Alternator rotor came off following removal and refitting to change seal.

[Simon_London]

Any advice appreciated.

About 10 miles after changing the front crankshaft seal the charging light came on. I pulled over to hear an alarming sound coming from behind the alternator cover.

The rotor had come off. Luckily the crankshaft seems ok

I don't really understand what I did wrong - how tight can you make an 8mm bolt..........? The split washer was fitted.

Any advice appreciated as I don't want a repeat. I can't find a suggested torque value for the rotor bolt. I don't really want to use locktite. Is gentle use of a hammer necessary to seat the rotor prior to tightening the bolt?

[Simon_London]

Ahh - Guzziology quote 25 foot pounds torque for the alternator bolt.............

[rodekyll]

Do not use Loctite!

Getting the torque right should solve the coming loose problem.  Did the rotor contact the stator?

[Simon_London]

Yes Rodekyll I think it did. A wire to one slip ring is severed. I thing there is some scoring to the inside of the stator but my multimeter suggests the windings are intact.

The thread in the crankshaft needed some cleaning up and there seems to have been some slight rubbing / fine scratches scratches to the taper of the crank shaft. I don't think it will be a problem.

Think I just need new brushes, rotor and rotor bolt.

[brider]

Do not use Loctite!

WHy not? Not even if you use the non-permanent type?

[guzzisteve]

Not if you want to take it off. It will get on the end of the shaft. Usually they are bad if you want to get them off OR they go bad trying to get them off. I use a stainless tool and rubber mallet and bingo.

[wirespokes]

What bike is this? I assume this is a Bosch alternator?

What I've heard is that tapers are supposed to be meticulously clean on assembly. I've heard of guys cleaning really well, and then cleaning some more with alcohol to remove any trace of oils. Tapers are used a lot in Machining work.

I haven't been that meticulous, but probably will be from now on. Once seated well, the retaining bolt doesn't do much to keep it on - I've heard of tapers losing the retaining bolt and still functioning as if nothing changed. When a taper seats, a bit of force is required to separate.

Why not solder up the slip ring lead? I'm betting the rotor can be repaired.

[Simon_London]

Wirespokes - yes Bosch.

That's interesting about how tapers work. I didn't remove all traces oil from the taper surfaces

[chuck peterson]

Be very careful if you decide to clean that crank nose thread with a tap

I had a horrifying experience with a broken tap, when I realized just what that means, like remove and replace the crank

Luckily I discovered what tap extractors are for, in the McMaster Carr catalog

I am a lucky dog...

[Rough Edge racing]

 If you use blue Loctite the bolt can be loosened .You can always heat the bolt head to about 400 degrees if necessary.... I had the rotor come loose from my 84 Cali...It was a permanent magnet brushless conversion done by a previous owner...I had to clean the crank threads with a tap....it was tough going...I used Loctite and it's been fine for years...

[Groover]

The old Vespas use a combination of taper and woodruff key to hold their flywheel/rotor-magnet on the crankshaft. If the flywheel would become loose and break the woodruff key and if you're lucky, you change the woodruff key tighten things back up and you're back in business. If a lot of scoring would happen to the tapers on both the flywheel and crankshaft (mostly caused by the woodruff key shredding), it would go all down hill from there,meaning the likely-hood of the flywheel coming loose and constantly shredding woodruff keys would increase with scored tapers. The ultimate solution was to replace crankshaft and flywheel to ensure a good taper lock, but that is not always feasible, so you'd start with cleaning the taper and getting them as smooth as possible as suggested above. If that didn't work, most of the time replacing the flywheel would solve the problem. Since Guzzi rotors don't use a woodruff key, being the piece that would do most of the damage in a slipping case, a good polishing with 600 grit emery cloth should do it. If not, then replacing the rotor should.

Also, don't over-tighten that rotor bolt. They do break (don't ask how I know). I see right there and I agree 100% that you should not use Loctite of any sort there, cause that would have been a sure engine rebuilt in my case, but instead I was lucky to use a needle tool to rather easily back out the broken bolt piece.

The original black bolts are harder and better than the later gold ones found at places. Take care of that bolt since they are near impossible to find.

Good luck

[rodekyll]

WHy not? Not even if you use the non-permanent type?

In a perfect world you don't need Loctite, and in an imperfect world the Loctite gets in the way of removing the bolt should it become broken or damaged.  If you examine the bolt you can see a number of ways that could happen.  You can also see why it's not a good idea to turn down a standard bolt as a replacement.

One cause of taper contacts failing is crank whip.  I had a (garage sale'd) drill press chuck spin off of its taper shaft the other day when I failed to center an end mill.  The whip and wobble got it in the amount of time it took me to spin it up, notice that I'd foul-chucked it, and shut it down.  Guzzi has a lot of crank forward of the front main bearing.  Unless the rotor and crank nose are very well balanced, true, and centered, there will be whip.  I'll speculate that whip is why Guzzi uses both a belt (taper) and suspenders (bolt).

[John A]

if there is crud in the rotor, I mean down in the hole, old locktight etc it may prevent the rotor from seating then the crank will whip that sucker into the weeds after the bolt loosens. I don't know how much clearance is there, just trying to come up with reasons it would come off. I like Rodekylls explanation

[Groover]

I think the whip effect makes complete sense. Based on the flywheel experience I mentioned above, the problem would most occur in our racing engines, which all of them had cut crankshafts in order to change the intake timing & duration (crankcase/crankshaft intake port), which would make the crankshafts out of balance just doing that, increasing vibrations. The woodruff key was to there to place the flywheel cone in the right spot as it had the lobe for the points.

[wirespokes]

The BMW airheads have the exact same Bosch alternator and the same bolt retaining the rotor. If you have problems finding an original bolt, you might try BMW. I haven't gotten a new one lately, so can't say whether they're the same black bolts as the old days, or gold as has just been mentioned. As a point of reference, when joining two parts with a taper connection (like machinists), it's common practice to give the part being attached a good thwack with a plastic or lead mallet to seat it really well. I've never assembled a rotor that way, just bolted it on normally. The output transmission flange (airheads) is torqued to 160 pounds, and I've heard of bikes going many thousands of miles before it was discovered the retaining nut had been missing the whole time. When tapers seat, they can be extremely difficult to break loose. I've learned the hard way not to use anything but a hardened bolt to remove the rotor - and when the rotor breaks free, be ready to catch it because it can jump away and hit the floor, probably damaging it.

[Chuck in Indiana]

Uhh, this is pretty  basic stuff. A "holding taper" will do just that. If clean. Do a good job of assembling, and the bolt is almost superfulous. You have to get a drill chuck out of a drill press or lathe with a drift behind it. When you use "the tool" to remove a rotor, you are doing the same thing.
If a rotor comes off.. sufficient care wasn't exercised in assembly. <shrug>

[Simon_London]

Everybody, thanks for your input. Really helpful.

I am pretty sure the reason the rotor came off was the bolt wasn't adequately torqued down, but I also carefully degreased the taper before reassembly this time.

New rotor, brushes, seal and bolt and bike is back on the road.

[wirespokes]

You should be fine now. In fact, I think this is the first time I've ever heard of a rotor coming off - though it probably has happened before. I just never heard of it happening, and I've been reading BMW forums since 1991 - the airheads have the exact same alternator.