L-78 Spark plug choice

[Chevy454]

After much reading, I have *yet* to find any charts that agree on corresponding part/stock numbers, and the cross reference charts are all different it seems also. But here's what I could dig up from my AC book and the NGK site...(NGK #s below are listed as part number/stock number, hottest plug @ top)...

<u>Z28 &amp; LT-1 with 186 heads:</u> my AC book calls for a C43 or an R43 plug, and the NGK site says-

Standard, V-power (3/8" reach, 13/16" hex, projected tip, gasketed seat):
XR4 / 5858
XR5 / 3332

Race, V-power (3/8" reach, 5/8" hex, projected tip, gasketed seat):
R5670-6 / 4449
R5670-7 / 5034
R5670-8 / 5657
R5670-9 / 6468
R5670-10 / 6702

<u>L78 &amp; L72 with 840 heads:</u> my AC book calls for a R44 or a 44N plug, and the NGK site says-

Standard, V-power (3/4" reach, 13/16" hex, projected tip, gasketed seat):
GR4 / 2635
GR5 / 2851

FR4 / 5155 (same as GR plugs, except for 5/8" hex)
FR5 / 7373 (same as GR plugs, except for 5/8" hex)

Race, V-power (3/4" reach, 5/8" hex, projected tip, gasketed seat):
R5672A-8 / 7173
R5672A-9 / 7405
R5672A-10 / 7942

R5671A-7 / 4091 (same as 5672, but with non-projected tip)
R5671A-8 / 4554 (same as 5672, but with non-projected tip)
R5671A-9 / 5238 (same as 5672, but with non-projected tip)
R5671A-10 / 5820 (same as 5672, but with non-projected tip)
R5671A-11 / 6596 (same as 5672, but with non-projected tip)

<u>L78 &amp; LS6 with 291 heads:</u> my AC book calls for a R43T, and the NGK site says-

Standard, V-power (.460" reach, 5/8" hex, projected tip, tapered seat):

UR4 / 6630
UR5 / 2771
UR6 / 7773
YR5 / 7052 (same as UR plug except w/ non-projected tip)

Race, V-power (.460" reach, 5/8" hex, projected tip, tapered seat):

R5674-6 / 4449
R5674-7 / 5034
R5674-8 / 5657
R5674-9 / 6468
R5674-10 / 6702

R5673-6 / 2405 (same as 5674, except w/ non-projected tip)
R5673-7 / 2817 (same as 5674, except w/ non-projected tip)
R5673-8 / 3249 (same as 5674, except w/ non-projected tip)
R5673-9 / 3442 (same as 5674, except w/ non-projected tip)
R5673-10 / 4050 (same as 5674, except w/ non-projected tip)

[Keith Tedford]

I have a chart somewhere listing the AC spark plug coding. Off the top of my head, the "N" designates a long reach plug. "T" is for taper seat. "S" is for extended tip. There was other stuff as well. Any sparkplug problems that I ever had stemmed from carb problems, carbon tracked distributor caps, or bad plug wires.

[Chevy454]

The "N" suffix means it's the same 3/4" reach, but only 3/8" threaded, as opposed to the "XL" like the 074 heads got that has 3/4" reach and is fully threaded.