The Stovebolt Site Forums The Shop Area Engine Shop 305 V6 spark plug reach

I have a 1960 GMC 1/2 ton with the 305A V6 in it. It has been in my family since new, and I feel confident it has not been changed significantly from stock. Whenever I search for spark plugs, I am given two sets of choices: plugs with 3/8" reach and plugs with 3/4" reach. When I inherited it, it had 3/8" plugs in it and I have continued their use because I am not sure of the clearance to the piston head. Does anyone know for sure that a 3/4" plug would be ok? The closest I have gotten to an answer is on the 6066GMC site which recommends a Delco AC4 (3/4" reach) for the 305E, which I believe was after 1964. Were there any clearance differences in the combustion chamber from 305A to the 305E? Thanks for any help or input.

If you use a plug that is too long, it can get carbon buildup on the threads that are exposed in the combustion chamber, and you may not get the plug out again without damaging the threads in the head! If the plug is too short, carbon can build up in the "unused" threads in the head, and ignition of the fuel would be poor due to shrouding of the plug tip.

You can probably check thread depth, crudely, with a piece of wire with a bent tip, or a dental pick, inserted into the plug hole. Feel for the end of the threaded hole.