Tuesday 2 February 2021

Fixing the Gearbox in Place

Whilst the engine was now securely fixed in the frame and there is large “hole” in which to insert the gearbox, the gearbox requires securing to the engine plates. Although the gearbox fitted in the “hole” the amount of space available severely limited the movement for adjustment and the curved edge on the front of the plates was therefore flattened to allow longitudinal movement.

The flats on the engine plates have allowed movement to give approximately 2 links of adjustment of a 5/8” pitch chain.

At this stage, the gearbox is resting in position on a couple of blocks of wood and held with G-clamps. The next stage is to machine an aluminium block that would be sandwiched between the engine plates and with through-holes (or, to be more precise, slots) through which would pass one set of gearbox mounting studs.

This was made of 6082 Aluminium alloy – a medium strength aluminium and the highest strength of the 6000 series alloys.

Longer studs (EN24T) were made for the gearbox

and the alloy support block together with the engine plates were drilled and tapped to accept 3x 5/16” high tensile BSW bolts on one side


and a steel clamp with 2x slots for the gearbox studs to pass through and 3x  5/16” BSW  cap screws on the other side.

This entire collection of bits

now provided the means to secure the gearbox to the engine plates and in the correct transverse position for chain alignment and vertical position for the chain to avoid the intermediate rear frame tube.

However there was one final job to make this work. When the gearbox was slid into the fully-forward position it was found that the gear lever (referred to in the Sturmey Archer spares book as the “Rocking Shaft Lever”), located at the bottom of the gearbox, would foul the engine plates when selecting top gear (the fully up position) and a new curved lever would be needed to avoid this problem.

The lever was first sketched and cut out in cardboard


and then a new rocking shaft lever was made in 2 pieces; the curved lever was made using 4mm thick mild steel (leftover scrap from the engine plates) and the cylindrical portion from silver steel. The 2 pieces were fixed together with a square section and then silver soldered. The tip of the rocking shaft lever that engages with the Sliding Gear Fork (also SA terminology) was hardened and tempered.

There is one small outstanding job remaining, namely to make 4 dome nuts rather than the off-the-shelf nuts used to hold the gearbox in place but that can wait for another time.

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