Hello, John. Sorry for the late reply. I am currently restoring an AJS G8 (1926) which has its original carrier and have the following measurements should they be of use to you.
The top frame, of 1/2 inch tube measures 16" long x 11" wide with two cross pieces. The front one is positioned 3 1/2" from the front end of the top frame and has a small lug for attaching to the top of the mudguard, the rear one is 11 3/4" back.
The front cross piece coincides with the position of the vertical support - 5/8" tube and the tool box space is 9" x 4 1/2". The vertical (bent) support is of 1/2" tube.
There is a vertical strap from the centre of the rear of the rack which serves both as a numberplate mounting and continues down the mudguard ending in the clip for the rear stand.
I'm at the stage where I'm still being held up by the cold weather with regard to painting the tank and chaincases etc and I think I'm running out of time to get a new stainless exhaust system made in time for this year. Still, it'll give me time to save up enough guineas to have one made next winter
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Hope this helps a bit.
Steve
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The verticals do taper in towards the mounting points.