Hey guys!!! I was wondering what is the best type of spark plugs for my '78 with a 351m. I running an HEI type distributor. I also read that I should increase the gap to .050 with that type distributor? Any input would be appreciated. Thank you

 

"Best" might be hard to answer. The 2004 Motorcraft catalog, for the 1977-78 E & F series Ford truck with the 5.8L (351) engine, shows ASF42CF4 as the standard service plug and the ASF42PF4 as the alternative platinum plug, both with a gap of 0.044".
If the HEI is generating higher secondary voltage than the original system, you can likely increase the spark plug gap, but you may not be able to use the 7mm wires. Bigger gaps might require larger diameter wires. How much might be a trial and error process to determine. Larger gaps usually mean better combustion and more power but too big means no combustion and no power if the system can't fire the plug. As you said, many use 0.050". When GM started with HEI they specified 0.080". After they sobered up they backed off to 0.060". The right gap depends on a lot of factors; too involved to cover here. If you make the gap too big it will not fire and you will feel the misfire.


Replacement spark plugs for Motorcraft ASF42CF4:

AC Delco 2 AC Delco 44TS AC Delco CR43TS AC Delco CR43TSM AC Delco R43TS AC Delco R43TSK Accel 276 Accel 576 Atlas C36R Atlas C36R4 Autolite A25 Bosch 7575 Bosch H9B Bosch H9BC Bosch HR9B Bosch HR9BC Bosch WA125T40 Bosch WA125TR40 Bosch YR9B Champion 12 Champion 121 Champion 18 Champion 2018

Champion 2406 Champion 4018 Champion 52 Champion 5942 Champion 83 Champion BL13Y

Champion RBL13Y Champion RBL15Y Champion RBL15Y4 Champion RV15YC Champion RV15YC4 Delphi 22131 Delphi 22531 Denso 5025 Denso 5026 Denso T16P-U Denso T16PR

Denso T16PR-U Denso T16PR-U11 Mighty RF42 Mopar P-55-7P Mopar P-55-7PR Mopar SP0RV15YC4 Napa X741 Napa X941 NGK BP5FS NGK BPR5FS NGK BPR5FS-11 NGK UR4

Precision Tune P4RF42 Prestolite 14F42 Prestolite 14RF42 Splitfire SF2E Splitfire SM2E Valley Forge 24 Valley Forge 24R Western Auto 64-1907-1



Replacement spark plugs for Motorcraft ASF42PF4:

Brand Model Autolite AAP25 Bosch 4107 Bosch 4108 Bosch 4207 Bosch 4208 Bosch 4499

Bosch 6108 Bosch HR9BP Bosch HR9BP4 Bosch HR9BPX Bosch HR9BPX4 Bosch HR9BPX6

Champion 3018 Mighty RF42P NGK 2841 NGK UR4VX

 

I always put in whatever the truck was built with. The motorcraft ASF42C plugs are a hard bargain to beat and are OEM specified.

I have also heard many good things about the NGK V power plugs, though I have yet to try them.