No worries, I understand what you mean. The amount of advance appears to only be about 2-4 degrees so the difference between retarded and advanced can be compensated with gap timing alone.

MarkT your replies made perfect sense. As the owner of a 76 DT175 it was perfectly clear to me what you were saying regarding putting cam in a fixed position. Your 7000 plus posts are a testament to your knowledge and level of helpfulness.

Yep agree, because to get timing right with cam on retard i'd wildly guess you'd need a 40 thou gap.--or something not feasible. Can you slot the back plate a little & get away with it or would replacing countersunk screws with flat head screws or bolts rub on the flywheel.