Zenner Diode Supplier

... please let us know, although I think Al Osborn of this parish may have one or two decent used ones lying about.

There is NO Lucas Zener any more being manufactured since the mid 1980s There is no substitute! There is one or two chinese item flying about but they have no specification so I do NOT recommend them. I very much doubt you will find any dealer with NOS bear in mind that is some 40yrs ago. BUT if you do come up with an apparent unit I can test. I also sell the regulator/rectifier but it is not recommended to use the original warning light assimilator with these as they can make the reg/rects fail. I do sell two units you can use instead, one retains the red idiot lamp, the other give you a lot more information.
You say your MKIII is original with regard battery charging, there is quite a few strange things with this circuit see my web site  aoservices.co.uk for the in-dept analyse with regard to MKIII charging.
A failed single zener would give you half charge with the original rectifier (from Lucas). Replacing the Lucas rectifier with a full wave bridge would put you up to slight over charging. Your choice.

An observation you might want to comment on Alan...

The original Mk3 rectifier setup uses the two zeners as one half of the bridge and two regular diodes in the rectifier pack as the other half. Assuming silicon, there's a 0.6 volt drop across each diode. Current passes through two diodes each side giving a total drop of 1.2V.  If the Mk3 rectifier is replaced with a modern full bridge rectifier, the current passes through two diodes in the rectifier and one zener, so the voltage drop increases to 1.8V. The zeners still clip to the original voltage but the voltage at the battery in reduced by 0.6V. This will result in slight undercharging of the battery.

Negative case power zeners are available, I wonder whether an insulator between the diode and footrest plate would work.
 

In the MKIII as supplied the two zeners are two legs of the bridge. if one fails (open circuit) then the power transfer is 1/4. If the 1/2 bridge is replaced with a full bridge then the zeners are in parallel with two diodes, so if one zener fails (open circuit) then the bridge carries on charging and partially unregulated, so increases the charge rate. All of this is explained on my web site.
I wouldn't consider negative earth zeners, they are NOT available and I am trying to advise safe systems for use  of  any/all of my customers-and/or Norton owners, Getting the average motorcyclist to understand the find points about earthing on motorcyclist is almost impossible.
If Stan wants to try this let me know what happens. IN fact if you had two negative earth zeners than converting the whole bike to negative earth with a full bridge rectifier would work-but would need quite a lot of considerations and then you you could end up with a RED Negative earth wire on the battery-CRAP. So a rewire with a BLACK earth would be needed as well.

The zeners are in parallel with the bridge diodes - doh!

So no difference, they just share the forward load.

Yes the Zeners are in parallel with the zeners IF you fit a non standard bridge, as I recommend. They are on the AC side (another Lucas bit of trickery from way back) If not you use the original rectifier the zeners are part of the bridge, and operate on the AC side of the bridge, so they only handle half the power each, one zener fails you are down to half power for charging etc.

I had to rewire my Mk III about 12 months ago as the harness was rotten. At the time the rectifier and zeners were fine. I did not reuse these items and was intending to put them on fleabay. If you would be interested in all, or any of the items, let me know? 

I did have a similar issue with my 99, The  Commando  Alternator puts out more amps than i can use in the summer , So i have a decent size flooded acid battery that can cope with a bit of excess charge , The Zener is mounted on a proper  heat sink up front  in the cool airflow and i run with the lights on which is helpfull with being seen. A 60 watt HL bulb and an EI box  helps soak up the charge ,. In the winter I can power a heated jacket . The Battery needs an eye kept  on fluid level., but has lasted for many years. The rectifier has a decent heat sink now after I melted one. The zener is 30 years old.I should probably cary a spare ! I also have the Blue thingy capacitor which means It will start and run with a flat battery or no battery!.. As i have the PSR8 switch I could  go back to using the charge switching arangement , perhaps Al would give his thoughts?. Am I playing with fire and should i carry an extinguisher ? !.

Yes you are playing with a 'box of matches' but all what you are doing is going the right way, BUT zeners and rectifiers are semiconductors, and as such when they are run outside their specification they do not 'take prisoners' ie they will fail much quicker than the fuse. BUT bear in mind the fuse is there to blow if the electricity generates a short circuit thus stopping all that battery electricity getting into the wiring and bursting into flames. If/when any semiconductor fails, depending how ie short circuit or open circuit, then the semiconductor fails far quicker than the fuse. So Robert to be a bit safer in your case monitor the battery voltage at all times. The instant you see a potential high charge then put the lights on.
Also the Commando (Not MKIII) has a single Zener on the DC side in parallel with the battery and its DC. The MKIII has a completely different arrangement with the two zeners on the AC, a strange half bridge rectifier to cope with an RM23 (180W) alternator.
Malcolm-if you read my words and my web site you will see that if you have the original MKIII set up with one zener disconnected you will be into half charge. You will not harm the alternator or the battery. At the end of the day monitoring the battery voltage for under or over charging is your answer.
Robert-going back to the PRS8 part charge switch could be a good idea, if you had the RM19 3 wire alternator, but that will be within spec of your 100W zener anyway. If the alternator came off your Commando then it would be a 2 wire so you can't use the PRS8. But a good idea. 

With any/all of these rectifiers, Zeners even rec/regulators and my dynamo regulator I can test these. All I need is return postage costs ie £5-00

Forgot that you need a 3 wire alt to use the PSR8 feature !. I could fit a mini fridge to keep the beer cold and mount the zener in that? !!  The only issue i have now  is i like the bright white of the new LED's but they use too few amps  to keep my system happy. 

Slightly off topic but related to negative earth zener comments above. I used my old '65 Atlas Lucas zener as the voltage regulator on a negative earth Honda CB750 K2 with its original Hitachi SB6B-7 rectifier. Alternator was the standard 210W/17.5a. Rode without problems for around 15k miles from 1976 to 1978 with a 14ah Yuasa Yumicron battery.

In 1978 I fitted this same setup to a negative earth 1978 Kawasaki Z1R. Alternator was 240W/ 20a so the zener operated way beyond its normal range, trouble free for the next 10 years, around 105k miles.

Zener mounted to a Triumph finned heat sink on the lower yoke, directly in the airstream.
For negative earth use, the heat sink was isolated from the frame with two thin nylon washers and a short sleeve. A thin copper plate with a 6mm spade tab underneath the zener held the earth wire with the feed on the normal spade at the top.

Zener function check was easy, just rev the motor and feel it getting warm. Only occasional glitch was a fracture on one of the wires due to the steering movement but otherwise a completely fit and forget mod. Yuasa batteries lasted around 5 years.

Interesting that the zener could cope so well, Lucas stuff then not so bad after all.   Back then !! Not sure now about the make.

13.4V is too low, the nominal voltage needs to be above 14V. Lucas (the real Lucas before 1980s) have said that 14.7 is a good voltage or 14.2V. I have seen them up to 15V and the battery and lights survive.

If I turn the mounting nut tighter will that adjust the voltage so I can get another 120k miles???

NO as the Zener has a copper/brass stud you will hopefully snap it off!

Lighten up son, wishing misfortune upon others is never a good idea. Bluster and pedantry aside, at least you now know there may be no need to change the wiring if using different polarity zeners and they are far more durable than you thought. At least with a decent quality rectifier and battery, however I would never let the voltage get anywhere near 15v for decent battery life. 

Must be off, have to trim the hedges, tidy the shed and finish the Commando in time for the hols. No time for any further comments here but appreciate your time.