Crankshaft Alternator Keyway Repair

Proper repair: weld and recut keyway. Requires engineering workshop.

DIY fix: clean up keyway, fashion a key to fit as best you can by filing down an engine sprocket key.

Key doesn't drive the alternator rotor, just locates it for the timing marks which must be reset whichever route you go. Ensure rotor is bolted up tight to recommended torque and correct spacers are used so that nut is not bottoming on thread.

Just get a new key way cut opposite the old one.  Most small engineering companies can do this kind of job. This saves messing about with brazing or welding then re-cutting. The new slot will not weaken the shaft. A key can be fashioned to fit the new slot.

end threads could do with cleaning up, but probably not essential.

Why not make a key that fits snugly in that slot?  It's not heavily stressed but needs to be snug so the nut doesn't vibrate loose.

A woodruff key will/should be normal mild steel.

As Stan says, it is there purely to locate the rotor at the 'correct' position for the timing marks. 

Drive keys are (should be!) a different shape and made from a much tougher steel.

Again, as Stan said, make sure that the nut cannot bottom out on the thread when you tighten it. 

George 

What's going on with the lip on the shaft, where the taper meets the parallel  shaft?  The parallel shaft where the alternator rotor sits looks badly worn.  I would suggest that the keyway is the least of the problems with that crank. 

John

The taper portion doesn't appear to show any sign of the sprocket spinning on it.  Since taper and parallel portion must have been machined in different operations, the lip looks odd but not serious.  But it might make fitting the oil seal a bit of a problem.  Does anyone have pictures of that end of others?  Better be sure the sprocket sits on the taper and isn't drawn up to the shoulder.

Better do some measuring with a good micrometer.

It seemed for your photo that the lip was just dirt. If it is raised metal you've got a problem to sort out.

Looks like you will need to replace the crankshaft.   My instinct would be to go for a later shaft in the hope it might be a better design.  The Commando experts here would be able to answer that one.

Took another look at the photo there are clear signs that the sprocket has been moving on the taper, fretting.  What's the condition of the sprocket keyway?

Have you tried filling off the step first?  It's quick and zero cost.  Especially compared with a new crank.

Before you look around for a different crankshaft assembly try the firm in the clubs' "Services" section to see what they can offer and to let you know what your long-term options are.

If you go the eBay way, ask the vendors for good quality close ups of all of the journals and fixing points. If they can't or won't give them a miss, saved me a lot of hassle in the past.

There are quite a few companies around who can make good a worn crankshaft by Hot Metal Spraying.  This does as it says on the tin by coating a worn surface with layers of molten metal. These layers can then be trimmed back to recover original journal sizes or shaft tapers. 

It was at one time a cost effective repair for crankshafts compared to purchasing a used replacement. 

to Matthew H, th crank end may not be a total loss, a classic racer, running no alt. ,may be able to machine it for that use. .

barry. .