Chrono Numbers

[Slymo]

With my 58 Super Rocket came a couple of instruments that were close but not perfect. The tacho was a backwards drive one from a RGS or similar which I have modified to run the right way. The speedo is an SC 3305/08 of which I can find no mention anywhere.SC3305/04 being the proper instrument. Does anyone have any idea what it is off? I can make it tell the right speed with the addition of some weights but the odometer needs to be right for 1440 revs per mile.

[JulianS]

The Smiths lists show 3305/08 as being fitted to the 1958 B33 Special, whatever that might be.

[Slymo]

Ok cool. Right year then. Seems the odometer is right too, made sense when run up on the lathe.

[KiwiGF]

Ok cool. Right year then. Seems the odometer is right too, made sense when run up on the lathe.

After all this it’s quite possible the number on the dial does not have any relation to what’s inside  so testing on lathe is a good idea.....as it’s not hard to replace a dial, a local instrument guy prints the dials off cheaply with no number or one from a limited range, but he won’t do “special number” requests  it can be hard for him to change the ODO accuracy, but it’s easy to get the speedo accuracy right. He will take on pretty much anything tho (apart from getting the number right!)

[Slymo]

Yes my mate who is a genius with these things managed to convert the later backwards driven tacho to the right direction and with the addtion of four weights got the speedo reading a proper speed for te rpm. Lucky for me it seems the odo is already set up for 1440 revs per mile

[KiwiGF]

Yes my mate who is a genius with these things managed to convert the later backwards driven tacho to the right direction and with the addtion of four weights got the speedo reading a proper speed for te rpm. Lucky for me it seems the odo is already set up for 1440 revs per mile

Id be interested in the details on how to adjust weights to change the speedo, my b31 reads 80mph when I’m doing 60  at first I thought I had the quickest (std) b31 ever.

[Slymo]

There are clip on weights on the oscillating shaft that you can see swinging back and forth when you drive the speedo. I believe (although I could have this backwards ) that the more weights the slower the oscillation which increases the sample size making the needle go further. You should by that logic have too many weights there. If not then ive got it wrong. It the part described as the regulating washer in the diagram.

[KiwiGF]

There are clip on weights on the oscillating shaft that you can see swinging back and forth when you drive the speedo. I believe (although I could have this backwards ) that the more weights the slower the oscillation which increases the sample size making the needle go further. You should by that logic have too many weights there. If not then ive got it wrong. It the part described as the regulating washer in the diagram.

Thanks, I’ll have a look inside it 

[Bsareg]

There are at least two types of escapement wheel. The brass centre weight is thicker on the 120 than the 80mph so it's difficult to change an 120 into a 80 without changing the wheel, but normally, by adding weight the recorded speed will be raised.

[KiwiGF]

There are at least two types of escapement wheel. The brass centre weight is thicker on the 120 than the 80mph so it's difficult to change an 120 into a 80 without changing the wheel, but normally, by adding weight the recorded speed will be raised.

I’m fairly sure the speedo is a 120mph, part of the speedo inaccuracy I have is due to a bigger than std rear wheel sprocket, 46 teeth instead of 42.

Will I have to source a new weight (s)? Or will I be able to adjust (remove?) some that are already there?

[Bsareg]

If there are weight clips fitted, they are easy to remove. I think you need to remove a tenth of the weight from the wheel (42/46= need to increase escapement wheel speed by a tenth in order to decrease needle reading by a tenth). Probably best way to check is to drive it by a stable speed (lathe etc) and the aim for a ten percent reduction. To change the wheel needs a steady hand if you're not to damage the hair spring.