The frame lugs are malleable Iron castings (cast iron that has been heat treated to make it more flexible) and the frame tubes are mild steel...The tubes and lugs were assembled, pinned together and then hearth brazed...I don't know for sure but I suspect a low temperature braze was used...
So, when heating the frame to make corrections try to keep away from the brazed joints as much as possible...If it is very localised damage it probably will require heat....If it is more of a 'twist' of the general frame structure that can often be corrected by bending 'cold'...The frame will need to be clamped down and suitable levers used for that process, acting on the frame where it is most practical...
I assume one of the plunger columns isn't vertical from behind, based on your description...Is it the one on the brake side where the frame has received a blow?...
Whenever I have tweaked frames and used heat I have always allowed them to cool naturally at room temperature...
I have a drawing of the plunger frame which might help with deciding where the rectification is needed...I'll try to dig that out today....Ian
Keith, I'm afraid I inadvertently sent you the frame drawing for the plunger B Series Models...I'll send you the correct one for the M Series Models tomorrow...Ian
Many thanks for the drawing Ian.
I did find the M series frame drawing online.
I will bolt in a crankcase with crankshaft and a complete gearbox, and the back wheel,
and I should be able to figure out the length of the spacers required for the gearbox mounting plates.
This frame straightening project will be a big mission.
Thanks Keith
