G11CS Speedo Drive Gearbox

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G11CS Speedo Drive Gearbox

Postby DavidHulse » Tue Jan 05, 2021 4:51 pm

When I acquired my 1958 G11CS basket case twenty years ago, it had a rolling chassis which included, fitted on the rear wheel (a 1995 one with metal drive pins), a silver-coloured streamlined speedo drive gearbox. This was in the "underslung" position, driving from the bottom of the spindle rather than the usual top mounting. I also got with it a spare back wheel (1958 type with cush drive pins), which had a black-coloured square shaped speedo drive mounted on top.

When I finally got them both dismantled, I discovered that the black one is a 2:1 ratio, while the silver one is 19:10 ratio. I've decided to use the black 2:1 box in my reassembly (with the 1955 wheel as I don't have a cush drive hub/drum), as this appears consistent with the pictures in the manual. But what does the silver 19:10 box belong to, and could I use it? Could it be from a "lightweight" model, which is pictured in the manual? I note that the speedo cable originally on the bike was mangled and broken; could this be because it had been running backwards?

One other point to note is the bike was fitted with a 150 mph speedo, which is in poor condition and needs replacing or reconditioning. Should I change this for a 120 mph unit while I'm at it?

I'd be interested to hear some views. Thanks.

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Re: G11CS Speedo Drive Gearbox

Postby clive » Thu Jan 07, 2021 2:09 pm

David I also have a 1958 G11CS which I rebuilt in 2011/2 as it came partially dismantled. It too came with the 1955 metal pin QD hub and brake drum. The competition supplementary spares list for 1958 shows that the hub should be a solid bolt on brake hub 021634. This was only used for competition machines. I wonder if replacement availability was poor or if the QD element of the later hub was considered useful, the pins will bolt straight into the same hub I think. Anyway mine is fitted with the QD. The Smiths catelogue shows that your bike should be fitted with the square ended 2:1 speedo drive and a 120 mph speedo. The ratio of the speedo would depend on the size of your rear wheel (19 inch standard mine came with 18 rim and spokes so that is what I built up) and tyre size and profile but 1548 would be about right.

The 19/10 is a later drivebox and was fitted to some other bikes, cannot remember which but not AMC I think. If I can help at all with your restoration let me know I put together quite a bit of info to get mine done pm or email me at adverts@jampot.com. The speedo cable should be 5ft 5ins long and fitted straight up from the rear wheel and along the top frame rail. I don't like the look of that and have run mine the more conventional way alng the bottom of the frame. Its tight especially with the oil pipes and exhaust pipe tucked above the footrest.
clive
if it ain't broke don't fix

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Re: G11CS Speedo Drive Gearbox

Postby ajscomboman » Thu Jan 07, 2021 2:53 pm

I believe that the correct speedo gearbox for a standard L/W with 17" and 18" rims depending on model size was 19:10 ratio and it was Pt No 042553 or Norton number 22888

Only the L/W CS models used the 2:1 ratio, had the same 19" rims as the H/W range hence the same drive.

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Re: G11CS Speedo Drive Gearbox

Postby clive » Thu Jan 07, 2021 7:01 pm

Yes I hadn't thought about the lightweights and their scooter sized wheels.......
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Re: G11CS Speedo Drive Gearbox

Postby DavidHulse » Fri Jan 08, 2021 10:20 am

Thanks for the replies, guys, and thanks for the offer of help, Clive. Much appreciated. Cheers. Dave.

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Re: G11CS Speedo Drive Gearbox

Postby DavidHulse » Tue Feb 09, 2021 12:11 pm

On further investigation, I now realise that the underslung gearbox, which would have reversed the direction of rotation, was compatible with the non-standard speedo head that was fitted, as that had a right angle gearbox on the back, which would also reverse the drive. That was a Smiths S660/L which, according to the Smiths list, belongs to a 1955 Vincent Black Shadow! I can't install the speedo with the right angle drive so, as it was a bit knackered anyway, I'm trading it in for a rebuilt one, S491/3/L, correct for my bike. I just thought I'd let you know the outcome of the story. Thanks for all your help.

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