I have picked up a nice BM20 plunger basket case to restore, and I need some advise please.
The rear frame section and the gearbox mounting plates are a problem. (I'm sure the most obscure problem encountered with a basket case)
The rear frame has the cast outboard stub that the brake pedal pivots on.
I dont have the gearbox plates to frame spacers, as the dimensions of these will dictate final drive alignment and also plunger column angle duality, (not sure of the right word here) Angular Alignment, relative to each other.???? ie The axis measurement through the plunger columns are different to each other top to bottom about half an inch looking from the rear, so I need to get that right.
If I can find the correct dimensions for the spacers, and also the size and threads for the frame through bolts/nuts.
The stub section of the rear frame for the brake pedal looks like it is been in a collision with a solid object from the front.
Not serious but the through bolt holes are slightly out of alignment especially on the brake pedal side.
I think I will need some serious heat to tweek it back.
What is the best advise for heating these sections to cherry red/plastic state. Heat the tubes either side of the casting or the casting itself. Not sure?
and also, Just let ambiant cool, or quench with water, or controlled cooling. I understand the old BSA frame iron is Austenitic.
I'm not sure if the frame iron, and the casting iron is the same.
Thanks for any advise.
Regards Keith Atkinson
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
