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Gearbox sprocket - 19 or 20 teeth?

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Hi.

I’m about to make the directors of RGM much wealthier as I’m now ready to purchase all of the parts needed for my 1960 Dommie 99 gearbox rebuild.

I believe that the standard gearbox sprocket is a 19-tooth item but was reading that increasing this to 20 or 21 teeth would give a more relaxed motorway cruising speed experience (I have the standard engine and rear wheel sprockets).

I don’t want to ruin the ‘getting away from the lights’ acceleration or cause the bike to have problems going up steep hills, so is it worth making the change to the gearbox sprocket? If it is worth it, will a 20-tooth sprocket give me a noticeable rev reducing benefit at motorway speed and still retain the ability to get away briskly from a standing start? The 21-tooth sprocket sounds a bit too much of a change but I’m prepared to be corrected on that one. Also , would such a change mean a slightly longer than standard drive chain was needed?

Thanks

Regards

Tony

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I also have a 1960  99.  And I also would appreciate a more comfy cruise. I don't think raising the overall ratio's will do it. You may spend more time in third gear as a result.  I think your best bet is to get the ignition timing spot on on both cylinders to make the motor as smooth as possible. When you have the motor down get the pistons lightened and balanced and the crank also.Whats really needed is an overdrive 5th gear . I have looked into this and it appears that no one is yet making such a gearset. A close ratio set is not suitable. I have a 500 sporting single with a raised ratio gear set and its a real pain to ride .I will be following this thread with interest . 

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I dabbled around a bit with various sprockets in an attempt to improve the performance of my bikes. On my 99 I fitted a Commando rear drum/sprocket with 42 teeth and electronic ignition, an SS cylinder head and a Concentric 928 single carb. The electronic ignition gave easy starting and much smoother running. The SS head along with a QR camshaft gave loads of power and the carb better fuel consumption.

When I fitted a 650 engine in place of the 600 I kept the same changes apart from the carb which went up to a 930 Concentric. I can cruise at 70mph at just over 4000rpm and often get over 70mpg.

Later on I changed my AMC 1 gearbox by replacing the Layshaft 24T third gear with a 23T cog. This is a smart move as it raises the 3 lower gears and diminishes the previous big jump between third and top.

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When I had my 850 Interstate me and a mate went to Reg Deardens in Manchester and I asked him for the biggest gearbox sprocket he had to fit the Norton.The fella behind the counter used to stutter badly and he said "Oooookkkkk" and off he went into the back, after what seemed ages and a lot of clanging and banging he came back with his hands behind his back then dropped what I thought was a dinner plate with teeth on the counter. "ththere  you go" says he. "What the f***k is that" I asked  "It's a 24 tooth sprocket,I couldn't find a bigger one"!. Well I didn't have the heart to say I didn't want it after all the time he had spent in the back of the store room so I took it home and banged it on.

It certainly dented the acceleration but what a motorway cruiser it became , even if my mate did laugh for weeks afterwards.

 

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Thanks everyone. Looks like it’s not worth worrying about changing the gearbox sprocket from the standard 19-tooth one. I’m going to do the ‘N2’ gearbox update to my ‘N’ gearbox - from what I remember, there are two pinions that need changing (4th gear mainshaft and layshaft 3rd).

Robert, if you’re after a 5-speed cluster for road use, I believe that Mick Hemmings sells one, but it’s about £1500 I believe.

Regards

Tony

 

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Its certainly true  that more power  allows higher gearing and  less feeling that you are pushing the bike.We have geared up the Atlas but the shakes  were still horrendous.(perhaps its one of those that AMC tried 84% balance factor on) .  One of my bikes will break 140mph before top is needed,but I'd still rather be rumbling along at 70 on a Norton with an overdrive 5th.

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On my 1967 650SS I have gone to a 20 tooth gearbox sprocket though it is better I wonder if I should have altered the engine sprocket (primary) instead.

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Hi Alan,  The bigger box sprocket  in theory is better for the rear chain  (more teeth to share the load )  ,bigger engine sprockets can get the chain too close to the alternator housing. I think they also give the gearbox a harder life .

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Not sure whether this is relevant to the Norton community but in a similar, but converse, discussion on my Ducati OC forum to reduce gearbox sprocket by one tooth to improve acceleration concluded that one tooth on the drive sprocket was too much and the change should be made to the number of teeth on the driven sprocket at the wheel end, which is less sensitive (more teeth). Clearly , it's simple to establish the effect on mph/1000rpm in top gear, as this is the same % as the ratio change. However, as you all say, the effect on acceleration/performance feel in the lower gears is more difficult to predict and a more 'suck it and see' approach is required. It depends what trade-off you are prepared to accept.

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I changed the gearbox sprocket on my 650ss- engined Wideline from 19 to a 21 tooth. I noticed no penalty in driveability lower down the speed range, apart from a slightly higher 1st gear as one pulls away. It feels far less fussy at open road speeds, yet certainly does not feel over-geared. Back in the day the lower ratio may have made for better acceleration round town (!), and ability to rev out in top, but we don't really ride them like that now, do we?

Ian

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Hello all  well if your wanting  motorway speeds and comfort riding  then you need to keep the revs  down  and to do this  change the  engine drive sprocket up one tooth makes all the difference  so on a 650 a 22  tooth sprocket is the biggest you can get for alternator without chewing up the alternator housing,  and then you can go up one on the gearbox to  if needed  which will make the ratio just around  4:1 overhaul,   but this will be trial and error  until you have this how you like it, I get around  85mph  in comfort  on the Motorways at  3800rpm to 4000rpm just buzzing away, without any stress    you just have to keep a eyes out for boys in blue,  or speed cams,   But we go motorbiking looking like a streak of lighting,               yours  anna j    

  

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I put a 21 T sprocket on my Commando. When I let the clutch out it feels like taking up up the slack on QE2 during launch. It's better in top gear than my 1973 Jap bikes.

On my Model 19 rigid I could only buy a 19T sprocket, so I made a 21 sprocket myself, and that bike really lops along at low revs at 60mph. Firing, as they say, at every other lamp post.

These are torquey bikes and can handle a high top gear.

The few seconds slipping the clutch in town is easily outweighed by the hours of relaxed cruising in top gear.

Just my humble opinion though.Take it or leave it.

hello peter, yes the model 19 are the REAL Nortons,  The good old Model 19  I love one  is On my Christmas list  when one becomes available,  they will  last 3 lifetimes  if you look after them well, do enjoy your rides out,  yours  anna j     

 

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