SPEEDO DRIVES, CHRONOMETRIC AND MAGNETIC
British bikes with Chronometric speedos , say up to 1963 use 2:1 speedo gearboxes. The size of the hole for the spindle may vary and the drive direction can be reversed by taking it apart.
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SPEEDO DRIVE

TELBG5330/168
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Ratio15:12
Triumph T120, TR6 1968-70

SPEEDO DRIVE 2:1
TELBG5330/287
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19mm hole

SPEEDO DRIVE 15/12
TELBG5330171
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speedo drive, ratio 15 : 12 (06-0627)
Norton Commando 1968-74 (NOT MK3)

SPEEDO DRIVE 2:1 HOLE 16MM
TELBG5330/247
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SPEEDO GEARBOX AJS/MATCHLESS
52283-5
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Hole 15/16"
Ratio 2/1
All AJS and Matchless heavyweight singles and twins.

SPEEDO DRIVE 125:1
BG5330/164
!
Ratio 125:1
Triumph 650/750 1971-77