[dave dale]

If I may put in my ha'p'orth - in the early 2000's I used to run my 403 on the NGK C7HSA plugs which are recommended for ' touring ' engines, and suffered occasional plug fouling. I switched to the recommended hotter running NGK C6HSA for ' sports ' engines with great success. The critical thing is to make sure that the hotter plugs do not cause pre-ignition ( not always audible ). Quite how how I was supposed to detect non-audible pinking without taking the head off was something that I never found out!

For what it is worth, may I also share the remainder of my meagre knowledge. The NGK CR6HS and the C6HSA plugs run at the same temperature - the 'R' denotes a 5 k/ohm built in resistor to suppress ignition noise on radios, televisions etc. The 'A' in C6HSA denotes a difference in length of the cylindrical collar between the hexagon and the gasket, which is not important in Bristol 2-litre engines.

If you fancy trying N.O.S plugs ( if you can find them ), the original plugs for the 403 were KLG P.TEN L80 or KLG PEN L70 at first. Later, Champion Z 10 plugs were used.

To the best of my knowledge the above is accurate but please don't rely on it!
Dave Dale.