A: There are two types of main spark plug seating design. Due to engine swaps & head changes.. its worth taking a minute when you have time.. to become aware of the plugs fitted to your special interest car.. particularly those with pre-80 model vehicles.
B: There's a SEAT difference mainly as show below.. it's how the plug rests into the engine head.. to seal up the pressure.
C: There are two types of main spark plug seating designs.
D: Due to engine swaps and head changes.. it's worth taking a minute when you have time.. to become aware of the plugs fitted to your special interest car.. particularly those with pre-80 model vehicles. Cars with engine swaps or upgrades are also effected. More then a few decent people order by year or in haste & get to find out on the weekend etc..but this simple one its not unavoidable.
This image below might help in the future