Friday 7 August 2020

The Gearbox

The Gearbox casings that were fitted to all the 350cc models – SV, OHV and OHC are the same as far as I can tell as I have examples of each. The gearbox on the OHC K7 was stamped “OHV” and that suggests that the OHV and OHC engines were fitted with the same gear cluster and the factory stamping was to differentiate between SV and OHV/OHC gearboxes so that they didn’t inadvertently build the wrong gearbox into a bike. I also know that the K7 gearbox was the one originally fitted to the bike as the original brass plaque with the correct number is still attached to the front cover.

There is a substantial difference between the gear ratios of the SV and the OHV/OHC models.

The SV has the internal ratios: 1:1, 1.68:1, 2.56:1

Whereas the OHV/OHC has:  1:1, 1.23:1, 1.88:1

Having ridden this bike in its original SV form, I can attest that the ratios are extremely wide and really not suited to what is intended to be a sporting vintage motorcycle.

My sketches of the number of teeth of the gears clusters from the SV and OHC/OHV gearboxes are shown below. The mainshaft is on the left and the layshaft on the right.


By a stroke of luck, I subscribe to an ebay alert for “AJS Big Port” and some time prior to the AJcette project, a set of brand new close ratio gears had come up on ebay. These had been made by a guy in Australia and I bought them without hesitation. I had intended them as a replacement set of gears for a Big Port project (I have 2 of these to do at some stage….) but they ended up in this bike.

The tooth-count on this set of gears is slightly different from the OHV/OHC gear cluster and have the ratios:

1:1, 1.36:1, 1.87:1

resulting in a slightly lower 2nd gear, but they are otherwise essentially the same, as shown below.

The quality of these gears is excellent and, together with a new set of quality bearings, the gearbox was rebuilt.

I have never met or had any correspondence with the Australian guy that went to the trouble of making these 6 gears and 2 shafts but, whoever you are and if you ever read this, Thank You.

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