Gear Design

It is interesting that the overhead camshaft drive layout really didn’t change from the very first K7 (and K10) in 1928 to the post-war 7R “boy racer”. There were certainly detailed changes along the way –the 350 and 500 models had different chain lengths, the magneto moved from sticking out the front to behind the cylinder in 1933 and the magneto became gear-driven in later years but the overall concept of a crankshaft pinion driving a gear/shaft with sprockets and chains connecting to the camshaft, together with the use of a Weller chain tensioner, remained unchanged.

This K7 came without any of these parts with the exception of a badly corroded crankshaft pinion. During the vintage years, it seems that AJS did not even change the detailed design which makes it a lot easier to reverse engineer and copy one from another bike. If it works, why change it!  Detailed measurements were therefore taken from the R10 engine and, together with a simple bit of gear design, new gears and sprockets were made.

The following simple measurements give enough information to work out the gear details:

Number of teeth on crankshaft pinion = N₁ = 20

Diameter of crankshaft pinion = d₁ = 1.375”

This allows the Diametral Pitch (DP) to be calculated by:

DP = (N₁ + 2) / d₁  =  22 / 1.375 = 16

This, in turn, allows the Pitch Circle Diameter (PCD₁) of the pinion to be calculated as:

PCD₁ = N₁ / DP  =  20 / 16 = 1.25

As the ratio of the number of teeth of the crankshaft pinion to the camshaft drive gear is 1:2 we know that the camshaft drive gear has 40 teeth and a diameter 2x the diameter of the crankshaft pinon, ie 2.625”. A similar calculation for this gear gives a PCD₂ = 2.5

The distance between centres of the crankshaft and the camshaft drive shaft is given by:

Distance between centres = (PCD₁ + PCD₂) / 2 = (1.25 + 2.5) / 2 = 1.875” ….and this can easily be checked by measurement.

These simple formulae and explanation of the various quantities can be found in the book below, which is all I have ever used to assist in making gears. There is also plenty of information online that explains all of the above.