My M21 project continues and I've started looking at the electrical/charging system. Electronics is is a black art to me and is something I've always struggled with so the thing I try to avoid at all costs is any form of roadside repair - reliability is everything!

If I understand it right, my D7 Bantam outputs an AC current, so I fitted a solid state rectifier to produce a DC current suitable for lighting and charging. No regulator is fitted so when I'm hurtling down the road at 40mph with the bike trying to shake itself to bits, the current goes over 6v and it blows bulbs - I want to try and avoid that with my M21!

I've sent the magdyno off to Priory Magnetos for a rebuild, meanwhile I'm trying to figure everything else out.

Now, if I've got this right, the magneto provides the spark and the dynamo charges the battery and powers the lights. Given the dynamo only has two terminals, I'm assuming the output is DC and there's no requirement for a rectifier? The silver box thing on the mudguard is presumably the regulator but as its older than I am I'm eyeing it with some suspicion. I want to keep the right look so plan to clean up the old regulator and fit some modern electronics inside.

Can someone please tell me if I've understood all this correctly and also let me have any suggestions regarding the regulator?

Gary.

email (option): gj.owen@hotmail.co.uk

Quite right in what you say. There are several easy to fit solid state regulators on the market. I've used several AO (Al Osborne) regulators over the years which have lasted me well. I gut the original MCR1 box and fit the AO inside and solder the wires to the original terminal posts. Everything looks exactly as it should from the outside.

Ron

email (option): ronpier@talk21.com

If you have a post 47 (I think) M21 it will have had the MCR2 regulator as standard fitment...

Pre war,WD models and early post war models had the MCR1 regulator..

You mention a 'silver box' on the back mudguard..If this is an aluminium, crimped on top for the unit, it is a late regulator, an RB108...

So, the first thing is, what year is your M21..?

As Ron says you can purchase an electronic unit to fit inside the regulator (after removing the original internals)...
If you have a bike that would have had an MCR1 as original fitment you have to check with the supplier of the electronic unit that it will fit in the small MCR1 case...Not all of them do..Ian

email (option): ian@wright52.plus.com

Many thanks for your help guys.

My M21 is a 1958 model with the MO1L magdyno, the box is indeed aluminium but is currently in a box buried deep in the shed so I can't tell which specific model it is.

Gary.

email (option): gj.owen@hotmail.co.uk

My 1958 parts list for the M21 shows an MCR2 regulator in the illustrations..

Both the RB107 and RB108 have crimped on aluminium 'tops', the MCR2 has a brass top, painted black and retained with a wire clip...

If you want to test/adjust/fiddle with your existing regulator you'll need a copy of BSA worksheet 804A which covers the later regulator types...Ian

email (option): ian@wright52.plus.com

Precisely what Lucas components are well known for!

I think the Lucas regulator is a good but much maligned unit...

If new ones were available today I'd take one any day in preference to an electronic regulator...Ian

email (option): ian@wright52.plus.com

The MCR1 was said at the time to have a resistor problem, hence the MCR2....I decided that I preferred electronics hidden in an original small box rather than a working mechanical unit in the big box with the bulge on the back.

In general, I like Lucas stuff too and can't work out where the bad reputation comes from. I've done 100,000 miles plus on a Commando with the RM21 / zener set-up and only ever had problems with wiring connectors. Compare that with Suzuki's combined regulator / rectifier units of the 1970s / 1980s.

Wipac flywheel magnetos are something else altogether.

I'm a 100% with you here Ian! Mins you I said "well known for", at least to some extent then implying that this is en evol rumor. I have many times defended the Lucas components in heated discussions among my garage-buddies. It was just to obvious a joke not to make it. Hope nobody took offense, in that case I of course am sorry!
My mother's Saab 9-3 is a fine car in many ways but to me the best thing about it has always been the inscription on the back on the "remote" for the key that says 'made by Lucas GB'.

The trouble with old bikes is all the stuff on them is old and has deteriorated/suffered over the years, if new stuff was available it would function just as it was designed to. Plus bikes are much more vulnerable to the elements and the well meant intentions of home mechanics, thus suffer more than cars which are better protected and the owners are less brave with screwdrivers.
My 1934 Morgan's electrics are mostly from the 50's and have never let me down in 40 years or so but bikes are a different matter. As someone who savaged (stole?) from the skip at the college I worked for, instruments from the electronics dept. just because they are beautifully made I'm slightly worried that people destroy the insides of regulars etc. to fit modern stuff. In years to come they will be little evidence of the original which is not a great loss to mankind but a shame none the less.
Richard

yes they are fine, when used as intended and maintained as instructed.
I have an old mechanical fitted to my M20 which according to the magneto guru is wrong for the dynamo never the less it worked perfectly when I rode the bike at least once a week , some weeks every day and at least 3 nights a month.
'However now that I really only use it 4 or 5 times a year it gives me nothing but grief so it is going to get superseded.

Now I am not one for always fitting the lattest & greatest and I am not afraid to put a spanner to the bike, but having to clean every set of points then readjust it just because I have not ridden it for the past 6 months is too much an inconvienance particularly as this regulator has to come off the bike for adjusting being that the difference between not working and melting the solder is about 3/4 of a turn.

Exactly, see last post