Speedometer

[Geoff]

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

[L.A.B.]

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. 

 

[Geoff]

L.A.B

Thank you for a swift & erudite reply !

Geoff

[Rex]

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.

[Geoff]

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

[Geoff]

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

[L.A.B.]

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").