Which Spark Plug for GT40?

[east tx skier]

764 is one range hotter plug. With a hotter plug there is a possibility of pre-ignition (pinging) that can take place. Since it is one range hotter, probably not
an issue. I'll let you know about the 103. I really do not expect much difference from a 764. The AWSF-22C in the engine are the original plugs with 667 hours on
them and they still look new. Hard to believe they are original but given that they have a trace of black engine paint on them as if they were sprayed when the engine was sprayed at the factory leads me to believe they are original. Anyway they have the perfect very light tan color on the insulator tip so why change when they work so well.

I understand what you are saying. I'm very interested in how the 103 works for you. Presumably, I would not notice much pining in the GT40 since it would probably retard timing to prevent that. However, I did run the 764 in my old MC with a carbureted 351 HO (GT40 heads) and I never experienced any predetonation. I change my plugs every other year and they always look good. So I'm a bit resistant to change something that hasn't been a problem. However, if there is a better alternative more suitable for the application, so be it.

[boo]

I investigated the 103 vs the 764 on Autolite's website. The 103 is the correct heat range substitute yet it has a full thread shaft. Autolight's representative told me the full thread works just as well as the half-thread. Pictures are available on their website if anyone is interested. This post is purely informational as I haven't tested a full thread vs the half thread spark plug.

[gtxragtop]

The original AWSF-22C look just like (thread wise) to the APP103 I just installed. I'll let folks know how they work out as soon as it gets warm in Maine.

[lik2foot]

Autolite 103C works great 1998 sport nautique GT 40
looking for a factory extended pylon??