Author Topic: Plunger Rear Sprocket  (Read 260 times)

Offline Swarfcut

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Plunger Rear Sprocket
« on: 02.06. 2019 08:19 »
 The rear sprocket is made in one piece with the brake drum, so the whole thing has to be replaced when the teeth are worn. 42 teeth for a solo bike, 49 teeth for a sidecar. 46 teeth also available.

 The gearbox sprocket is a standard 19 teeth, and is the same on plunger and  S/A boxes. 18, 20 and 21 teeth are also available. Always fit a new chain with new sprockets, as old and new parts will wear out quicker. To change the plunger gearbox sprocket the engine and gearbox  unit will have to be removed from the frame.

 Information provided for new forum member fearie.

 Swarfy.

Online Greybeard

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Re: Plunger Rear Sprocket
« Reply #1 on: 02.06. 2019 08:26 »
I'm running a 42 tooth rear sprocket with a 20 tooth gearbox sprocket. I'm happy with that combination, it gives a comfortable cruising speed and doesn't cause problems going up hills. I mostly ride solo.

Online Minto

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Re: Plunger Rear Sprocket
« Reply #2 on: 05.06. 2019 19:25 »
Swarfy
Thanks for posting this.
I knew from previous MOTs that my A10 had pulled a sidecar until recent years, I hadn’t realised it was still running on sidecar gearing.
So looking for a decent used rear sprocket.

GB what do you call a “comfortable cruising speed?” Mine seems happiest at around 50 to 55 with its current ratio.
Cheers
Jase
52 A10 plunger

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Re: Plunger Rear Sprocket
« Reply #3 on: 05.06. 2019 19:34 »
GB what do you call a “comfortable cruising speed?” Mine seems happiest at around 50 to 55 with its current ratio.
60 to 65. I have gone faster, to overtake for instance, but I wasn't happy to push the old girl like that for long. I go to about 50 in third before I change up.

Offline Swarfcut

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Re: Plunger Rear Sprocket
« Reply #4 on: 05.06. 2019 19:58 »
Minto. Think at what speed your motor is happiest cruising, then consider that for that engine speed your road speed will be that little bit quicker.

 If buying used, be careful as the early S/A sprocket looks the same, but has a smaller  bearing (the same size as the wheel bearings). So it will not fit as a straight swap. The plunger drum has a bigger central hole and a more rounded boss compared to the S/A drum.

 As the speedo is driven from the gearbox, road speed may not be correctly indicated, unless the bike was originally a solo, and retained the solo speedo head.

 Swarfy.

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Re: Plunger Rear Sprocket
« Reply #5 on: 05.06. 2019 20:10 »
As the speedo is driven from the gearbox, road speed may not be correctly indicated, unless the bike was originally a solo, and retained the solo speedo head.
Oh yes, I'd forgotten about that. I had my speedo recalibrated after I changed the drive ratio. The odometer is not very accurate now but the speedo is very accurate. A SatNav will show very accurate speed, (as long as the vehicle is on the flat) if you want to check your speedo.

Offline BSA_54A10

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Re: Plunger Rear Sprocket
« Reply #6 on: 06.06. 2019 13:57 »
The rear sprocket is made in one piece with the brake drum, so the whole thing has to be replaced when the teeth are worn. 42 teeth for a solo bike, 49 teeth for a sidecar. 46 teeth also available.

 The gearbox sprocket is a standard 19 teeth, and is the same on plunger and  S/A boxes. 18, 20 and 21 teeth are also available. Always fit a new chain with new sprockets, as old and new parts will wear out quicker. To change the plunger gearbox sprocket the engine and gearbox  unit will have to be removed from the frame.

 Information provided for new forum member fearie.

 Swarfy.

And eother buy enough bulk chain to make at least 3 chains or buy 3 .
Run each chain till it gets slack enough to need adjusting then swap for another that you have washed properly & lubricated.
So you are always adjusting on the first chain.
That way you will get 3 chains worth out of every sprocket set
Bike Beesa
Trevor

Offline cyclobutch

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Re: Plunger Rear Sprocket
« Reply #7 on: 06.06. 2019 16:38 »
And eother buy enough bulk chain to make at least 3 chains or buy 3 .
Run each chain till it gets slack enough to need adjusting then swap for another that you have washed properly & lubricated.
So you are always adjusting on the first chain.
That way you will get 3 chains worth out of every sprocket set

Used to do that when I was bicycle racing.
Various, including ...
'58 Iron Head Flash Bitza


Online Minto

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Re: Plunger Rear Sprocket
« Reply #8 on: 06.06. 2019 20:20 »
Thanks, I’d not considered the speedo discrepancy, but thinking about it I’ve a suspicion that it might be calibrated for solo. Indicated 55 feels more like not quite 50 to me. Of course, that could just be that I’m used to a much less sedate pace on bikes.
Cheers
Jase
52 A10 plunger