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#815790 07/10/20 4:32 pm
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My Trident has evidently had a stator change. It has the slim version found on the twins, RM19 or RM20 I believe. I haven’t ever really bothered so much about alternators and their nomenclature but the rotor is of the wider type I believe to be part of the RM21 version. My 71 R3 had this, and I believe it’s quoted at a 140w output some places.
Question 1: Is this correct?
Question 2: will the wide rotor possibility have lost some magnetism running in the slimmer stator?
Question 3: is the RM21 in reality much stronger than a RM19/20?

Thanks
SR

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Stein Roger #815850 07/11/20 1:13 am
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The following might be a useful guide to start to understand the various variants available:

Alternator types (using the LU54202299, 74 mm rotor). This rotor is RM20 and is 1 3/8" thick.
RM20 rotor is used with all stators RM19 and up.

47149 (RM15) Energy Transfer 5 wire batteryless ignition version used on T20, T100,
T120, TR5, TR6, A65, C15 etc.

47204 (RM19) 6 V, 3 wire 120W single phase replacement. Fully encapsulated, highest
output 3 wire, 6 volt unit.

47205 (RM21) 2 wire 10 amp, 120W single phase this unit can be regulated to 6 or 12
volts and is a popular replacement for the RM19. A solid state regulator must be used with
this unit if it is being used to supply 6 volts.

47239 (RM27) 12V, 2 lead single phase 16 amp. Improved output over the RM21.
Recommend 12v single phase unit, can be used with a capacitor (battery-less)

47252 (RM24) 12V 3 phase 3 lead, 3 phase 10.5 amp stator.
Fitted originally as standard to Triumph T140 pre electric start models (from 1979).

47244 High output RM24.12 V, 3 wire 180W 3-phase. A very popular and reliable upgrade
of the electrical system. It can replace all above stators, not only producing more
maximum power but importantly producing more power in the lower rev range.

Adjusting running clearance
When mounted to the engine there must be a minimum 0.2 mm (0.008”) clearance
between the rotor and stator all the way round. Less and the rotor may touch the stator as
it heats up. If this happens the stator will overheat due to friction and fail very quickly, this
type of failure is not covered by the warranty. Inspect the pole pieces and the rotor for
signs of touching after initial fitting. The clearance is adjusted by slightly loosening the
mounting bolts and - carefully - tapping the stator with a soft faced hammer then retightening.

NOTE: Some Triumph T20s have a taper on the crank, in order to fit the 74 mm rotor you
will need to have an adaptor sleeve machined. T20SM models have a parallel fitting on the
crank. When ordering for Triumph Cubs please check the crank for a 19 mm parallel fitting.
Lucas is a registered trademark of Lucas Industries Limited
©Rex’s Speed Shop 2018. All rights reserved. Rex’s British Alternator Info Rev 3 nov 2018

Stein Roger #815861 07/11/20 4:41 am
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FWIW All my Brit bikes have had the RM21 alternator. My '74 T150V was adequate with a sealed beam auto headlight. The one on my Bonnie is adequate with an LED headlight (aftermarket regulator.) So far, the totally stock system on my '72 T150V keeps up with the H4 headlight. The one on my A65 was barely adequate except in town. I found an RM24 on eBay and replaced that.
As mentioned, the great advantage to 3-phase is at low RPM. The 3-phase comes up to full voltage just off idle.
Age will reduce the magnetism of the rotor. When I bought the RM24 I tested the rotors just by placing a nut between the two, the nut went to the newer rotor (original stamped '71, later stamped '79.) I can't see that running in the wrong stator would affect the magnetism, age will.


Knowledge speaks. Wisdom listens.

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72 T150V "Wotan"
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Stein Roger #815867 07/11/20 7:38 am
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Thank you very much both.
However, it is stated that the RM20 rotor was used on all alternators from RM19 and up. But my rotor is quite a bit wider, from memory maybe by around 3/8”. Which rotor is that, and is it available today? The stator is wider by a similar amount and has 3 leads from 6 poles. My well stocked dealer don’t have these, and knows little other than that they exist. According to him they didn’t offer a whole lot more power. On my bike the rotor is wide but the 2 wire stator is slim. The installation is vey neat but probably not original. The cable harness has got 2 leads to the rectifier, but one of the leads got a double connector, consistent with a 3 wire stator with 2 of the wires coming together.
I’m 250 miles from home so I can’t easily obtain measurements and pictures just now.

As I have a new spare RM20 rotor I’m tempted to buy a 3 phase stator and a modern regulator but I’m still curious about the “wide” alternator and if they offer a significant improvement over the RM21.
I suppose not, as they were discontinued.
The RM27 as I recall came much later and were physically of the same dimensions as the RM21?

Edit:
Info from the Lucas website speaks of RM23 single phase 2 lead alternator with 16A output.
It seems to employ the same RM20 rotor as all the others (disregarding the very early ones).
So what about the “wide” alternator, I can’t find any info on it at all, on the web or in the manuals.
I’m confused, though that in itself isn’t something new.

