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Spark plug question


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The stock recommendation for spark plugs on my 54 Cadillac is AC 46-5. I was running AC

46 and it is definitly a short plug in as how far it extends into the combustion

chamber. I have switched to some Autolite plugs that are in the same heat range

as the 46-5, 46, etc. Even though these Autolite plugs (A82) show to be used in

place of AC 46-5, etc, it is noted that A82 is an extended tip spark plug. With

these extended tip plugs the electrode is definitly more exposed (sticking out

at least 1/4 inch longer). Initially I made sure there was no contact with the

piston. That was fine and I would think the extended tip would give me better

combustion, but I thought I would check to see if anyone had any specific

knowledge of the workings of this type of plug in these engines. It runs fine

now but I would hate to burn a hole in a piston when the car is hotter and

engine is working hard. Any thoughts would be greatly appreciated.

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I'm surprised your Cadillac calls for a AC 46 heat range plug. The only car I have that calls for a 46 heat range is my 76 Olds, but that was because of the emission control system and they wanted a hotter plug , I get better results with a 45's in the Olds these days. My street/track( road racing) Pontiac's ( a 1962 428 Catalina & a 1969 455 LeMans use AC 44's and they never load up on the street. Extended tips are fine, but make darn sure that you look inside the combustion chambers to make sure no threads from the plugs extend into the chamber ( this is why I use A/C plugs ) or you will have to shim and index them so they don't. If you allow any plug to extend it's threads into the chamber you will allow carbon to build up on the exposed threads and when you go to pull them out you'll damage or pull the threads out of the head. This is especially true with aluminum heads. On my air cooled VW's I always use Bosch plugs for the same reason- use a extended tip Autolite with exposed threads on a air cooled VW-and pull the plug hot instead waiting for the engine to cool down and I can guarantee you will be doing a head repair. Those air cooled VW's use the same thread length as a old 14mm domestic GM car. You'll notice that most of the Japanese cars when they started using aluminum heads (mid 60's in Nissan's case) extended the length of those 14mm thread plugs to about a inch long for safety reasons.

For your Cadillac I would use exactly what Cadillac calls for because they know the relationship of where the plug to the combustion chamber is and how the flame front is to be propagated in the chamber. Extended tip plugs were around in those times too and cost the same to manufacture so that is not a issue.


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After looking into it, I too am a little surprised the 1954 Cadillac recommends an AC 46 heat range since from listening to other 50s Cadillac owners regarding AC, Champion, and Autolite as well as the specific recommendations of Autolite in their 1958 spark plug catalog, one heat range is specified for Cadillac 1949-54, then the next coolest range for Cadillac 1955, and finally the next coolest range for Cadillac 1956-58, so basically 3 ranges in three years. I am just wondering what factors are causing this drop in recommended plug temperature. Especially puzzeling to me is the drop between 1954 and 1955. Any thoughts would be appreciated. Jim

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I got it online, so it must be current. It lists other brands than AC if you want to try another brand...B

What's so strange about it is that it covers three heat ranges for one year of one motor. I'd call that totally noncommittal. That must match all the warning labels they put on their products these days. In other words, steer clear of liability.

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