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My 1954 359 L8 currently utilizes Autolite copper resistor 308 spark plugs. The gap is 0.025. Thread size is 14mm. Reach is 3/8". Heat range is 12 (cold). Seat type is gasket and tip is short projection. All works very well. Does anyone know if there is an iridium or platinum cross reference? What are the pros and cons of a modern hotter plug in a 1954 359 aluminum head should there be a cross reference? Holley MSD has a 14mm, 0.4375 reach, heat range 6 (hot) iridium plug. Will a longer reach be detrimental? Will things go boom and melt? Thoughts are welcome.

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Absolutely, positively do NOT use a longer reach plug or serious engine damage will result!!

With no lead gasoline the standard Autolite 308 plug should give good service for 12,000 or more miles. Just my opinion but I can't see where a more exotic plug makes any sense.

As O_D said, you definitely want to do a quick measurement with the valves fully open before using any longer reach plugs. The plug hole is directly over a valve (exhaust I believe) and with the valve fully open there may not be enough room for anything longer than what you have.

The valve clearance to the head is so small issues with valve edges hitting the head have also been found when heads were slightly milled to clean up the surface. In some cases people had bolted a freshly cleaned head down without checking clearance and then cranked the engine and did substantial damage.

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The plugs in my 1954 Patrician are the multi-tip type that require no gapping at all and have been in it for over 40 years. Still look new but unfortunately they were from JC Whitney and I know of no modern equivalent. Not saying there isn’t one but I have no numbers.

When I first put those in and also put in graphite oil, the idle speed started getting faster and faster to the point that there was no need to push the accelerator to go 35mph and, once readjusted, mileage greatly improved. Sadly, it has regular oil in it now as the graphite was discontinued. People apparently didn’t like it because it was black to begin with so difficult to know if it was dirty. I still have an unopened can or two, though!

Thank you Owen and Howard for your quick replies. I have always valued both of your opinions.

I was taking a shot and intrigued by a hotter plug burning off deposits better and the iridium plugs feature of a fine wire electrode designed to conduct electrical energy better and increase firing efficiency. With iridium being 6x harder and 8x stronger than platinum with a 700 degree higher melting point I surmise iridium lasts pretty long.

We all agree to stick with a 3/8" reach.
God Bless BigKev and thank you !

What makes you think you need a hotter plug? Is the center electrode ceramic showing problems? If in doubt, how about a good closeup photo of the electrodes and especially the center porcelain?

All is well with the Autolite 308s. Everything runs perfect. I was just fishing to see if anyone thought modern technology might help things run better. Eg. burning off deposits better, conducting energy better and increase firing efficiency. No worries though.

Hotter plugs are applied to prevent fouling due to service like short trip usage, worn rings, overly rich mixtures, and so on. I had an old BSA motorcycle that fouled the standard plug because the capacitance discharge ignition was weak. I used a hot long reach plug and the thing always ran afterward. I got more cheers on that BSA than any Packard I ever drove.

Cooler plugs are used for high speed driving. Given the Reinhardt styled Packards have significantly downsized radiators they already run hot.

As well, isn't that Slick 50 stuff some kind of graphite thing? Somebody told me that Harley-Davidson used that stuff.

Right now I'm dealing with somebody who was involved in NASCAAR, I'll try to remember to ask him what he thinks of graphite in engines.

Riddle me this. I'm almost over it and not to beat a dead horse with a stick. On JEGS NGK has an iridium 14mm x 3/8" reach plug. Albeit the gap is 0.059. The copper Autolite gap is 0.025. I'm wonder if there is a conversion chart for gap in copper vs iridium plugs? Gapping iridium plugs is very fragile I heard. Of course the next issue is heat range. NGK might go as high as 8 vs the Autolite 308 of 12. NGK shows a GM 1973 455ci engine as compatible. This all goes out the window if I have to adjust the timing and fuel enrichment correct?

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Others can better speak to any benefits of the various materials but one question I would have on the larger gap is if a stock 6v ignition coil would have enough spark to reliably jump such a large gap.

Many 6v coils have issues even with the smaller gaps particularly when the starter pulls heavy current turning the large engine and drops the available voltage. I would think the larger gaps would be more in tune with modern 12v coils that produce 40-50k or more and should be avoided in cars in the 25-30k range produced by a standard 6v coil.

Not only is the 359 a huge engine to turn with voltage drop to be expected but the higher compression also makes for a more dense atmosphere for the spark to try and jump thru.