just called ngk tech line earlier today for my heads and found out they take ngk plug number TR5 or motorcraft part no.asgf32c. they have a 708 thread, and dont use platnium plugs in the heads as they have a problem grounding(found that out from msd). also if you have a lower compression engine IE 9.x:1.0 order your plugs one step colder than the motorcraft p-n. And also make sure that you have your heads grounded with a 10awg wire from head to head then from head to negative batt. terminal (as aluminum does not conduct electricity as well as iron and you will have a terrible miss if you dont) the bolt size to fit in the un used holes in the head are 3/8 x 1". the heads will not ground in the same way as the factory heads(through the head bolts) and that is why you need the direct ground. I just wish i would have had this info around 2mos ago............... would have saved me money in plugs and wasted time chasing wild geese. just for future reference the gt 40x heads take many of the same parts as their cast iron counterparts atop the 93 lightning 5.8. hope this helps and good luck
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