Edmunds head sparkplugs

[martybose]

Awhile back on the old forum I had posted about my surprise at finding that I have been running the wrong length plugs in my Edmunds aluminum head. Like a lot of people here, I had been running Autolite 306 plugs, and was very happy with them. Then I inadvertantly stripped the threads on #1 cylinder, but was intrigued to see that there were more threads below the stripped ones. So I dug out my OEM iron head, and it indeed was 3/8" thick at the sparkplug threads. I then measured my Edmunds head, and found out that it was 1/2" thick! So in the Edmunds head, instead of the 306 sticking out into the combustion chamber, it was actually inside the sparkplug threads.

So I called up Autolite tech support, told them about my engine mods, and said I wanted a 1/2" reach plug that was one or two heatranges cooler than a 306, and would work well with my Langdon HEI unit, should I ever get around to installing it. They recommended an Autolite AP425, which is a platinum plug. I finally received that Back-Tap that I had ordered, and last weekend I cleaned up the threads and installed the new plugs (still using the stock ignition system).

The interesting thing to me is that the engine runs even smoother now than before, and I found that I can get in the car, turn on the ignition switch and hit the starter button without even touching the throttle pedal, and the car will start up and idle. I used to have to fiddle with the throttle to get it running, but not any more! It was so smooth that I wound up turning the idle speed down.

I would strongly recommend that anyone running Autolite 306 or 308 plugs in an Edmunds aluminum head on a 218/230 check to see what their thread depth is; if it is 1/2" like mine, it might run better with an AP425 plug!

Marty

[bob westphal]

Installing the old plugs in a high comp head is a very comon mistake. The thing to remember is that the the spark was designed to be in the combustion chamber not up in a hole outside of the chamber. By the way what is the compression ratio of the Edmunds head?