AMC gearbox doesn't fit

[Dave Sharp]

After fitting the engine into my 1956 G80CS I then fitted an AMC gearbox using the genuine steel plates and not home made alloy ones, but the rear of the gearbox catches on the off-side swinging-arm cotter pin which pushes the gearbox completely forward in it's adjuster slot.
Anybody else had this problem?

[John Jarrett]

The 1956 G80CS originally had a Burman gearbox, not AMC and the Burman gearbox is very tight to the swinging arm. I have never tried to fit an AMC box to that frame. Others may have done so and be able to assist.

[Dave Sharp]

I originally fitted a B52 gearbox but it too wouldn't fit. The rear of it catches on the swinging-arm. That's why I decided to fit an AMC gearbox.

[dave16mct]

Strange, I would have thought either box would fit in a 56 frame. I had a friend who had a 56 with an AMC box. It was a G3LS but same size frame etc. Have you got the correct engine plates? I usually fit the plates, gearbox, then the bottom end, then build the engine up. How far does the cotter pin protude? I can only think you've got the wrong plates?? Do they project back and fix to the frame with 2 studs at the top or 1?
Dave.

[Dave Sharp]

The plates are correct as far as I know. They came out of a 1956 G3LS and have two studs fixing them to the frame at the rear. The cotter-pins were purchased from the club spares scheme so I have to assume that they are the correct length and don't protrude too far.

[dave16mct]

Start by fitting the engine plates with the 2 studs at the back and the footrest bar. All loosely fitted. Then fit the box. Then the engine.

[Dave Sharp]

I'll give that a go and see if it makes any difference. Thanks

[Dave Sharp]

I've found the problem!!!
The problem is with the main frame. It's the swinging-arm pivot that's different in that it sits proud in a slightly forward position compared to other frames that I've compared it to, which are more or less 'flat' with the vertical seat post part of the frame.
Thank you for all your suggestions

[dave16mct]

It sounds like you've got a 1955 frame, registered as 56. This would have an alloy casting that the swinging arm fits in. If so you need 55 engine plates which are shorter.
Dave.

[GOLDSTAR]

my frame is from 1954 what I call the three part frame that is that is has the removable alloy pivot casting, I have a 1957s/s C80cs fitted and an AMC box originally I had a Burman but the AMC fitted without any trouble, kind regards