cooler zener diode

Tomas, 

Here's a suitable design from Lucas. 

Glad that a sensible maker would look at the drawing  and not read the text! 3/8" thick is a big bit of metal! 

Well spotted George.  Another factory publication with a typo.  Not uncommon. 

For my 650SS, I calculated the surface  area from this article but used an alloy wheel removable centre from a locally made Oz car.

Has worked perfectly for 15 years.  

BSA  bikes were fitted with a compact cast  heavily finned sink , got one for the 99. 

... if I was making one I probably would go for 3/8" thick. The Z plates on Commandos which were used as heat sinks are pretty substantial!