Re the discussion of cylinder heads and secondary spark plugs, it's worth mentioning that when installing the plugs it's a good idea to shim them (if necessary) so as to bring the plug tips flush with the surface of the combustion chamber. This in such a way that the tip of the center electrode is even with the chamber surface and the ground electrode is positioned in such a was as to not shield the spark from the firing area. This is called "indexing the plugs". It may make a small difference in efficiency. I did mine in this way when I re-did my heads and I can't say it made a noticeable difference. If one is using the Champion G59's for the secondary plugs, they have a long reach that will extend quite a ways into the combustion chamber if not well shimmed. This is a situation just asking for pre-ignition. On the other plugs (magneto), the plugs were recessed somewhat - the "reach" of the plug wasn't long enough to bring the plug face flush with the surface. The spark was slightly buried into the head. I don't remember what plugs were in the engine that caused this situation but I replaced them with Denso's with the proper reach. Since the heads are off might as well index the plugs, was my thought. Mike Stirewalt KSEE
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