Gallery, Projects and General > How to's

Norton Crank Repair, (or how to stop a grown man crying)

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Ned Ludd:
Hi Guys,
After being a member of this forum for some time, I thought it was time I put something up myself, instead of commenting on others efforts.

Have you guys noticed that once it is known you have a workshop and some idea of how to use it, that people find "little jobs" for you to do for them. I don't mind when they do this as it often gives me chance to do things a little out of the ordinary. To cut a long story short, a mate of mine rang up to ask if I would look at a crank from a Manx Norton that belonged to a mate of his. Although my mate has a workshop of his own he did not feel able to fix the problem, hence the call. As with all such calls I said I would have a look but would not commit myself till I knew what the problem might be. So the story moves to the Norton owners house a few days later, and all was revealed. The problem was the end of the crankshaft had been beaten severely about its end with a club hammer. For those interested in why, well, the owner had fitted new main bearings and to over come slight wear on the shaft he had used Loctite, as recommended by the bearing suppliers. Only trouble they had recommended the wrong grade and supplied a permanent one, not a bearing fit one, now this would not have been too much of a problem had he put the cases back together quickly enough but he didn't. The outcome was that the cases would not close fully because the bearings were not where they should be and the Loctite had set. Said owner was almost in tears when he was explaining all this, how he had tried to get the crank out with a hammer and how the previous owner of the bike would be spinning in his grave at the way he was treating his old treasured classic.

Photo ! shows what a Manx crankshaft looks like.

Photo 2 shows what the damage looks like (not good). After sympathizing for a while, I could see a pathetic pleading look in face, so I said I would fix it for him and with luck he would not be able to see the damage.

Photo 3 shows the crank set up in the lathe. The timing side was held in a collet chuck and the drive side was first set in a centre, and the crank was checked to see if it was still true, which unbelievably it was despite the damage. I then set the fixed steady up to hold the crank while the repair was done. You can't see it in the photo but the con-rod is attached to the ceiling by two bungee cords, so it would not flap about while the lathe was turning.

That's all for now as the pixel limit is running out, and as "they" say always leave them wanting more.
Ned

John Stevenson:
Ned, I have a set of two piece split dies for doing just this, you open the dies out fit over the damaged thread and tighten down and wind off, they are reverse cutting, do this a few times tightening down a bit at a time and you reclaim the thread 9 times out of 10.

John S.

Ned Ludd:
Hi John,
Back in the old days I had a set of thread repair rollers that would also have worked but that was then.

To continue;
Photo 4 shows the end of the crank after turning to size and the signs of hammer damage cleaned away. I know that some would view the turning away of the belling to be "not best practice" but the end justifies the means. I had thought to swage the belling back where it should be but the thought of causing more harm than good might be done stopped me trying.

Photo 5 shows a threading tool aligned with the good part of the thread. The thread is 20tpi but my lathe is metric the nearest pitch to 20 is 1.25mm (actually the pitch is 1.27mm) and as I was only planing to cut a few threads I did not feel too concerned and I had a "cunning plan" to correct any errors.
The way to set up the thread is to start the machine, but with the tool clear of the work, and run the tool till you get to the good part then stop the lathe. I was not using the set over top slide method so all I had to do was to move the top slide till the tool could be put into the thread. From now on the half nuts would have to stay engaged, but his is no problem as my lathe reverses with ease. I am sure most of you know how to thread cut so all I will say is I cut the thread in the usual way but stopped before going to full depth.

Photo 6 shows the thread cut, doesn't look too bad but the nut didn't quite fit smoothly, so bring the cunning plan into action.
More to follow in the final thrilling instalment.
Ned

Bernd:
Nedd,

Do you have any way of making your pics smaller, like 600 X 800. That's one of the reasons you ran out of space.

Bernd

Ned Ludd:
Photo 7 I did not cut the thread to full depth because I had it in mind to use a thread chaser to take off  the final shavings to give the final size and the correct thread profile. A few passes with the chaser and the result is Photo 8, the caption says it all. The English expression "sweet as a nut" seems apt.

So back it went to the owner who was pathetically grateful, and when he asked how much I said I did it for the fun of salvaging a piece of motorcycling heritage. This he did not accept, so I had to think of a price, which I did and he paid me three times the figure I suggested. I guess he was still feeling guilty of causing the damage! I got a phone call a few weeks later to say that the bike was back together and running better than ever, and to say thanks again. Sometimes a "thank you" is reward enough, but it is still nice to get petrol tokens, too.
I hope I have not bored you with this little tale of woe.
Ned

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