454 heads on 396 block. Which Spark Plug?

[mosel70]

Truck is a 1970 Chevy C10.

I confirmed during tranny change and with valve covers off that it's a 1969 396 block and 454 heads stamped HI-PER PASS.

The spark plugs that were in the truck when I bought have shorter threads which match plugs listed for a 1970-1972 402 and a 1973 454.

The plugs for a 1969 Chevy C10 with a 396 have longer threads.

Any suggestions which ones are right for my set up? I don't want the plugs hitting the pistons.

[geezer#99]

What’s the casting number on your heads?
That’ll help decide.

Don’t get excited by that ‘hi-per pass’. Even the 265 hp 2bbl 396 had that.

[sweetk30]

long thread in your pic is gasket seat .

short thread in your pic is tapper seat .

2 different style plugs . long thread is more modern version .

early plugs were gasket seat with short thread body .

casting info will help . but if you look you will see tapper cut in the head for tapper plug or flat surface for gasket seat plug .

[Mike C]

The 454 all came with open chamber heads near 120cc while the 396 had closed chamber heads through 1970. Unless someone has put pistons for the open chamber heads, the compression on your motor with those heads is less than 8:1. Since torque is a function of displacement and compression ratio, your motor is giving up a lot of potential FWIW.