BF82 (FORD AUTOLITE - MOTORCRAFT) was the correct heat range for the Y-BLOCK. With PERTRONIX (or higher compression), I think you would want a cooler plug, BF42 or equivalent.
BF82 and AUTOLITE 46 are the same heat range.
ASF42C is a  14mm suppression plug with a copper core. The heat range would be the same as a BF42.

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I think Motocraft BF82 are the same as Autolite 46, the other I don't know.

I'm no expert on plugs, but the 82s were used in the old 170 in six from the sixties and the ASF, don't know about the C, were used in later 351W and M motors which I thought used to smaller plugs.  My guess is that the 82s are a little hotter and would be ok in the 'Bird especially if it is getting as tired as mine.  If I'm correct about the ASFs, it'll be obvious, but earlier 351s used BF42s which might be a little colder, maybe ok for the 312.

I hope this helps, 
Don

55 Bird,  56 F100 in progress still

A friend, who sold his early Bird. gave me a bunch of different spark plugs.  All Autolite  46, ASF42C and BF82.  I usually use the 46's, but are the others a good plug for a basically stock 312 in a 56  TBird and a warmed over 312 in my 54 Ford.    The Bird has a Pertronix II, the 54 a stock points dizzy.

Thanks for any info.


54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