SR

Last edited by Stein Roger; 07/11/20 10:07 am. Reason: Added info
Tridentman #815880 07/11/20 12:21 pm
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Originally Posted by Tridentman
47239 (RM27) 12V, 2 lead single phase 16 amp. Improved output over the RM21.
Recommend 12v single phase unit, can be used with a capacitor (battery-less)

47239 is RM23.

Stein Roger #815881 07/11/20 12:46 pm
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Thanks for the correction, Les.
I thought that RM27 was wrong but copied in the whole extract from Rexs Speed Shop just to get the discussion moving.

Tridentman #815911 07/11/20 5:56 pm
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Quote
Question 2: will the wide rotor possibility have lost some magnetism running in the slimmer stator?

it may have lost some magnetism over time , but not from misalignment.
the rotor magnet-geometry is pretty much self stable . ...

...it does not need to be kept in a keeper or a stator
( a keeper just keeps it cleaner )

If you examine the inside of a rotor ... through the potting
you would see
the inside is automatically kept by the steel hub
and outside has the magnetic domains concentrated and directed by the laminated iron caps .
[Linked Image from i888.photobucket.com]

L.A.B. #815912 07/11/20 5:58 pm
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Originally Posted by L.A.B.
Originally Posted by Tridentman
47239 (RM27) 12V, 2 lead single phase 16 amp. Improved output over the RM21.
Recommend 12v single phase unit, can be used with a capacitor (battery-less)

47239 is RM23.
Clear as mud, but a 16A single phase stator with over 50% higher capacity and the same physical dimensions sounds too good to be true.
I see no mention of 5000 rpm as often quoted with the others.

SR

Stein Roger #815924 07/11/20 7:24 pm
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Sorry to use thick opaque mud!
I think the RM27 in the article I quoted from is a misprint for RM23 as Les pointed out.
I think 5K rpm is the "standard" speed for quoting output.

Stein Roger #815926 07/11/20 7:47 pm
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Quote
50% higher capacity and the same physical dimensions sounds too good to be true.


50% is what to what ?

Lucas , back in the day " filled " the stator core to meet an expected load capacity .

we dont known how much fill space was left on the table with older stator models .

wire gauge , quality of wire insulation ... bobbin insulation space
All play A Part on how much wire can be wound on each stator pole .
so some upgradability is understandable ... with modern materials .
and there may have been a few coil turns left anyway to make it easier to Assemble ?
the space is limited
so
it does take more time and skill to wind and fill the space more efficiently .
( I wonder at what point does this cut into reliability )

are you in the market for more/most power ?

Sparx , at one time marketed an extra high powered rotor .
( if they're still on the market , it may need reaming to fit .
the had a reputation for being a couple thousands under sized )

Stein Roger #815960 07/12/20 1:25 am
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Or, for a Trident, if you want maximum power then talk to or search for an article on [on this forum] using a GSX 600 or 1100 alternator conversion. I can't remember the output but somewhere around 240-280 from memory.
I think Romac and DM have both done it at some stage.

Stein Roger #815990 07/12/20 2:38 pm
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This is not necessary - 3 phase Made in England stator + old rotor + cheap Chinese regulator / rectifier gives all the power needed for my Trident without costly and complicated conversions. $120.- investment.
I have to keep lights on all the time,and live in big town.

Stein Roger #815992 07/12/20 3:41 pm
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Use the high output RM24 on my bikes, one with a pod regulator and the other I’m building now with the Boyer power box (big unit) the stator has proved reliable and has always started the bike with a flat or dead battery, no problem with running lights at night etc etc, found the RM21 just couldn’t keep up with the 55w headlight whilst running unless you were in a position to keep the revs up, not the best idea at night in built up areas.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

Allan G #815993 07/12/20 3:52 pm
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BSA A65 , I'm using an RM23 2 wire stator , notably with new rotor, gives 13.5v at tickover , hits 14.2-14.5 by 2000 rpm. I'm mostly running around London, i.e. stop/start , low revs and lights on winter time, 55w headlight, never a problem.

Stein Roger #816101 07/13/20 4:21 pm
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I think the RM23 came on Mk3 Commando, and used dual Zeners to handle the extra current.


Knowledge speaks. Wisdom listens.

72 T120V cafe project "Mr. Jim"
72 T150V "Wotan"
92 BMW K100rs "Gustav"
Stein Roger #816165 07/14/20 9:59 am
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Sorry I’ve been indisposed for a few days. Thanks to all for their contributions!
I’m not worried over power output as such, only curious to be honest.
My main question is really about the wider alternator, what it was called and what nominal output it was quoted at.
I’ve run an LED headlamp for years now, drawing a mere 20 W, so my RM21 stator energized by a “wide” rotor (which as of yet doesn’t seem to have its own RMxx code) was able to cope with a Lucas RITA on 12 V coils. If not by much.
I have a “wide” stator (early edition RM23??) on the shelf which should, unless I’m very much mistaken, give me some additional power and keep my battery happy with lights on and in town.
Or I can stick a 3 phase in there if I want. This wasn’t meant as a “worry” post, but I’m happy with the response, even if the mystery of “the wide alternator” remains...

