I have just fitted one of these plugs to my BSA WM20 motorcycle.
So far I am very happy with the bikes performance on the new AV86C. The bike starts well and has mostly stopped backfiring when the throttle is shut quickly. The heat range seems about right too. I can't comment on fuel consumption as low compression military side valve 500's were never known for their fuel efficiency.
I have just ordered 4 more for my Austin 10.