classic motorcycle forum
Motorcycle Discussions => British Bikes => Topic started by: Geoff on April 09, 2012, 11:16:55 AM
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Hello from a newbie.
My '69 BSA A65 Thunderbolt has a kph speedo which I would like to change to a normal UK mph type . Is it as easy as just changing the instrument over or do I need to change a drive gear somewhere too ?
TIA
Geoff
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Yes, it should only be necessary to change the speedo.
However it is wise to check the speedo drive gearbox ratio. If the ratio is 2:1 (2 cable turns to 1 wheel turn) then you'd need a MPH speedo marked "1600", if it's 1.25:1 then you need a MPH "1000" speedo.
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L.A.B
Thank you for a swift & erudite reply !
Geoff
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Doesn't really matter whether it works or not on those old bikes. Being both cheaply made and 40+ years old they soon died a death, as will the new one, should you replace it.
Not that Smiths were any worse than any other maker, and it wasn't until Nippon Denso came along that a working speedo was considered a possibility on anythinmg more than a couple of years old.
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Yes, it should only be necessary to change the speedo.
However it is wise to check the speedo drive gearbox ratio. If the ratio is 2:1 (2 cable turns to 1 wheel turn) then you'd need a MPH speedo marked "1600", if it's 1.25:1 then you need a MPH "1000" speedo.
On inspection the kph face is marked "1000" ...does that indicate that a mph speedo with 1000 on the face is correct ?
Geoff
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Doesn't really matter whether it works or not on those old bikes. Being both cheaply made and 40+ years old they soon died a death, as will the new one, should you replace it.
Not that Smiths were any worse than any other maker, and it wasn't until Nippon Denso came along that a working speedo was considered a possibility on anythinmg more than a couple of years old.
My kph speedo has worked well for the last 8 years ...
Geoff
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On inspection the kph face is marked "1000" ...does that indicate that a mph speedo with 1000 on the face is correct ?
No, it means you have a KPH "1000" cable turns per kilometre speedo which would match a drive box ratio of 2:1 (and a KPH 1000 item probably indicates that the drive box is a 2:1 unit however I would still check it to make sure).
1 Km = 6/10 mile (approx.)
Thus a "1000" turns per Km speedo or a "1600" turns per Mile speedo both require a gearbox drive ratio of 2:1.
A "600" KPH speedo or a "1000" MPH speedo would both require a gearbox drive ratio of 1.25:1 (or "15/12").