Engine sprocket sizes

[Nomad54]

Looking for advice on sprocket sizes. I have an ex police alternator a10 which comes with a 19 tooth splined engine sprocket. The bike is in early stages of restoration so looking for as much info as I can get. Was told that these bikes were quite lively low down but revd there heads off at top end. Has anyone got one and can you tell me best sprockets to use. I have tried to get a 21tooth but no one seems to have any.

[Topdad]

Originally when I put my 59 A10 bitsa on the rd mine rev ,the motor had been completely rebuilt ,rebore 8.25 pistons spitfire cam and followers lager exhaust valves  so had a fair bit of go in her whilst keeping that lovely low down torque but at 60 she revved I uped the engine sprocket to I;m sure 21 (but honestly is some time ago )  result for me was pretty good life at 60 sounded a lot less fussy anfd acceleration was fine in the lower gears and she goes up hills in top very nicely . Think I got mine sprocket from either C&D or dragonfly, best of luck.Bob

[chaterlea25]

Hi Nomad,
There are several topics about engine sprocket sizes on the forum.
An A10 will easily pull a 22 tooth sprocket
With the advantage of not needing a half link in the primary chain.
I am running a 23 tooth on mine
4k rpm at 70mph, 3400 or so at 60
A lot of the time I ride two up and have no issues.
It maybe an issue getting alternatives for alternator engines though

John

[Nomad54]

Hi John problem is with the alternator models they don't have the Cush drive 2 or 4 lobe sprockets. The crank is splined like the b33 and nobody seems to have sprockets bigger than 19 tooth. Looking for info from anyone who is running an alternator model if there are any out there.

[Colsbeeza]

Hi Nomad, I tried the 19-tooth (1961 Flash) and I thought it revved its tits off at 50mph on Australian roads - wide and fast, so now have a 21-tooth. I think it is still too revvy at 60mph, but I have a steep driveway and when I get home, I have to ride the clutch under power to get up. Still that is only for a few seconds each ride. I'd like to get a 22 tooth, but as I spent my dough on a Steve McFarlane electric start with the drive cog riveted to the 21-tooth engine sprocket, I'd have to send a new 22 tooth to UK to get it modified. Still thinking about it - probably forever .
Col

[Nomad54]

Hi Col
My bike uses a different type of sprocket from the normal dynamo model A10's this is the reason I am looking to try something different to alter the gearing if I can't get a bigger engine sprocket. By the looks of it I seem to be the only one with an alternator model still running so I will just have to play around with clutch drum, gearbox sprocket and rear wheel sprocket until I get it right.
Tom

[JulianS]

There were alternative engine sprockets listed for the alternator bikes. Just a matter of finding one.

If you have the 18 tooth one I would image that the bike was geared  for acceleration and/or urban area use?

Below from the Factory issued booklet "Accessories and special equipment" from the early 1960s.

The 18 tooth one also used on the alternator B31 and the 19 tooth one also used on the alternator B33.

[Nomad54]

Cheers Julian my sprocket is a 19 tooth. I notice on the accessory booklet that there is a 20 and 21 tooth but the problem I have is nobody seems to have any. I could go down the route of getting one machined but I think I will wait for the restoration to finish then try it out. Just thought someone out there might have came up with this problem before.
Tom

[chaterlea25]

Hi Tom,
I have a thought that pre unit Triumphs might have the same spline pattern???
It might be worth doing some research
I also remember seeing a B33 with a duplex primary using Triumph parts

Some years ago I rebuilt an alternator B33
I seem to remember that it had a bigger sprocket welded to the splined hub

John

[muskrat]

G'day Tom.
I was thinking the same as John just suggested. Machine off the teeth then bore out a blank of your desired tooth count and weld on.
Similar to how I had change rear sprockets on my racer/Cafe (38, 40, 42). I used alloy blanks so welding was out. I used 6 scotch keys in the form of bolts, but the same principle.
Cheers

[Richard Hinton]

18 was for sidecar use
21 for standard use
23 , 24 for long motorway use but have to ride the clutch on take off, so it puts a strain on clutch bearing. I had a 24 on mine but bearings didn't last long due to high speed and plates wore down quick. But i did clock up 100mph on M5 just let it build up slowly.

So on last rebuild went back to standard of 21 Tooth. Use it on motorways 50 - 60 cruises along ok. have run it at 80mph just to test engine rebuild, nothing blew up or fell off. Did the ACE cafe London to Brighton run last year on the 21 tooth great day and ride.

So i have a friend who has several engine sprockets etc, ill ask him if one of them is a 21 tooth and is for sale. in the next few days ill post the feed back on this.