Cylinder base compression plate

Giver you the facility to lower/adjust the engines compression ratio, with barrels and heads having been skimmed over the years they are useful for lowering engine C.R. down to something like std. Or lowering a combat engines C.R.

Its also helpfull to ease kicking over if in age your strength is falling and Arthritus is looming !. Some bikes like the Nomad were fitted with very high comp pistons that almost need race fuel to avoid pinking and knocking at low revs . 

…was at the forefront of their minds. What was variable in the day was quality of fuel in all parts of the world. 
J