Atlas alternator

The alternators are interchangeable between large twins.  So...yes if you have alternator crankshaft.  No, if you have dynamo engine with original type crankshaft 

Standard from 1958  inc Singles, until Mk3 Commando 1975 on.(Electric start)

Happy Norton' and riders for the New year. Ride them while you can.

Although the general physical size of the Alternators fitted to Norton Twins remained unchanged for years there are a couple of practical points which need to be considered. 

The Dominator Post Dynamo era (1958ish) saw RM15, then RM 19 and RM 21 fitted with a couple of others inbetween.  The early alternators would have offered 6 volts from a 3 wire output. Whereas the RM 21 was really suited to a 12 volt system via a 2 wire output.

There is really nothing to stop a mix and match plan if it considers the wiring system fitted to the bike, the light / ignition switch and whether the 1960s 6 volt set-up is being retained.

Best move is to get hold of a wiring diagram for the bike and also speak with someone (like Al Oz) who knows about conversions between models.

The Atlas alternator may have a rotor with a larger diameter so keep that with the correct stator. It may also be narrower so you made need the rotor spacer to fit between the engine sprocket and the rotor.  It's a flat washer about 3/16 inch thick and it's usually hardened.  Sometimes the thickness of the stator also varies so you may require longer or shorter studs (3) to fit the stator holder casting. Good luck, Howard

If this is for a 12 V conversion, with voltage regulation, all well and good.

If you’re keeping the (not very good) 6 V system with the light switch controlling the stator, then the headlight bulb wattage should really be matched to alternator output.

I kept my first comment brief but just to say there are two Dia of magnets (Rotors) not much between them but the air gap or lack of makes a difference. You need the rotor to match the stator.

I have spares of the smaller one.They will work if you keep the revs up and have a charged battery for traffic etc,

I have 20 teeth on the Engine on my 88 Dommie racer. Can't say till Spring if  this is right.

Bacon says 20 on the engine and 19 on the G / box for 88.

But always to suit riding style.  For me, 18 would be a high rev motor = Faster acceleration at the expense of lower top speed and lower MPG.

Your call.

19 tooth engine sprocket for an 88, 20 for a 99, 21 for 650 and Atlas.

I wouldn't argue with you, Gordon.

Just a case of what works for you. Starting points.......

Happy New Year.

  there are also options of 22t & 23t engine sprockets in th parts list, for 750s. l wonder if anyone supplies these-?

I had a very good 88 with std gearing and it was a speedy smooth bike , gearing it higher would have just inhibited its perfomance. 

I have a 21 T engine sprocket on my big bore (630cc) ES2.. Works well.

RGM has a good selection. (3 offsets for singles)

I had a very good 88 with std gearing and it was a speedy smooth bike , gearing it higher would have just inhibited its perfomance. ​​​​

Yes, overgearing, in a quest for mythical “relaxed cruising,” has turned many an old bike into a slug and caused more than a few engine seizures.

Hello all contributers - This thread seems to have got off course a bit since it started as a query about  swapping alternators and is now discussing gearing options for 88s.  As you seem to agree most 88s are happy with a 19T engine sprocket but for those wise enough to invest in an 88SS model a 19T engine sprocket is a bit small,  especially if the bike is used on fast runs.  On standard gearing of 5:1 at fairly fast speeds the engine will be revving far too high.  Even with 99 gearing 4.75:1  it's revving very high.  It makes me wonder what gearing the factory had installed when it was road tested in 1961 at 111mph.  Even on 650 gearing at 4.53:1 it would be revving at over 6800rpm.  For those of you who still enjoy a track day don't undergear your pride and joy.  Give it stick - Happy New Year, Howard     

Hello again Sean and Happy New Year - A standard Model 88 from 1960 would have had a 19 T engine sprocket fitted by the factory.  This equates to a gear ratio of 5:1.   Good luck, howard

To you all. Someone had to respond to the well wishers!

Surely it’s as Howard and Neil suggests; it’s dependant on your expected use and the terrain you live in.  East coast flat lands will require less gear reduction than a North Wales mountain pass with switch backs and hills.  You may run two up, tour, sprint or just ride from cafe to cafe. You may even want to explore the limits of your machine. To do this you select the appropriate gearing for your machine.

 The John Player Commando had great acceleration due to the factory fitting a lower tooth gearbox sprocket (19t?) but it sapped the top end. I run 22 because it’s flat and open road where I live.

Glad you solved your alternator issue :-)

Jon

According to Bacon's 'Norton Twins Restoration' the engine sprocket on the 88 changed several times in the 50's between 18 and 20, finally settling on 18 in 1964.  Since that would have been 88SS, perhaps they were aiming at club racing (or why not buy a 650?), so they wanted it to rev out at max bhp and speed together?  My 88SS has 20.  I'm perfectly happy with it.

Hello David and the rest of you readers.  Your 64 88SS figures can be misleading in so much as in 1964 they changed from a rear chain with 5/8 X 1/4 inch size to 5/8 X 3/8 inch size.  The 1/4 inch width usually accompanied a 43T rear wheel sprocket and 19 T gearbox sprocket however with the switch to 3/8 inch wide chain a 44T rear wheel sprocket came into play and 20T and 21T gearbox sprockets were available.  One size fits all went out of the window.   Cheers, howard

I have got all of the Norton Dominator Service Manuals from 1949 up to 1968. For the Model 7, 77, 88, 99, 77(mk2) 88SS, 99SS, all 650 variations and Atlas bikes. The rear wheel sprocket is always quoted as having 43 teeth.  The gearbox is generally 19 teeth and the clutch basket as 42 teeth. The engine sprockets vary according to model and whether a sidecar is hitched to the respective frames. 

Up to 1956 the engine sprocket on both Model 7 and Model 88 was kept at 19 teeth for standard road bikes. Then Bracebridge started to mess about with compression ratios, camshafts and valve sizes and up went the BHP and engine sprocket size on some engines.

The standard Model 88 was given a 20 tooth sprocket for a few years then back to 19 and then up to 20 again. Whereas the 88SS stuck with a 19 tooth engine sprocket all of the time.