Packard Forum Index » V-8 (1955-56)

On the otherhand, i belive it was BH that indicated that AC Delco had recently changed their code system for AC plugs. Probably why GM went bankrupt. But the MMH codeing system for the AC's was used from about 1955 up thru late 80 that i know of. Maybe a wider time range than that.

No other spark plug can make that claim that i know of.

Alright guys, now I'm overly confused. If they are listing both plugs for the car then which ones should I use? Is it dependant on the cylinder head numbers or can I just freely choose to use one of them?

Check either your vehicle number or the cylinder head casting number against the data I gave in post #6 to determine the "reach" and therefore the plug required for your cylinder head.

PS - if you're still not clear on this, I suggest you read the Service Counselor which covered this subject:

https://www.packardinfo.com/xoops/html/downloads/SC/SC-VOL29NO7.pdf

Use a small scale with a slight hook on the end or some other suitable measuring device to measure the length of threads in the CYLINDER HEAD. MEasure one hole in EACH that is BOTH heads that is on the car.

U mite do it an easier way by using uing two eyes:
Tell us the casting number(s) on the cylinder head that is directly over the center exhaust port of the head. CHECK BOTH HEADS!!!

Note that it is entirely possible that after 50+ years that it could be ANY combination of 55 AND/or 56 heads on a 55 or 56 block.

Let me put it another way:
There is one thing, exactly one thing and only one thing that determines the which reach of plug that is needed. That is the length of threads int he heads.

Short of measuring the threads in the head then there is one thing exactly one thiong and only onle one way to determine the length of thread needed and that would be those very top secret casting numbers on the heads that noone ever seems to be able to read,reveal or for some reason seem to believe are so eternaly mystical.

those very top secret casting numbers on the heads that noone ever seems to be able to read,reveal or for some reason seem to believe are so eternaly mystical.

Huh....?

Would someone please "Unconfuse" me!

Machined dome and cast dome spark plugs.

Okay, I went out to the 400 and determined that the part numbers on the heads are "440689" (dang six looked like an eight) because of being thickly painted. So I then go to my Packard parts book and see that under 5.0503 (page 89) "440689" correlates to 5540. Hummm.......but I have a 5587-4182 ....and the SC doesn't get into that ? I'm confused.

So a couple of questions and assumptions....

# 1 -I can assume that I definitely have a machined head which calls for a 7/16" reach plug and not the 3/4" reach that is for 1956 and late production 1955's?

# 2 - What determines late production other than the number in the heads present on the car?

# 3 - Heat ranges - Why are they important to me? These AC 43LS plugs I have on the shelf are at the "cold" end of the chart that Owen provided? Should I care? Hank said in a post that his early production heads had H-10's in them? Will I injure my engine if I don't get this right?


# 4 - In a perfect world, if another one of you guys had a car much like mine ......with it's newly rebuilt engine, 4GC Rochester carburetor, Delco distributor, what would be the best choice for me living here in Arizona at 2300' elevation and warm temperatures most of the year?

Many thanks in advance.

I'll leave it to others more knowledgeable than I about the particulars of the cylinder heads.

But to address your #3; YES! heat ranges of plugs should be of importance to you, very much so. Too cold a plug will quickly carbon up, foul, and fail to perform properly, engine missing being the most likely outcome. Too hot a plug will lead to preignition and probably self-destruction of the porcelain insulator. Once you determine which reach plugs are correct for you, you can get advice on which plugs perform best for your car from others on the site, or just go the AutoLite website, find their sparkplug recommendation and lookup chart, and select accordingly. You'll have several satisfactory plugs to chose from, though not necessarily the same make and number as was originally supplied. I've had 3 or 4 different plugs in my 56 with good results, but they are all of the proper heat range.

Pulling a spark plug after a few hundred miles of driving and just making a visual examination of what you see should tell you whether the heat range is reasonable. Look at the inner porcelain sleeve surrounding the center electrode; you want to see very light tan to nearly white. Darker or sooty is too cold, too hot is a bit harder to spot but the porcelain may be scorched hot (extremely white), perhaps cracked, and the electrode wear will be advanced.

Answer to question #1 should be Yes. 7/16. NOT 3/4.

AFAIK all cast dome heads have "CD" directly after the number that is cast into the head. If yours does not have "CD" then it's a machine dome head.
thERE ARE SOME stb'S AND SC's that coverf some of this cast dome vs machine dome.

According to my AC plug book 1976:

"1956 - 55 V8 7/16 reach 43LS gap .035.
3/4 reach 45XLS gap .035."

According to the AutoLite website, the currently recommended plugs matching the OE specifications for 1955 Packard are:

#437 for 7/16 reach
#65 for 3/4 reach.

I run #65 in my 56 Packard V8 with excellent results.