Rotor alignment a warning from the basement

Posted by benedict_spencer at May 02. 2010

I'm rebuilding my 1972 750 Commando in the basement, don't have a shed or garage.

There's been a lot of discussion on getting 8 thou clearance between the rotor and stator.

But I didn't see any mention of checking clearance more than once. By which I mean check clearance, turn the engine a little check again and so on to see if they is a tight spot anywhere. I did this and found a tight spot that moved with the rotor. My conclusion was either the rotor was ovoid or the cranskshaft was bent. Please do not let it be the crankshaft.

So I thought the rotor should go onthe lathe and be turned with a dial guage touching to see how distorted the rotor was. I don't have a lathe but I know a man who does.

However as I undid the rotor-crankshaft nut I thought I saw the rotor move relative to the steel centre.

The rotor comprises three parts, the alloy housing, the magnets and the centre steel cylinder.

I put the rotor in the vice, I used a couple of large nuts as "padding" on either side of the steel centre.

Sure enough the rotor was loose and could move relative to the centre steel cylinder. So that old rotor can be a paperweight and I'll buy a new one.

So my advice is this, put the rotor in the vice (with padding) and check it is not loose, then if that's ok make sure you check clearance all round, turn the engine a little and check again etc.

Re: Rotor alignment a warning from the basement

Posted by paul_senior at June 14. 2010

Hi Benedict,

Thanks for the input on commando rotors, it really tickles me that there is so much first hand knowledge out there, to be shared about such old bikes.

Mintage !

Regards,

Paul.

Re: Rotor alignment a warning from the basement

Posted by jack_blakemore at June 19. 2010

Stupid question here I suppose, but how do you know if your rotor is shot?

I have just been dismantling mine to relpace the sprag clutch and crank gear, and found that the stator is completely u/s.

I cleaned the rotor up and it seems to be ok, but does anyone know if there are there any specifications or measurements that can be made to verify that it is OK?

I have seen some advice regarding an upgrade to a 3 phase alternator on another thread that I will take, you never know if I am lucky, I could even get the electric start to work and the battery to charge!

By the way, when buying a MkIII, if you get the answer"Naahhhh mate! They all do that!!"; to the question "Should that red alternator warning light really be on at 2,500 revs?"; be aware that someone might be telling you an itsy bitsy fib...the b*****d!

Re: Rotor alignment a warning from the basement

Posted by tony_ripley at June 20. 2010

Jack

There is a special tool for measuring the magnetic force generated by a good stator. It consists of a piece of steel (NOT Stainless) about 3/8" diameter, it will usually have a wooden or plastic handle to allow comfortable use. I think the technical term is a screwdriver.

You hold the screwdriver horizontally and the see if the rotor can support it's own weight anywhere around the circumference of the stater with the exception of the joins between poles.

If it can then you have a good one.

Regards

Tony

Re: Rotor alignment a warning from the basement

Posted by jack_blakemore at June 21. 2010

Outstanding technical description - now, do you know where I may find a "screw-dri-ver"?..

I half expected someone would chime in with a comment about testing field strength using a fluxmeter or something, in which case I'd just have gone down to the pub.

The rotor has developed a strange attraction for the vice actually - so it could be OK. I'll test it after work. Thanks.

Re: Rotor alignment a warning from the basement

Posted by perry_harrison at June 23. 2010

Previously wrote:

I'm rebuilding my 1972 750 Commando in the basement, don't have a shed or garage.

There's been a lot of discussion on getting 8 thou clearance between the rotor and stator.

But I didn't see any mention of checking clearance more than once. By which I mean check clearance, turn the engine a little check again and so on to see if they is a tight spot anywhere. I did this and found a tight spot that moved with the rotor. My conclusion was either the rotor was ovoid or the cranskshaft was bent. Please do not let it be the crankshaft.

So I thought the rotor should go onthe lathe and be turned with a dial guage touching to see how distorted the rotor was. I don't have a lathe but I know a man who does.

However as I undid the rotor-crankshaft nut I thought I saw the rotor move relative to the steel centre.

The rotor comprises three parts, the alloy housing, the magnets and the centre steel cylinder.

I put the rotor in the vice, I used a couple of large nuts as "padding" on either side of the steel centre.

Sure enough the rotor was loose and could move relative to the centre steel cylinder. So that old rotor can be a paperweight and I'll buy a new one.

So my advice is this, put the rotor in the vice (with padding) and check it is not loose, then if that's ok make sure you check clearance all round, turn the engine a little and check again etc.

Thanks Benedict - I have exactly the same problem on my 74 Commando because the previous owner didn't align the primary chain. For the sake of 50p.'s worth of shim behind the clutch housing, the chain destroyed the inner chaincasing mounting as well as my alternator. Ho hummmm !!

Perry