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Hi all,
Does any body know what the speedo drive gearbox ratio is on a yellow widecase desmo single with ceriani forks and disc brake.
Cheers, George

Hi George

i think you need the one with a 20/10 ratio . that is the one for a 18 inch wheel and Smiths or Veglia clock

Eldert

Hi Eldert,
I have what I hope is the original speedo drive gearbox for this bike, but I didn't know the ratio.......what I am trying to do is replace the rubbish original smiths speedo head with a better quality 1960s black face 100mph smiths speedo head from a bsa b40 which I believe is a 2/1 ratio.....I have already replaced the 'rubbish' rev counter with a decent smiths 12000 rpm black face race instrument, so this would give me a matched pair, I have of course put the two original 'rubbish' instruments away safe for a future owner someday!
thanks for the info.
cheers,
George

If you have the original you can count the input/output turns. I did mine at the weekend, so can tell you a 175 has a ratio of 1.8. Drive tang is broken off mine, so I'm looking for a replacement.

If there's still some of the drive tang there, it can be repaired to usable condition by soldering or brazing on a new piece of sheet metal to it.

Same as Jordan, I have repaired several of these with a damaged or broken tang by silver soldering a new bit of steel on.
Cheers,
George

The whole tang has disappeared so I'm not sure there is enough material left, but will have another look later.
Are there any bits inside which would be damaged by heat?

The tang is part of the sheet metal pressing that lifts off the gear.
So only the sheet metal gets brazed.
Usually the tang breaks because the gear dries out of grease and puts stress on the tang which is the first piece to wear and break off

Graeme

Persuaded the steel drive plate off carefully. I think I'll make a new one. Hopefully can get one of the CAD jockeys at work to make a DXF profile so I can get it laser cut, but could do it by hand if needed.