I recently picked up a B & S service and parts book from a fellow that used to work on small engines.
Although most of the information only goes back to the middle 80's I did find somthing I found very interestion.
A B & S service bulletin #367 and dated 5/10/60 explains that the factory has received several engines that had premature failure.
The engines had been on snow plows and the engines had been run with the choke mostly closed resulting in fouling of the spark plug.
The spark plugs were removed and cleaned in a spark plug "blaster" with abrasive material. The plugs were then cleaned with compressed air, re-gapped, installed and used.
The engine failure was from small particles of the abrasive material still on the spark plugs and ending up scoring the cylinder walls and the crankpin journal.
B & S repair shops were instructed to NOT use the abrasive blasters and to clean the plugs with solvent only.
I was thinking, just throw the plug away and replace it...duhhhh
In 1965 a gallon of gas cost $0.31 so a spark plug was probably $0.50 or less, but the income was only $7500 so that is why people cleaned and re-used their spark plugs!
Man I must be bored to be reading a repair manual for NOTHING I even own!