SR

Stein Roger #816195 07/14/20 5:33 pm
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Hi SR,

I find this all very confusing!

I would have expected the Trident to be fitted with the RM21 as standard.

This would be the encapsulated 2-wire stator, with a 74mm OD rotor and 35mm wide.
The width of the stator is only marginally more than the rotor, just the encapsulation I would think.

All the Lucas alternators from RM19 upwards that I have encountered have had these basic dimensions.

For this series of alternators, I have not encountered a “wider” version. That’s not to say they haven’t existed, I only have experience of a modest number.

The pre-encapsulated range of alternators (something like RM14 – 17) were of different dimensions, as I’m sure you know.
The rotor was of smaller OD (70mm) and the only one I have known (on my 1st Triumph, a late 50’s 6T) was of longer thickness than the 35mm of the “modern” variety.
The old stator was correspondingly thicker than the modern variety.
With the single chain engine sprockets of the time, these thicker alternators had room to fit in the chaincase. I can’t give any thickness dimension for these early ones, mine was thrown away 42 years ago, but it was substantially thicker.

When I went to duplex primary on the 6T, with the wider engine sprocket, I’m doubtful that the wider, older alternator would have fitted. I don’t know for sure, as I didn’t try it – but the alternators that I’ve fitted ever since (an RM21, then RM24 fit with no issues).

Does your long rotor have the usual close fit to the stator (ie the 8 thou or so gap)?
And how long (wide) is it?

What is the width of your “wider” stator? Is it much beyond 35mm?

I’d be surprised if the timing case of the Trident allowed much spare room for much wider alternators than standard, but that is only a guess.

Not much help I’m afraid SR, best I can do. Dave.

koan58 #816199 07/14/20 6:38 pm
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never seen one ,
but supposedly there is limited batch
of wider rotors made at one time
for police work , on bikes fitted with radio .

Stein Roger #816204 07/14/20 7:06 pm
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So they presumably had wider stators as well? Otherwise what would be the point of having magnets outside of the stator?
Any further clues on this quinten?

There definitely were differences sometimes for cop use. The later 50's police bikes had both dynamos and alternators to support the electrical load.

The 60's units didn't have that option, so a "super cop alternator" would be perhaps the only way to go? Though I've never heard of it, not a whisper.

And bearing in mind the very limited space inside the unit chaincase (the alternator nut doesn't have to loosen much to grind the case), I can't see a much wider stator/rotor squeezing in there.

Stein Roger #816208 07/14/20 8:24 pm
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Last edited by L.A.B.; 07/14/20 8:39 pm.
Stein Roger #816212 07/14/20 9:00 pm
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Yes, the 3rd Ebay pic does show what seems to be a much fatter stator. It says P/n 47209.

Have you come across such a thing LAB?

If the rotor spacer is related to it, it suggests the stator was 6.4mm thicker than usual.

Living and learning!

(and for some reason 3-wire, odd)

Last edited by koan58; 07/14/20 9:06 pm. Reason: extra last comment
koan58 #816214 07/14/20 9:18 pm
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Originally Posted by koan58
Yes, the 3rd Ebay pic does show what seems to be a much fatter stator. It says P/n 47209.

Have you come across such a thing LAB?

No, as later triples had the RM21.

Originally Posted by koan58
(and for some reason 3-wire, odd)

I'm not sure there was anything particularly odd about it but some RM20 (if not all?) were 3-wire single-phase (like the RM19).

Stein Roger #816216 07/14/20 9:32 pm
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So when did they adopt the RM21? I imagined all Triumphs and BSAs were 12V by the time the triples came out.
Did they still use 3-wire RM19s in the early models?
I've not noticed an RM20 specified in a Manual, is that something particular to the early triples?

koan58 #816219 07/14/20 10:03 pm
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Originally Posted by koan58
I imagined all Triumphs and BSAs were 12V by the time the triples came out.

Yes, they were all 12V.


Originally Posted by koan58
Did they still use 3-wire RM19s in the early models?

Still use? In triples? No, not that I'm aware of.


Originally Posted by koan58
I've not noticed an RM20 specified in a manual, is that something particular to the early triples?

Well, it was fitted to early triples.

[Linked Image from i.postimg.cc]

Last edited by L.A.B.; 07/14/20 10:04 pm.
L.A.B. #816232 07/14/20 11:07 pm
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I've not noticed an RM20 specified in a manual, is that something particular to the early triples?

i believe so .
the RM 20 in my 69 bsa parts pdf. is ....47209 ( just one source )
stator is ... 54213903 .
it has more stator laminations per fleabay pic.
[Linked Image from img.auctiva.com]
[Linked Image from img.auctiva.com]
but it is a 3 wire stator
[Linked Image from img.auctiva.com]

the 3 wire , 6 pole RM19 and RM20
and 2 wire , 6 pole RM 21 ... are all listed as 10.5 amp stators ... so it appears the the fatter RM20
is a transitional design and Lucas settled on the RM21 pattern .

Last edited by quinten; 07/14/20 11:08 pm.
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