Crank rotor keyway worn

I'm about to reassemble my primary but the keyway/slot on my crankshaft seems to be worn and the rotor is able to rock back and forth by a couple of degrees each way. I see from other posts that Lucas rotors are "sloppy", but my woodruff key fits the rotor pretty good, the sloppiness is in the keyway on the crank. Considering how tight this gets done up, is this a problem? And if it is can I do anything about it without pulling the whole engine apart?

Headfullofsnakes said:

I'm about to reassemble my primary but the keyway/slot on my crankshaft seems to be worn and the rotor is able to rock back and forth by a couple of degrees each way. I see from other posts that Lucas rotors are "sloppy", but my woodruff key fits the rotor pretty good, the sloppiness is in the keyway on the crank. Considering how tight this gets done up, is this a problem? And if it is can I do anything about it without pulling the whole engine apart?

Click to expand...

I sheared a woodruff key on the crank of a 750 fastback....but the one on the taper for the engine sprocket, which takes more torque!...this was the result of the rotor nut coming loose...and yes the movement had caused some damage, but a new key worked fine in the old keyway, though I did check the tightness of the nut a bit more after that....and of course on the engine sprocket the taper is also working for you.

I would say that in the circumstances, fit a new key in the alternator rotor slot and tighten to the specified torque, ensuring you use a new serrated washer.

Messing with the key and keyway any way, and the slop you already have, raises the need for at least a thorough check of the timing mark or a new timing mark.....as on the other thread, check with a degree wheel (and make a new mark).......

When you are happy with the timing run it, but check rotor tightness every ride at first, then at longer intervals assuming it does not come loose. If it does come loose, and surely you won't go on that long cross country trip until you are sure its OK, now will you?.......

First option if it does come loose is to make a woodruff key with a step so that it fits tight in both parts.......which would most likely fork fine after redoing the timing mark.....if not the work to repair the crank after trying both of these options will be pretty much the same as it would have been anyway, cut a new keyway, which yes, will involve an engine strip for the machining....and a recheck of the timing mark.....

Thanks for educating me some more Steve, I've not had loose grooves but have had 2 rotor nuts come off on ride, to tell ya they only do, come off, at low rpm, but can then jam under sprockets to bust primary cover and jam engine to a stop, at low rpm. If not discovered at low rpm then will make nasty noises but likely chew up the nut and nothing else. Point of my message being its the loss of nut clamp force that is the concern here rather more that mis-timed anything. The clamp on tapper should hold torque rather better than the key, which is mainly just alignment function. I have a coulpe-3 spare serrated washers to lend as stopped using them as seems to lose clamp force easier than plan or no washer. Make sure you have tools to remove the primary cover on the road side to clear a lose nut and limp home w/o the noise. Hi temp loctite might help but remember to add hi heat to remove nut again.

hobot said:

Thanks for educating me some more Steve, I've not had loose grooves but have had 2 rotor nuts come off on ride, to tell ya they only do, come off, at low rpm, but can then jam under sprockets to bust primary cover and jam engine to a stop, at low rpm. If not discovered at low rpm then will make nasty noises but likely chew up the nut and nothing else. Point of my message being its the loss of nut clamp force that is the concern here rather more that mis-timed anything. The clamp on tapper should hold torque rather better than the key, which is mainly just alignment function. I have a coulpe-3 spare serrated washers to lend as stopped using them as seems to lose clamp force easier than plan or no washer. Make sure you have tools to remove the primary cover on the road side to clear a lose nut and limp home w/o the noise. Hi temp loctite might help but remember to add hi heat to remove nut again.

Click to expand...

When the engine sprocket woorduff sheered I lost the drive immediately, which was confusing until the primary cover came off, engine ran freely but no drive....but the sequence of events was most likely...nut came loose...alternator rotor came loose.....sprocket came loose on taper...torque sheared woodruff key...

It seems in your experiences the taper held, see below, you were probably applying less shock :roll:

The alternator nut was where it should have been, but loose...I found those stupid serrated washers work....but I reckon just the once...

But of course others have removed the nut and still cannot force the taper to let go....

Shall we say that I was running 7000 in 4th, braked and changed to second for a right hander, then 7000 on the way up the box in 3rd....7000 into 4th....no drive....still leaning pretty hard right...it didn't want to turn anymore.....1st season of racing...steep learning curve....both mechanically and riding experience

Shock is definately the way to loosen an engine sprocket on the taper....the woodruff key is the likely collatoral damage though :shock:

On my next bike we cut the alternator rotor section off of the crank and just left the taper section with a tapped hole in it and used an allen bolt and large purpose made washer, with just and ordinary spring washer, no loctite.

IMHO, A couple of degrees is a lot, this amount of play in an item fitted on a cylindrical shaft will overcome any amount of fastener tightness as well as the effectiveness of any lockwashers - radial play must be zero for things to stay tight.
You have two choices:
1. Take the crank to a machine shop and have the keyway properly repaired- no more loosening nuts; or
2. Using a new key, fill the gaps between the key and the keyway with JB Weld, slip the rotor onto the crank until the JB Weld cures, remove the rotor and scrape off any surplus JB Weld, apply some Loctite sleeve and bearing retainer to the shaft, refit the rotor with a new lockwasher and torque the nut - should hold for quite a while but you will have to heat the shaft when you next remove the rotor.
Good Luck

Shall we say that I was running 7000 in 4th, braked and changed to second for a right hander, then 7000 on the way up the box in 3rd....7000 into 4th....no drive....still leaning pretty hard right...it didn't want to turn anymore.....1st season of racing...steep learning curve....both mechanically and riding experience

Click to expand...

OH SHIT! I don't race but know that loss of drive or drag in a turn causes bike to rebel to turn in horror. Glad you lived to tell, me too of course. Not all survive those initial surprise steep learning curves, wheweepeepee.

A proper taper fit, if that exists in Norton variety is enough to twist the crank into a candy cane. Loss of clamp force plus shocks is what one does to take a good proper taper fit apart, duh, plus some heat.

I found a fastener testing site and vibration loading machine that showed too plainly any washer of any type allows nut to back off, split washers the worst but other spring washers and textured washers only delayed the same loosening a bit longer. Flat hardened washer was better to keep torque applied but sans washer was best, if bearing on a hard enough seat of course.

Racers have it so easy to just work around the factory faults us ordinary factory guys must strain to retain.

Here's the damage I found from an incorrectly repaired keyway when I opened up my Bonnie

The repair consisted of center boring the crank and then welding on a new solid threaded end.
I'm seen two different ways to avoid this, (both on Triumphs)
One is to fabricate a lock washer with a section that sits in the keyway that can then be folded over the nut, in addition to this the nut is then lockwired to a small screw that has been screwed into a shallow hole in the rotor. This comes from the book "Triumph Tuning", I can post pics when I get home this weekend.
The other is to drill two shallow holes in the back of the rotor to engage with two studs that can be screwed into the puller holes in the primary gear, this way there is no load on the keyway, the nut is only used to stop the rotor from falling off.

All the best

Webby