am not sure that he can get access to the front end of the crankshaft thread? I think the nut he is talking about is a second nut that a central spinner retaining screw threads into.

Crankshaft locking as Dwain describes is quite an accepted technique, it carries a bit of risk but right at the moment Nick is looking over the cliff in anycase.

[Nigel R]

Is a new crankshaft expensive?

I'd be tempted to grind down (dremel) the damaged end threads, and replace the entire crank. Those two threads are already gonners, from your description.

At least all the prop, nut, spinner and cowl survive intact that way.

[Nigel Dell]

Posted by Peter Jenkins on 11/01/2021 23:35:45:

I would recommend buying a die and "rearranging" the thread rather than using a second nut as the die will win and the nut might not! Just tighten the prop nut and use the prop to hold the shaft steady as you run the die up the shaft.

All good suggestions here but I used Peter’s way to sort a very similar problem with a MVVS 1.44 twin and saved the threads and it has been good ever since.

[Engine Doctor]

Posted by Dwain Dibley. on 11/01/2021 22:52:02:

You need to buy the appropriate piston locking tool for your engine.

The other thing is, can you thread a new nut on, to recut the threads on the shaft.

Thirdly, post a pic as that will help us a lot.

D.D.

Edited By Dwain Dibley. on 11/01/2021 22:52:25

What ever you do DONT use a piston locking tool as they can seriously damage the piston and wreck an engine .

Sorry DD but I have seen good engines wrecked by those things . They can often weaken or crack the piston crown that breaks up later when engine is running and the whole engine gets wrecked . even light pressure can damage a piston crown .They should be banned from sale .

If its only the last couple of threads try to clean them up wit a dremel with a thin diamond disc . The nuts supplied to hold on spinners are usually made from brass and it may be that the brass is just stuck in the crank threads . cleaning this out should restore the thread .If you still need to lock the engine turn it inverted and fill the cylinder with engine oil . Using the oil method to lock your engine may be enough to allow you to undo the prop nut and reform the thread Be careful as too much force can damage the crank.

Once you get the nut off you can get the a thread cleaned properly or even chamfer the damaged threads off if they are beyond cleaning .

To fit a spinner after this you can get a long nut to replace the prop nut that will take the spinner retaining screw .

Good luck

Edited By Engine Doctor on 12/01/2021 10:57:00

[Nick Stock 2]

Thank you all for your responses and so swiftly too,

I’ve added photos to illuminate things more:

first photo showing the nut in it’s position when screwed up to the prop (which I intend to replace anyway once all this is done)

second photo showing the nut unwound as far as it goes before the engine simply turns over.

Third photo showing how little clearance there is for an adjustable spanner etc to grip to prop driver.

photo showing how cowl cannot move backwards due to the centre section of wing being in the way. I will be removing the centre section now to see if I can get the cowl to move backwards in order to revert the driver. I’m going to try and grip it with an adjustable spanner with some soft leathery type material to see if I can get a decent but safe grip.

have ordered a piston stop from Amazon which is due to arrive tomorrow but will almost certainly send it straight back.

im hoping I can just force this nut off the thread by working it forwards and back.

thanks all again

nick

[Martin Harris - Moderator]

If you can get to the spark plug I'd go with ED's hydraulic locking suggestion first but if that's not the answer then if it was my engine I'd attack the prop hub with a dremel to weaken it, break the prop off across the hub and wind the nut toward the driver clear of the end of the shaft so that you can address the problem with the damaged thread which is not accessible at the moment.

Edited By Martin Harris - Moderator on 12/01/2021 12:04:11

[Keith Berriman]

Maybe a trip to local garage or handyman who may have a nut splitter to hand

[Martin Harris - Moderator]

Nut splitters are pretty effective but often, propnuts are something similar to tool steel so may not split as easily as a normal steel nut.

[kc]

Taps and dies are not that expensive individually so buy the proper thing if you need. Tracy Tools is a good source for high quality taps & dies at good prices.

[Andy Stephenson]

The first thing I would try is to use a lubricant because if there is any galling between the nut and the crank it may just be enough to get past the problem.

The second thing I would do is to remove the prop by cutting into the hub and breaking it out. This would give you access to the non-threaded part which could then be gripped or alternatively screw the nut on further to allow you access to the damaged thread.

A.

[john stones 1 - Moderator]

I would put a slight taper on the end of shaft then clean up with a needle file, pull prop from one tip and try again.

[Nick Stock 2]

Succeess!!!

with the cowl screws removed, I was able to wangle the cowl back enough to squeeze in some mole grips around the prop driver. May have scratched it a little in the process but very minorly. It then did not take much force really at all to twist the nut off with a spanner. It’s difficult to make out where the damaged threads are - on the prop shaft or on the nut itself.

ive just managed to screw the spinner nut on though at it went on without much fuss.

my question now follows, is this nut the appropriate method of securing a prop I.e without the prop retaining nut that come with engine? I presume yes but please could someone advise definitively!

thank you all so so much for your support

best wishes

nick

[Nick Stock 2]

[Engine Doctor]

If the spinner nut is made from steel it should be fine Are you sure its the proper thread ? Make sure and tighten thoroughly.

If you ever fit a wooden prop then make sure you check tightness regularly as wooden props compress with time and then loosen .

[John Stainforth]

From my experience of having had endless prop nuts come undone, I would say you will be lucky if this is adequate. Getting washer, prop nut and spinner combinations that are compatible and work is a pain in the neck. I would try to put on a thinner washer (without the taper), then a thinner nut and then the spinner attachment nut as a lock-nut.

Another problem (that Jon of Laser may be able to answer), is why do so many engines come with threaded crankshafts that look barely long enough for the job?

Anyway good luck with it.

[Nigel R]

Can't see why it wouldn't be ok - if it is a proper steel nut.

Other more knowledgeable heads may say different.

Or, use a main nut that is thinner then the existing one, and then the long nut has more thread to wind on to.

[john stones 1 - Moderator]

I have engines with only the spinner adaptor nut on, no issues.

[Martin Harris - Moderator]

Posted by John Stainforth on 12/01/2021 14:03:33:

From my experience of having had endless prop nuts come undone, I would say you will be lucky if this is adequate. Getting washer, prop nut and spinner combinations that are compatible and work is a pain in the neck. I would try to put on a thinner washer (without the taper), then a thinner nut and then the spinner attachment nut as a lock-nut.

Another problem (that Jon of Laser may be able to answer), is why do so many engines come with threaded crankshafts that look barely long enough for the job?

Anyway good luck with it.

In your case John, maybe high nitro content fuel in Saitos may have something to do with it?

But generally, prop nuts come undone when people don't tighten them enough. Far too often, I see people using short adjustable spanners and 10 minutes later repeating the exercise. Propeller shafts and nuts on reputable engines are more than adequate to resist a bit of eye bulging tightening with a correctly fitting standard length spanner. My only concern would be with the material quality of the spinner nut.

[Mike T]

Try one of these: spinner adaptor/extender

You'll need a new washer to suit and you'll need to ream the prop out, but you will be using much more of the crank thread to secure everything.