Repairing Crankshaft Thread for Cush Drive Nut

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paul.wirdnam
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Repairing Crankshaft Thread for Cush Drive Nut

Postby paul.wirdnam » Thu Dec 13, 2018 11:51 am

Looking for advice on what to do with the poor thread on the end of my 4G crankshaft. It should be 11/16" 20TPI but as you can see, it's not in a good shape.

Thankfully, the centre hole on the end is still true, so I can hold the crankshaft in my lathe between chuck and live centre. I'm thinking that maybe I should put the lathe in screw cutting mode and restore the original 20tpi thread but take it down to 5/8". The thread is not too hard, so I think my HSS tool bit will be OK. I do have a 5/8" 20TPI die but its only a carbon one and I not confident that I could get it on straight with that flat in place.

Not the first time I've seen a damaged thread here, so what's the answer?

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Re: Repairing Crankshaft Thread for Cush Drive Nut

Postby adrie.degraaff » Thu Dec 13, 2018 12:27 pm

I would weld it back to size and make a new thread.

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Re: Repairing Crankshaft Thread for Cush Drive Nut

Postby cmfalco » Thu Dec 13, 2018 4:45 pm

adrie.degraaff wrote:I would weld it back to size and make a new thread.
Personally, I would avoid doing that because the heat will distort the end of the crankshaft and cause it not to run true. While rebuilding material with welding is OK on some components and in some circumstances, it's best not to have the engine sprocket wobbling.

It's not so much that metal has been lost from those threads, it's that the crests have been pulled down into the roots. Before considering drastic solutions, like reducing the diameter to 5/8", I would make a "thread rolling" tool similar in design to a lathe's knurling attachment but with a narrow 55° wheel instead of wide knurlers. Set the lathe at 20 tpi, locate the wheel in one of the good grooves, and manually work it back and forth to push the metal back where it belongs.

Thread rolling with multiples wheels is a common way of making threads in the first place. For your crankshaft a simple, easily made tool, would do the job.

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Re: Repairing Crankshaft Thread for Cush Drive Nut

Postby paul.jameson » Fri Dec 14, 2018 6:58 pm

Cmfalco's advice sounds excellent. Having followed it, use the later type cush drive with the castleated nut to hold it on and split pin rather than the two nuts used originally with the early cush drive.
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Re: Repairing Crankshaft Thread for Cush Drive Nut

Postby paul.wirdnam » Sat Dec 15, 2018 10:01 am

Agree...great advice from Charles, so I'm giving the "rolling" method a go. I've got as far as making up the wheel from the head of a large high tensile bolt and have hardened it. Next step is to make a holder for it:

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Re: Repairing Crankshaft Thread for Cush Drive Nut

Postby paul.wirdnam » Mon Dec 17, 2018 1:59 pm

Great advice from Charles (Cmfalco) and Paul....and my hardening of the wheel seem to stand up OK.

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Re: Repairing Crankshaft Thread for Cush Drive Nut

Postby alan.moore » Mon Dec 17, 2018 3:03 pm

Nice job Paul
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Re: Repairing Crankshaft Thread for Cush Drive Nut

Postby cmfalco » Mon Dec 17, 2018 7:08 pm

paul.jameson wrote: use the later type cush drive with the castleated nut to hold it on and split pin rather than the two nuts used originally with the early cush drive.
Despite having been torqued properly on my '28 Model C, those two nuts came loose and cost me some miles one of the day of the Cannonball. When I have the engine out of the frame an upgrade to the later type is on the to-do list.

I've been waiting for two months for a backordered exhaust valve to arrive, at which point I'll install a hardened seat and a new guide that I'll make from Ni-Resist cast iron. But, I digress...

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Re: Repairing Crankshaft Thread for Cush Drive Nut

Postby andyhughesflattankarielpre25 » Tue Dec 18, 2018 9:00 am

Hi chaps,Sykes Pickavant used to make a roller thread repair tool,back in the early 70ies when I was an apprentice,they were fantastic things.A bit late now Paul,sorry.

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Re: Repairing Crankshaft Thread for Cush Drive Nut

Postby paul.jameson » Tue Dec 18, 2018 9:56 am

paul.jameson wrote:
use the later type cush drive with the castleated nut to hold it on and split pin rather than the two nuts used originally with the early cush drive.
Despite having been torqued properly on my '28 Model C, those two nuts came loose and cost me some miles one of the day of the Cannonball. When I have the engine out of the frame an upgrade to the later type is on the to-do list.

I've been waiting for two months for a backordered exhaust valve to arrive, at which point I'll install a hardened seat and a new guide that I'll make from Ni-Resist cast iron. But, I digress...


The later type cush drive with castleated nut and split pin is a Square Four fitting. The singles used the two nut system with tab washer between right through to 1959.
Paul Jameson
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.


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