I am having trouble locating steel sprocket blanks, in 5/8" x 1/4" for the final drive on my B31 project.
42 tooth for the drum, and an 18 tooth for the gearbox. Is there a good supplier in the UK?
Only 5/8" x 3/8" in steel available, but not 1/4"
If I can find one, what would be a better option. Machine out the sprocket centre with a shoulder to match a shoulder machined on the drum, and then braze it on, with the idea that there would be more surface area for the weld to hold, or just machine a vee groove on each and weld it with a stick welder or a TIG
Would I weld it all the way around, or just a tack every inch or so.
Any-one have experience with this.
Thanks Keith

email (option): Atkinson

Hi Kieth..Rigid, plunger or swinging arm?..and early or late gearbox if it's a rigid?.Ian

email (option): ian@wright52.plus.com

Well Ian, maybe I'm not sure what model.
It has front frame YB for 1948, and plunger rear.
I have the later gearbox with speedo drive out the top RHS, just behind the lug for the clutch cable adjustment.
Fitted to M & B group 1938/63 (draganfly catalogue)
The engine is YB.
The steering head takes 3/16" balls rather than 1/4"
The front brake plate is 8" alloy.
I see that plunger models start at ZB for 1949, but YB is for rigid frame 1948, not plunger.
I think I have a bitza.
Front frame 1948 rigid
Rear frame 1949 or 50-57????
Front wheel off B33 1953-55
Keith

email (option): Atkinson

You do indeed have a bitza Kieth!...The sprinter I am building is a similar, but different, assortment of parts.
Ok, so the gearbox sprocket for B Series models is a 19T not 18T as standard. The 19T in 1/4" width is readily available.If you have the later version of the Speedo Drive box it will have an oil seal fitted at the sprocket end and will use a sprocket with a plain shoulder. If you have the earlier version of the box (not common) it doesn't have an oil seal and you will need a sprocket with the oil scroll machined onto the shoulder..as per WD M20. WD sprockets for the M20 are 18T..If you are in a rush C&D Autos should be able to supply one...and probably Draganfly as well. If you get stuck I can supply one but I currently don't have any in stock and won't be ordering from that particular supplier for a few weeks.
Plunger rear sprocket/brake drums are also available..try 'Monty's Classic Motorcycles'..01822 617010...He sells the UK made ones which are good quality..42T is common to nearly all BSAs of that period but don't forget to specify B31 to get the 1/4" chainwheel width..Unrelated but possibly of interest, the smaller early B31cups and cones for the steering head are the same size as Bantam ones which are still available. Also, if you have an engine with the crankcase breather situated on the drive side behind the clutch cases it is a good idea to blank that off and fit the later ,post 48, timing cover with the breather fitted there..it is much easier to get at if it requires attention at any time....Ian

email (option): ian@wright52.plus.com

Thanks for the advise Ian, especially regards the gearbox sprocket.
After restoring 5 bikes, I am getting down to the last of my parts now, and I was going to build up a B31 plunger bobber style, with bits I have left.
I will need some wheel rims eventually, and the pair of 18" WLA ones I have might do the trick (for example)
I don't want to outlay any more expense on correct parts. ie 1948 rigid or 1949 plunger.
Thanks Keith
ps I would have thought machining a sprocket blank would be a good economical alternative to buying a complete brake drum?????

email (option): Atkinson

Also consider that welding sprockets onto drums often distorts the drum even if opposite tacking (American tack) is employed.

Hi Kieth..How economical a repair would be depends on whether you have the facilities to carry out the work yourself..if you have to buy blanks and then pay for machining and welding they could work out not to be very cheap. In the case of the gearbox sprocket I think they are only about £17-19 so it hardly seems worth it. The rear brake drums are quite a bit dearer so that is the one to consider more closely.
As Fred rightly pointed out though, doing the job is not without potential problems re. the drum concentricity, weld pentration on the fairly thin drum and the strength of the welds on the finished job.
I put a sprocket ring onto my 51 B33 (for which there are no drums available)..Everything went OK but 3 years later on the road between Tongue and Thurso in the North of Scotland and 16 miles from the nearest town or village the welds fractured.. ...Ian

email (option): ian@wright52.plus.com

I take your advise about welding.
These drum/sprockets must be formed by forging, then machine finished, sprocket and all.
Mine still has good thickness in the drum, but just noticable wear on the front side of the sprocket teeth, unfortunately.
I'm going to use it like it is. It still has lots of miles left on it.
My first bike was a Yamaha 650 twin, and if accelerated too much, the chain would jump, what used to be teeth, and the chain would do 6 or 8 laps, before the revs dropped back, and I got chain traction back again. Done 100's of miles on it like that. Chain never came off though.
Thanks Keith

email (option): Atkinson

That's funny that Yam chain story, i also as a teenager considered preventative maintenance something for old people with too much time on their hands, now i AM one of those.
I even examine when turned how central the chain enters the sprocket now days as correct wheel alignment doesn't guarantee correct sprocket alignment specially when bits have been swapped about.
Ive just shimmed an engine sprocket 3mm on a pre unit bsa which was out enough to cause rapid wear.

I set the drum from my royal field wd/c up to a guy in shrops. he machine the old sprocket off put a new blank on and machined the teeth. looks good and seems to work fine.

email (option): v8zspeed @ yahoo.com