I haven't seen the show...In the past I have seen the alternator from a Kubota mini digger used more than once as this is compact and has a moderate output...
Many car alternators are physically too large and have an output that far exceeds the electrical requirements of an M20 or similar...
As generating electrical power costs horsepower it's not really a great plan to produce more than you need on an M20..
...Ian
email (option): ian@wright52.plus.com
I have seen a long time ago (Wish I took photos..)
An M20 in which the guy took the wrap off of the dynamo armature, and located a pulley
About half way, on the axle that was left.
(Or maybe simply replace the armature with a plain rod with a pulley in the middle?)
Anyway, he cut away a portion of about a third of the dynamo housing, where the pulley was. (So you can see the pulley exposed trough the cut)
OK.. bare with me, if you understood so far, cause it is complicated..
While the axle with the pulley was out, he inserted a portion of the belt to the dynamo
Housing, then inserted the axle, so the belt now rested on the pulley.
He fixed the "dynamo" back in place, and took a small Suzuki car alternator.
From Suzuki "Vitara", or "Alto" or "Swift", (At least, that's how they named here)
And fixed the alternator side by side to the dynamo.
This (Short) belt is driven by the pulley on the dynamo, and in turn, turning the alternator.
I guess that this is the case where 1 photo equals to 1000 words...
Noam.
email (option): noam10@gmail.com
Hi Noam...I did a similar conversion on a swinging arm B33 back in the 80's.
In that case I removed the brush end castings, brushes etc. from a dynamo and fitted another drive end casting in their place...After removing the field coils this just left a tube with a bearing at each end...
I then made up a drive shaft that replaced the armature and went right through the dynamo and fitted a pulley on the outer end...
This provided the belt drive to the alternator which was mounted off the frame...There is more space here on a swinging arm bike...I also moved the battery from there to the toolbox to create some extra room...
I had to remove the spring from the magdyno clutch and lock that solid to stop it slipping under the additional load....
It worked OK and you could run a searchlight as a headlamp with the output of the car alternator... ....Ian
email (option): ian@wright52.plus.com
Nice, Ian !
The B33 had an extra 10HP as stock, compared to the M20,
And being your bike with your modifications,
I guess it had 25HP more, so you could do even with a Leyland alternator..
Noam.
email (option): noam10@gmail.com
As generating electrical power costs horsepower it's not really a great plan to produce more than you need on an M20..
...Ian Not necessarily. An alternator that consumes a quarter horsepower, running at an inefficient 75% efficiency will produce over 130 watts. I'll gladly take the quarter horsepower "hit" if it makes me more visible on the road.
I've been searching for a car unit that would fit into the dynamo slot, either with or without some modification, but so far nothing. If anyone finds something that will fit I hope you'll let the rest of us know.
email (option): jonny.rudge@verizon.net
That seems a reasonable figure...About 2% of an M20s power output running at it's full potential of 13BHP...
I understood typical alternator efficiency to be nearer 60% but I don't profess to be an expert...and of course the dynamo consumes power as well.
It would be entirely possible to fit a car alternator with far more than 130 watt output though...in fact I doubt there is a modern one with such a low output...
In that case the higher than required power output/consumption would be squandering some of the limited engine power unnecessarily...Ian
email (option): ian@wright52.plus.com
Hi Ian, John.
If there is a spare power, not needed by the electrical system,
Then much like with a dynamo, the axial resistance of the alternator will drop, So it would not take more than needed.
John, there is the "Alton" direct replacement, which I use.
Many of the "Alton" users are disappointed, some dealers
Who used to sell them, would not touch them with a stick now,
And on the other hand, many happy users.
The "Alton" (From France) The manufacturer declares:
"They provide 150 watts at their maximum and at least 90 Watts at ordinary cruising speeds"
http://www.norbsa02.freeuk.com/goffyaltons.htm
I use it for about 4 years, and it work fine, never failed,
Charges very good, but annoys me that in low rev (Low idle, practically)
There is an added rough noise. annoying especially cause I built the motor
From mostly NOS parts, and it used to run as smooth as silk with the dynamo..
Another conversion is the German "Powerdynamo"
The "Powerdynamo" maxes at 75 watt 12 (14.4)volt output with 60 watts at 3000 engine rpm.
