[matt68gto]

I have a 77 400 block with 69 heads and a 1975 HEI ignition. What plugs should I use? I bought ac delco r45s. Is this plug to hot? The book said r43s came with the points ignition in 1969. Someone told me if you use a plug that is to hot you can burn up a pistion. Is this true?
matt

 

[lars]

The plug does not have anything to do with how hot the combustion process runs. A hot plug cannot burn a piston - that's nonsense. A "hot" plug has a longer heat path from the electrode to the body of the plug, so the tip of the plug stays hotter to avoid carbon fouling. It has to do with the running temp of the plug itself. A cool plug dissipates its heat faster, so the tip of the plug is a little cooler. It will not affect engine temperature or combustion temperature. Use the stock plugs for the 69 GTO heads. I have a personal preference for the Autolites.