Spark Plugs

[961TonDually]

I have a 1996 7.4 Gen 6 that I rebuilt and added a 454/502 HO cam. I rewired it and I'm no longer using the ECM. The spark is no coming from an HEI. The fuel is coming fom a 750 Holley. I'm running the stock Gen 6 heads that had some work done. I'm looking for different spark plugs the are the same length and seat as the original but with a 40 thousand gap. GM calls for Delco MR43T in the 454HO engine but the seat is shallower and will not fit as deep into the head. I tried going to the part houses but they don't have the knowledge as the old days before computers. If it's not a stock part for that model, year and engine in their computers they have no idea what to look for. I'm using Delco Rapid Fire #8 with a 35 thousand gap at the moment but feel there may me a better plug to try. Anyone have suggestions?

 

[RichLo]

Some places still have walls of spark plugs and reference books that you can flip through. Try a large farm supply store, a store called Fleet Farm here has a huge wall of plugs and books from every spark plug manufacturer that they have in stock... lots of info in those books! heat ranges, thread pitch, seating style, more.

I used this method to find different heat ranges for my classic Harley, front cyl vs rear cyl needed different heat to burn clean and not foul. If I remember correctly, the front plug is for a 15hp briggs and the rear plug is for a 6-cyl Chrysler or something crazy like that, lol.

 

[SAATR]

NGK TR5 will be the same heat range as the TR55 spec'd for the factory engine, but gapped at .040" instead of .060". If you need a colder plug, go to a TR6. There is no advantage to platinum plugs in your application, so copper will perform just as well and save money.