This piece is a sister article to Your Bike’s Charging System – Regulator/Rectifiers where I dig into the electronics around the alternator, how they have evolved over the years, how they work and some of the offerings available.
This article covers issues with alternators – many I have seen manifest in recent years – often following a refresh, or an ‘upgrade’
In the main part, I see that alternator stators are getting damaged in two different ways – physical damage & electrical damage.
The physical damage I see is usually caused by the alternator stator and rotor fouling.
This can be caused by either poor alignment, or poor clearance.
When you fit a stator, it is important to make sure that it is centred about the rotor – achieving this may involve slightly enlarging the holes in the stator, or slightly waisting the mounting studs in order to give you the wiggle room that you require.
Hitting it (or the studs) with a hammer is not (in my opinion) a sound engineering fix – the mounts are often brittle, the threaded holes into the primary case or crankcase are often small, and are prone to pulling out, so try and treat it with a bit of respect.
Using a shim between rotor and stator while you are snugging everything up tight is a great idea – a plastic drinks bottle or a cut-up beer can both work well.
Lucas call for a minimum air gap of 0.008″ to 0.010″ (0.2032 to 0.254mm) evenly all around between rotor and stator. Make sure that you have tightened up the rotor using the sleeved crankshaft nut while you are centering the stator about the rotor, as this centres the rotor on the shaft.
The other thing to be aware of is the outrigger plate on the MK3 Commando – I see a pinch point at between the two o’clock and three o’clock position on every MK3 I’ve looked at. Gentle relief of this outrigger with a Dremel at the points arrowed in the below picture will help you centring the stator about the mounted rotor.
I am seeing that the new rotors and stators have less clearance than the originals.
This is to the extent that it makes me very uncomfortable about fitting them to a bike.
The original design of the Lucas Rotor and Stator was for an outside diameter of 74mm on the rotor, and an inside diameter of 75mm for the stator (those numbers were given to me by LAP)
This would give you 0.5mm air gap all around – so comfortably exceeding the minimum that Lucas originally specified back in the day.
I have been closer to 5 thou (0.127mm) on brand new Wassell/Lucas kit fresh out of the box – way tighter than I am comfortable with.
I have mentioned crank flex in the groups and forums before, and I have no proof that it is a thing on a Commando engine, but personally, I am not comfortable with such a tight tolerance, and feel that the stator and rotor could foul on the overrun if you snap shut the throttle from highish rpms.
We have been carefully measuring stator and rotor clearances on the new ones that I have been buying for the last few projects, and taking a light skim of the rotor on our Myford ML7 lathe.
You can see a clip of me doing this here – it is certainly not as scary as it sounds, and the rotor cuts beautifully. The trick is slow autofeed and lots of compressed air to clear the magnetic swarf, and you get a really decent finish.
I have seen no noticeable loss in power by doing this, plus it gives huge peace of mind.
As you can see from the picture of the ‘dissected’ rotor here, I am not actually machining the magnets here – it is purely the casting and the laminated keepers that are getting a light skim.
Here follows some pictures of rotors and stators that are exhibiting damage typical to physical contact and fouling:
Example 1 – Physical Damage, where the rotor has been rubbing against the stator
Pictures from B50.org
Example 2 – Physical Damage, where the rotor has been rubbing against the stator
Pictures from BritBike Forum
Further Examples – Physical Damage, where the rotor has been rubbing against the stator one with an explosive ending!
Pictures from Triumph Rat
Electrical damage is much less spectacular to look at, and usually does not result in smelly molten plastic and in the extreme case a locked up primary.
Typically, you’ll see that the resin cracks out with the heating and cooling of the stator windings.
The other symptom you’ll notice is hot spots (which people call shit stains) – in my experience these usually seem to be around the wires that link the windings together rather than the windings themselves.
Electrically, the stator can fail in two different modes:
Fail to ground – where one or more of the windings comes in to contact with the laminated metal core of the stator (and hence grounds at to the bike). This can spell curtains for the reg/rec straight away (particularly on the Boyer Bransden Power Box in my experience)
Fail open – where the windings become disconnected (either inside, between the wound bobbins) or outside at the flyleads. This means your battery won’t charge any more.
It is important to strain relieve the flyleads – they are prone to high temperatures and a lot of vibration and are delicate. It is not uncommon to see fraying around the point where the wires exit the resin encapsulant. The fraying can result in a short between wires, or ultimately the wires breaking away totally. You can of course dig out some of the resin and make a repair, but most people don’t bother, choosing instead to replace the stator.
I am absolutely of the opinion that a lot of this electric stator damage I am seeing is down to short-type regulator/rectifiers. A factory standard charging system is on a bike and is problem free for over forty years, then just after an owner upgrades to a new reg/rec, I shortly after get to hear about electrical problems and charging related issues.
This is a pattern I am seeing time and time again.
Here follows some pictures of stators that are exhibiting damage typical to electrical shorting:
Example 1 – Electrical Damage – burnt out stator windings or link wires between coil bobbins
Pictures from Triumph Rat
Example 2 – Electrical Damage – burnt out stator windings or link wires between coil bobbins
Pictures from Access Norton
Example 3 – Electrical Damage – burnt out stator windings or link wires between coil bobbins
Pictures kindly provided by Nigel Waring (Fast Eddie) on Access Norton
Example 4 – Electrical Damage – burnt out stator windings or link wires between coil bobbins
Pictures from Triumph Rat
This piece is part of a series that covers the charging system in general.
Hopefully these will fill in some of the gaps and questions you may have, but as always don’t hesitate to reach out if you need any help or advice.