AC Delco Spark Plug

[brianc427]

Does anyone happen to know what AC Delco spark plug was installed in the '69 COPO cars (with #840 heads) from the factory? Or what a good AC Delco replacement is?

Thanks.

[Keith Tedford]

I always ran the R43N plugs in our 427 and I believe that is what came in it originally. With the unleaded gas, the 43Ns last far longer than they did with the leaded gas, where the plugs weren't working the best after 5K miles. I picked up a few sets at a swap meet a few years ago. I've only changed the plugs once in our L78 in 32K miles.

[Chevy454]

From a previous post of mine...

[ QUOTE ]

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)

[/ QUOTE ]

[Keith Tedford]

The 840 heads use the long reach plugs which are signified by the "N". "R43N" as an example. Lots of good info there Rob.

[Salvatore]

Only reason why your plugs last so long is because the guy that honed the block and installed the rings did a great job! In and out of the trailer kills motors. You warm yours up so there is minimial cylinder wash down.

[Keith Tedford]

You may just be right Sam. The nearest cruise night/show is about 12 miles away. The engine doesn't get much idle time and I keep the idle up around 1200 rpm. I don't think any of our cars were ever on a trailer except to travel back and forth to the bodyshop in the winter. 32K on the L78 so far and still having fun.

[Chevy454]

Keith: just for grins, I'd be interested to see how you &amp; your car liked a set of NGK plugs...

[Salvatore]

I am running the NGK's in the Z/28. Seems to be doing fine. Don't seem to have anymore power/RPM range than the R44S's that I took out. May be faster on the track because you can't always feel a couple tenths but I don't seem to get more out of the NGK's (longevity) than I do the AC's.

[Keith Tedford]

Years ago I found that NGKs seemed to work better than anything else in our two stroke snowmobiles. No complaints at all there. They would probably work very well in the 396 as well. It's just that, so far, I've had a good supply of AC plugs. I just might go out and get a set. Nothing like having a car running better.