stripped crankshaft thread

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Aug 24, 2009
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Gidday. The thread for the rotor on the end of my MK3 Crankshaft has finally let go and stripped. Does any one know of a repair that doesn't mean stripping the engine? We can't figure out the thread type either. I'm hoping to maybe cut a new thread and machine a new nut to suit, but its hardened steel as well.
Any clues anyone, cheers Hillbone.
For a short term fix you can cut to the next thread down (which may be metric), problem is how you first get the right smaller OD before cutting with a thread die and have this concentric. Longer term when the engine is next stripped have the crank bulit up with weld or metal spray and re-cut the orginal thread.
Gday Hillbone, if your not sure as to what to do, I suggest you should contact Geoff Collins at "Offsetcrank, Toronta , Canada. Geoff remanufactures cranks and would most definately be willing to help out even with the information on metal spraying as Kommando states. Geoff heat treats (Nitrides) all his cranks so could even do the job if freight wasnt to expensive?
The pre-Mark III 850 rotor-end thread is a standard American size or close enough for government work. Chasing the threads with a high-quality die might get you through the season. You could also potentially Heli-Coil the rotor nut internal threads to a size that really jams up on the crank threads that are left.

Another approach would be drilling and tapping the crank end a half-inch deep or so for an internal holding bolt like the camshaft setup--plenty of metal and not much load out there on the end. I'd use a new rotor key, screw on the rotor nut to center things with high-test Loctite or possibly solder or spot-weld it--and cinch things up with the new internal bolt and a suitable washer.

You do not want that rotor nut to come adrift. Mine backed off, parted the chaincase to evacuate the primary and crushed itself on the chainwheel about ten years ago. Because I didn't carry a spare at the time I pushed the bike 10.7 miles home against needless resistance, having forgotten yet again to back off the rear brake cable to compensate for unladen conditions. That was the only time the Combat got over 70 mpg.

Tim Kraakevik
'72 Combat and two Norton E-Bay choppers
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