Hi more info required!
On 16h and big4 frames pre war, is there any differences in the width of the engine and gearbox mounting points.
At the front where the engine bolts in the frame at the casting with 2 holes.
Also where the gearbox bottom mounting to frame width?
The reason I ask is I seem to have one crankcase with 2.125" wide mountings at the front and 3.25" at the rear.
The other crankcase is 2.25" front and 3.3125" at the rear.
Also the gearbox shells I have seem to be different in widthat the bottom mount too. (Dollshead shells)
But the first ones have been bodged and welded so it is difficult to tell if they are correct. So I was wondering if there were changes.
I have checked three of the prewar engines I have; (16H, Big 4 and 19) and all have the same dimensions across the top of the crankcases.
All are 2.25 inches at the front and 3.3125 inches at the rear. As the frames only had the models stamped as part of the frame number post war then it is logical that the same diamond frame would fit all s.v. and o.h.v. model crankcases.
Hope this helps
The 1931-39 non cradle frames came in a variety of specifications, although the crankcase lug widths are all the same on SV and OHV.
The Model 18, 16H and Big4 frames are the same, but the Model 19, trials specification and Model 50 are all individually different. The 350cc crankcases are smaller diameter than the bigger engines, so interchanging the 350 frame/engine parts with other models in not easily done.
The pre-war numbering system is very basic, so a Factory Records search is more useful when trying to identify the major components.
Re. your gearboxes, these should all be the same width, but they were used in large numbers for grass-track, sprinters and specials after the war so most likely to have been altered.