Can anybody out there tell me what spark plugs they are running in there Caddy flathead engine. Most of all how happy they are with them. My engine is a 1938 with 1948 heads that have the small plug hole (3/8" diameter) cylinder heads as per my picture below...
Your heads have the small 10mm spark plugs. The 1938 V-8 was the only year Cadillac used the more popular 14mm plugs, after that they were all 10mm. I have heard of drilling out the plug holes and tapping in 14mm threads for more heat range choice. I have no direct experience with this. 10mm spark plugs are available. What plugs were in the engine? (o{}o)
Only plugs I've seen in heads that I own are AC 104. They seem a little short... Being an old engine which I have rebuilt from scratch I am not willing to accept that plugs fitted are correct since they could have just been screwed into the heads to help keep the the cylinders clean as it was not a running engine.
I've decided to run with the smaller 10mm plugs for now and just make sure the gap is wide enough. The plugs I have chosen are an Iridium plug part number I will check later and post. I am told the Iridium plugs will cope better with any oiling issues. The plugs I have were a little long so I made up some 4mm spacer washers to pull the electrode out of the cylinder slightly to copy the original electrode position. I also took some more modern after market plug lead holders and made them fit my flathead engine which has provided me with a cleaner look. The Cadillac flathead engines with the overhead exhaust stuff is not a pretty look or plug lead friendly, what with heat from hot pipes in the most part. I guess time will tell if my modifications will be practical...
I am going to run with NGK Iridium spark plugs part number CR8HIX3569 It is really just a shot in the dark, but I'll report back with the outcome once this engine is on the road...
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