Gearbox sprocket size

[bonny]

The old gearbox sprocket on my flash was 17 tooth, it had a trials gearbox though. I never rode the bike before restoring it as it was in a shocking condition. Somewhere along the line i must have bought a gearbox sprocket, as i have a new one which is 21 tooth, the manual says 19 t is standard. Will the 2 extra teeth make any odds ? Drop the revs for a given speed ? Also the gearbox is now standard.     

[Greybeard]

quote author=Greybeard link=topic=10950.msg82501#msg82501 date=1471297538]
I had a 19 tooth gearbox sprocket on my Plunger; presumably from when it had a sidecar. The engine always felt like it needed a fifth gear. I've recently changed to a 20 tooth gearbox sprocket. That change has made the machine much more enjoyable in traffic.
[/quote]

[ellis]

Does that not make the speedo run slow? I would have thought it should be the engine sprocket is the one to change.   

ELLIS

[bonny]

Tried the 21 tooth earlier as a trial fit, so far one problem is the fully enclosed chainguard, front section, will not fit over the sprocket. Not too worried about the speedo problem as i intend to use a cheap bicycle speedo as well as the smiths one.   

[Tomcat]

I put a 20 tooth sprocket on my Super Rocket and am now 100% happy with the gearing at 60 MPH. 1st gear is still OK. 21 T would be OK if you are doing a lot of highway speeds but with a loss of power. I've checked the speedo with a GPS and it is spot on. Looks like I 'Pulled a Homer' on the speedo! 

[Greybeard]

Does that not make the speedo run slow? I would have thought it should be the engine sprocket is the one to change.   

ELLIS

20 tooth gearbox sprocket fitted to my Plunger has made cruising at 60mph comfortable. I no longer keep trying to find a fifth gear! My speedo needed to be recalibrated. The odometer is no longer accurate but I'm not worried.

PS. The Plunger has duplex primary chain. Seems sprockets are not available so I changed the gearbox sprocket.

[bonny]

I'll stick with the 20 tooth sprocket. It will not scratch up my chainguard and probably be better on the road. 

[Dave c]

can you change gearbox sprocket with engine and gearbox in frame? my 52 plunger  a7 seems undergeared , i have an 18 tooth fitted at the moment

[Greybeard]

can you change gearbox sprocket with engine and gearbox in frame? my 52 plunger  a7 seems undergeared , i have an 18 tooth fitted at the moment

You cannot. Even if you removed the clutch and stripped the gearbox you cannot get to the sprocket nut as it's behind the primary drive part of the left hand crankcase. I had to take the engine out of the frame and remove the gearbox from the engine to change the sprocket. It was a PITA but worth the effort.

I wonder if someone ever cut a hole in the casing and made a trapdoor so they could access that nut. Sure to, I'm sure.

[Dave c]

thanks greybeard,i thought that was the case. i will have to wait until the engine needs to come out!

[duTch]

 

...I wonder if someone ever cut a hole in the casing and made a trapdoor so they could access that nut. Sure to, I'm sure.

 Yep- sure did, a bit fiddly in a bush shed with  diet floor and no power,  but works ok so far,  just need to make sure everything is perfectly concentric

 Then I used a cover from A65 (or Tri...) impossible to get the right combo seal,  so I used I think 1,1/2" x 11/16 (? shaft size),  and sleeved the OD with a short length of stainless exhaust tube... total bodge but it works, but haven't needed to remove the sprocket yet but makes it easier to get at the grub screw for the selector shaft. 

 GB, did I miss something or didn't yours also have a 47T wheel sprocket,  or was that someone else ?

[Greybeard]

GB, did I miss something or didn't yours also have a 47T wheel sprocket,  or was that someone else ?

Yes. When I got the bike it was set up for hauling a sidecar.

[Greybeard]

can you change gearbox sprocket with engine and gearbox in frame? my 52 plunger  a7 seems undergeared , i have an 18 tooth fitted at the moment

Dave, looks like this is your first post on the forum. Please, will you go to introductions and tell us about your self and BSA(s) that you own. Pictures are appreciated, (some might be of motorcycles if you like).

[duTch]

 

Yes. When I got the bike it was set up for hauling a sidecar.

 Ok, so you'd obviously already changed the rear in the rebuild- just curious as I originally inadvertently used a A65 bolt-on sprocket chainwheel with 47T and it revved like buggery (seemed like a good idea at the time & didn't know better )

[Greybeard]

GB, did I miss something or didn't yours also have a 47T wheel sprocket,  or was that someone else ?

Well remembered duTch. I had to search my posts to find this from ages ago, (2011). My second post here.

Hello folks.
 
I have a low mileage 1955 plunger Golden Flash A10 'mouldering' at the back of my garage patiently waiting for me to restore her.

I got the BSA in the early 70's as part payment for an Ariel Square Four that I had bought for £60 without a Registration document. The Square Four was a 1938 4G model with rigid rear end and girder forks, like this one, only not so shiny http://tinyurl.com/3mb8dzo.
Someone had made a start on doing the bike up but it was not running. After stripping paint from the front number plate I was able to make out the rusted outline of the original registration number. I asked the DVLC for a replacement logbook hoping that the bike had not been stolen. They sent out an inspector who told me that the bike had not been taxed since it was two years old! My theory was that the owner had been killed in the war and the bike had not been used since then! I never managed to get the beast to run; probably a weak magneto. Anyway, I put an advert in Exchange & Mart and got £60 cash plus the A10 in exchange for the Squariel.
I soon had the Beeza looking really nice. I replaced the sidecar ratio rear sprocket for a solo one and really enjoyed riding the bike. It looked gorgeous. I was very proud of it!

One day in about 1975, while the bike was on its centre stand outside my house an idiot in a car reversed into it and knocked it over, smashing the headlamp and denting the headlamp cowl. I was heartbroken and have not ridden the bike since then.

However, now I'm retired I intend to get the BSA going again. I just need to get a few things cleared out of my garage and then I'll make a start.

It's going to be expensive; I'll need wheel rebuilds, new exhaust system, new tyres and tank replating. I am able to do the mechanicals and paint myself; I used to be a car and motorbike mechanic.

I'll try to remember to keep in touch with this forum.

Neil Ives

I have kept in touch