This sounds more or less like the original dynamo, but the original
Lucas gave 60W/6V, and the Powerdyno is 75W (Lets say even 60W) On 12V,
So the output power is double the original.
The "Powerdynamo" unit is combined with an electronic self-power generating
Magneto. (No points, and no need of a battery)
Both alternators are of "Permanent magnet" type, not like in the car's one,
So you always have some resistance, even when not charging.
http://www.powerdynamo.biz/eng/systems/7197/719710main.htm
When goggling now, I found this interesting thread:
http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=246512
By the way John,
I saw a conversion where someone took a DC motor out of an electric kick-scooter,
Such as one of these:
Example
And used it as a generator.
These small motors can give usually 12-24V when you turn them,
But then you would need to have it regulated.
The problem is that a dynamo regulator would not work there,
As it is a permanent-magnet motor with no "Field" coil,
And an alternator rectifier/regulator would not do also, as it generates DC, not AC.
(It has to do with the way the collectors of the commutator
are arranges and connected to the armature's wraps)
But it would not be hard to have someone build such as regulator,
Or even find an already made one on eBay,
And it is the cheapest and easiest DIY conversion..
Noam.
email (option): noam10@gmail.com
Hi, Noam. It appears that about as many people are satisfied with the Alton unit as are dissatisfied. This leads me to believe that quality control is lacking, so some units work as they are supposed to, while others do not. For the price he is asking, I'm not willing to risk it. As for the Powerdynamo, it seems to put out only 60 watts, so it's no more powerful than the Lucas "long" dynamo.
BTW, it looks like you're confusing watts with amps. 60 watts at 6 volts is the same as 60 watts at 12 volts (except that bulbs, horn, etc., must be for the appropriate voltage). Watts = volts x amps. 60 watts at 6 volts means 10 amps output, 60 watts at 12 volts means 5 amps output, etc.
As for your comment about a regulator, don't forget that our bikes were designed long before solid state electronics, that's why they need mechanical regulators. Nowadays you can easily achieve regulation of either AC or DC. A simple Zener diode circuit or one of the more modern solid state bridges which dumps "extra" power to ground once a predetermined voltage is reached, is simple and inexpensive.
Anyway, I am convinced that there is unit out there, either a car alternator or some sort of starter motor that can be reverse-wired, that could be used in place of the Lucas dynamo. If only I could get a job at an auto parts store for a few days, I could locate it . . . 
email (option): jonny.rudge@verizon.net
Hi John...The problem with fitting an alternator directly to the mag is the fact that as well as the Lucas dynamo being relatively small in diameter its armature is also offset within the body to make space for the field coil...
This pretty much rules out all alternators, even smaller ones, from being fitted as they all have centrally mounted armatures..
So even an alternator as small in diameter as the Lucas dynamo wouldn't fit...
The Alton, at least in its original form used a gear running inside a ring gear to reverse this 'offset' and centralise everything within the body...Ian
email (option): ian@wright52.plus.com
Hello John,
Yes, you are correct regarding the Power output.
60W is 60W. I confused it with the current, which would be half
On 12V systems than om 6V, which would help to reduce voltage losses on the system,
And with better efficiency of a new alternator, I'd take it..
Generally, 60W output is OK for the BSA:
There is no starter motor, and even if a halogen 45W headlamp is used,
and 5W on the tail, you still have 10W to spare from 60W, and 25W leass of the max 75W.
It would not be a problem stabling the voltage of a DC motor using even more
Modern and accurate tools than the Zener diode, such as LAS1512 transistors or others..
I was just pointing out that a stock dynamo regulator or an alternator rectifier/regulator
Cannot be used for this application..
I never tried to open the "Alton" I use, but that planetary gear Ian mentioned does sound very logical.
I wonder if this is what makes the rubble noise.
More than once I thought of replacing the dynamo back, just because of the noise.
Interesting..
searching for ideas, I found now this Ural charging system,
http://cvkustoms.com/PDF/Ural_and_Dnepr_Gen_Alt_Part_III_G-414.pdf
Which looks like they where more than "inspired" the Lucas one,
And also few interesting conversion, which might work for the M20 (?)
http://www.russianiron.com/forums/index.php?showtopic=15142
Of course, it would not fit as-is, but maybe it would be a more simple
Conversion to use an alternator, or an higher output dynamo...
Noam.
email (option): noam10@gmail.com