Upright gearbox gear ratio hep?

Dear Nathan,
    British motorcycle gearboxes have top gear with input and output locked together 1 to 1.
The intermediate ratios could vary within limits, hence close ratio for racing and wide for trials. If your spare box selects all gears and has same case as existing one ( check width of lugs ) then all you really need to do is have same size sprocket as your existing one. This will give same top gear.
No doubt someone will have data as to what box came off, likewise what gearbox sprocket your bike had on when it left the factory.
Regards Martin

If 1st gear (kick start) pair are 34 teeth and 20 teeth, that would make it the unusual 'Daytona' first gear.  Which was used in kick start post war Manx boxes for Daytona sand racing (and incidentally makes the close ratio Inter box kick start box more usable on the road by shorting the otherwise big jump from 1st to 2nd) . Normal Norton box gear pairs all add up to 42 teeth but the Daytona pair uses smaller teeth and add up to 52 instead.

The rest of your gears as as standard road bike ratios.

Your ratios are 2.27, 1.78, 1.21, 1.00

Standard ratios (for comparison) are 2.97, 1.78, 1.21, 1.00

So you have the rather rare 1st gear pair in an otherwise standard box.  It should be absolutely fine for a  Model 18, although you will find 1st gear much higher than normal.  You might find sharp uphill hairpin bends a bit more challenging than you might like.  And certainly don't tow a sidecar or go far off road.  You might be better off buying the standard first gear pair (RGM has them) and offsetting the cost by selling your Daytona pair.  (RGM also have them).  Otherwise you could lower the gearing, but of course that would make top gear work more stressful.
 

Pages 40/41 gives good data on gears leaves and ID's
if your not familiar click on LIBRARY on our header, choose online content, scrol down to Singles Compendium and turn to page 40
 great document !
Jon

You posted before, I believe.  Certainly I'd go bigger.  The 16 was probably for side car use.  Or very steep hills...

Hi Nathan,

    I agree with all the previous comments, especially the Daytona bottom gear which will need lots of clutch slipping in heavy traffic or a pain if there are any steep hill starts. The standard solo sprockets for your bike are 20T engine, 42T clutch, 19T gearbox and 42T rear wheel which gives a top gear ratio of 4.64 to 1. This will give you a top speed of around 80mph under good conditions and a properly running engine, so 60mph cruising should be a good target speed. Later rear wheel sprockets are 43T which does not make much difference at 4.75 to 1. The 16T gearbox sprocket was used on the 500T trials machines and, I think, WD Big 4 sidecar outfits. On long open roads you could go to 21T engine OR a 20T gearbox for a slight over gear, but you might have to drop to third on up-climbs. First, check the engine performance is up to scratch.

Hi again Nathan,

    I think the layshaft is from a racing doll's head 'box which has no kickstart so the shaft runs in a bearing in the flat racing end cover. The road spec layshaft runs in the bush inside the kickstart axle shaft. Let us know how your first test runs go.