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Lucas RM23 Alternator

Joseph Lucas Ltd. 1975

Data on the Lucas RM23 alternator

Introduction of the Lucas
Model RM23 Motorcycle Alternator  		 ALTERNATORS 4/75
RE-ISSUED
AUGUST 1975
 
For 1975 the new 850cc Norton Commando and John Player Replicas are
fitted with high outout (15A at 6000 rev/min) RM23 alternators.

The charging system comprises the following parts:
 


 Stator Part No. 47239
 Rotor   "     " 54201143               hot riveted / welded
 Double Diode Rectifier   "     " 49181
 Zener Diode (2 off)   "     " 49345
 


The RM23 is similar to the RM21 but has heavier output cables and a modified rectifying / regulating arrangement. The wiring diagram is shown in Fig. 1.

 

       FIG.1  RM23  CHARGING  SYSTEM

 

RM23 vs. RM25 alternators

The yardstick for me of a good charging system is the practical measure of performance - how fast do you have to go to balance the headlight load?

Consider the RM23 fitted as stock to Mk.llls, versus the later RM25. This has same claimed maximum output as the RM23, 180 watts; but more importantly the RM25 claims 150 watts at 2400 r.p.m. whereas the RM23 claims only 100 watts at 3500 r.p.m.

The practical measure? My RM25 balances the headlight at just over 2000 r.p.m. or 35 m.p.h. in top gear, whereas the RM23 balanced at more like 50 m.p.h. As a bonus, the clearance between rotor and stator is 18 thou. which is heaps better for the crank whip than the 6 thou.of the RM23.

The RM25 can come as a conversion kit: alternator, rectifier, pair of matched zener diodes, wires, connectors, and spacers (for behind the rotor). I didn't use the wires, except the extra connecting lead for the alternator; nor the spacers, these are intended for converting older bikes. The leads to the zeners have to be the same resistance, to avoid unbalancing the currents taken through the zeners, and the Mk.lll is fitted with equal length leads anyway, so these do not need changing. The central bolt of the rectifier protruded rather to much for the health of the battery case so it needed cutting off. The rotor on mine needed filing to achieve constant 18 thou clearance, otherwise there were no fitting problems. I changed it seven years ago and it hasn't been touched since.

The bottom line? The cost of the RM25 conversion kit was GBP140.80 excluding VAT, from Fair Spares UK (ref price listed 1 Jan 98) plus postage, although it's also available elsewhere.

Alan Lewer (alanl@actrix.gen.nz) on NOC-L 23rd. Jun 1998

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