Thursday 10 March 2022

The AJS 33/7 Trophy Model Restoration: Engine Plates

Although the bike had come with the engine and gearbox in place, it had been immediately apparent that new rear engine plates would be needed. Apart from anything else, the plates fitted to the bike had been made to fit the Burman BAP gearbox and I planned to fit the correct Sturmey Archer ‘box.

I drilled an additional hole at the bottom of the engine plates to check the positioning of the SA EIV gearbox to ensure that it would actually fit OK.

Plenty of space ...unlike the V-Twin where I had to fight for every millimetre.

I had also found that the “fit” of the engine plates was poor – quite a bit of leverage was necessary to get them to fit at all. The reason for this is quite simple: the positioning of the holes was not accurate; the distance between the rear 7/16” holes was 0.030” out of position. It might not sound much but this amount is more than sufficient to make it nearly impossible to get the stud through the frame and plates. Secondly, it turned out that the width of the engine, shown below as being 3.57’’

was  0.092’’
greater than the width of the upper lug on the frame

The measurement on the lower part of the frame lug was the same. One would have expected the dimension across the engine and the frame to be the same, within a few “thou”, and I really have no idea how this could happen unless there was something of 0.046” thickness that was originally positioned between the engine plates and the frame on each side that would take up this amount of clearance, but I can’t imagine what that would have been. Manufacturing error? It’s hard to believe that the Collier brothers would have been this sloppy in making one of these top-of-the-range bikes. Did someone reduce the width of the frame lug at some time? I doubt it ....why? Another of life’s little mysteries….

Whoever made the engine plates that were in the frame had recognised this problem and milled a whole section at the crankcase-end of both plates to reduce their thickness to compensate for the difference.

I planned a much simpler solution than this.

The first step was to order 2 pieces of 300mm x 300mm x 6mm steel plate and using the original as a rough template for the profile, to mark out the shape with white paint.

Some modifications were made to the original shape by increasing the height to accommodate the upper gearbox mount and simplifying the internal contour. Each plate was then cut out using the plasma cutter and after removing the slag from the cut edge, the two plates were clamped together to clean up the edges on the milling machine.


The external rounded corners and edges were cleaned up with a flapper disc.

The centre-to-centre distance of the rear engine plate frame-mounting holes was measured accurately with the Vernier by inserting 2 pieces of 7/16” ground bar into the frame lug holes and then using the DRO on the milling machine to position the holes for drilling. The 3x 3/8” holes for the engine mountings were drilled in-situ with the engine mounted securely in the frame and with the rear plates bolted up rigidly to the frame mounting.

After drilling the lower gearbox mounting hole for the pivot, the upper slot – which needs to be at a radius of 7.75”, was machined by mounting the plates on the rotary table and rotating the plates to give the correct arc.

Finally, to accommodate the difference in width between the engine and the frame I found stainless steel washers that were exactly 0.046” thickness and silver soldered these on the inside of the plates to get the same clamping thickness when assembled. After grit blasting to clean up the black steel surface from the rolling process during manufacture and flux from fixing the spacing washers the plates were ready for first assembly into the bike.

I also decided to make new front plates to eliminate some of the additional weight-saving holes that had been introduced and all the existing studs were replaced with new ones made from EN24T.

A new magneto platform was made and a reconditioned BTH racing magneto fitted.

This magneto is not as tall as the one that came with the bike and I had already checked that it avoids interfering with the bottom of the carburettor.

The last job was to make spacers to position the gearbox in the correct place and to firmly clamp it at the top and bottom. A spacer has also been inserted between the upper gearbox support lugs to avoid a large bending load on the lugs that would inevitably result in them cracking.

There will be some further work on the engine plates at a later date for the footrests and primary chaincase but, for the moment, this is sufficient to set up the primary drive and the internal magneto and camshaft drives.

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