Re: L-78 Spark plug choice
 

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)

Re: L-78 Spark plug choice
 

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.

Re: L-78 Spark plug choice
 

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.

https://www.yenko.net/attachments/322679-plugs.jpg

Re: L-78 Spark plug choice
 

[ QUOTE ]
Bill, If you have your AC number they can cross reference that on the computers (I work PT at Advance) and that will give you some NGK numbers. Believe it or not we still have books and catalogs behind the counters but alot of the newer/younger employees haven't been introduced to them. If you can get one of the guys/gals to pull the spark plug catalogs, they have lot of information about heat ranges and numbering systems. I sell a lot of plugs to racers, they buy several heat ranges and try different brands. I even go through boxes of plugs with one guy while he finds the ones that index the best for him. he is nuts.... But very fast... I will be at work tomorrow, PM me with some numbers and I will look up some stuff for you. I stock and can order the Autolite racing plugs if anyone needs them too.

Tommy (believer in the catalogs) https://www.yenko.net/ubbthreads/imag...ins/3gears.gif

[/ QUOTE ]

Exactly. I've been working at Advance PT since '01, primarily to get discounts. The concept of a catalogs are vague to 80% of the people that work in the store. The only people that really use them are me, and the store manager. I have no clue why parts places hire people who have computer skills but no part/car knowledge. On old vehicles 90% of the parts aren't going to be on the computer. For those of you who don't know, advance has an interactive website with a full catalog system that is really easy to use to help you find parts. Most parts have images as well. Typically you can find more parts for your car online than the people can in the store. Find the part # you need online, then take it to the store with you and have them order it, or get it off the shelf. It takes all of the guess work out of it, and you get the right part the first time. https://www.yenko.net/ubbthreads/imag...iggthumpup.gif

NGK's are good plugs, like everyone has previously stated. I ran champions and autolites in my built 340 thats in my cuda and after 3 or 4 trips around the block, it would be skipping or missing. I put a set of NGKs in it about 4 months ago and have been trouble free since then. Colder plugs really helped my motor out.