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Old 07-08-2023, 12:25 PM   #1
jeepguy1948
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Default Question for the guys running aluminum heads

If you are a purist that believes that everything must be totally stock, please move on…. Here’s my issue; I’m working on a 1951 F-1 that had a fresh .60 over Merc. Motor installed (with Edelbrock aluminum heads) run about 1,000 miles and then parked for 40 years. I am attempting to bring it back to life (I have posted other questions about it here before). I have the motor up and running quite well but I’m trying to get it really “dialed in” something’s were not sorted out when the engine swap was done. It looks to me like the aluminum heads require a longer (more threads) spark plug than the original cast iron heads did (I admit that I could be wrong about this). I would appreciate some advice about a good spark plug to use. Heads are Edelbrock “F8 M 8.5” which I take to mean “Ford V8 8.5:CR? By my calculations, if the heads made 8.5: CR on a 239” motor it probably has about 9.5;1 CR on the current +/- 270” inch motor. Engine currently has a totally stock intake and 94 Holley, stock dist with a Pertronix module, “3/4 race” cam, Red’s headers. The original Ford Load a Matic vacuum advance system is in use and working correctly. I currently have it at 24 degrees TOTAL advance. Spark plug suggestions? And while we are out it: suggestions about jet size in that Holley 94 considering the specs of the motor? Thank you!
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Old 07-08-2023, 01:37 PM   #2
cadillac512
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Default Re: Question for the guys running aluminum heads

Since the heads are on the engine, the way I would proceed is to take a piece of wire and make a 90 degree bend at the end, maybe a quarter inch long...so it looks like an "L". Stick that in a spark plug hole and pull up against the bottom thread, and mark the top or pinch it right at the top with a pair of pliers. Remove it and measure how deep the threads are, and choose a plug with that thread depth, or use multiple spark plug washers to shim the plugs up if necessary.

If you post the measurement on here someone will have suggestions for a plug brand and number that will fit.


Terry


p.s. The carb jetting may be correct as is...but that engine sure would like more carb!
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Old 07-08-2023, 02:47 PM   #3
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Default Re: Question for the guys running aluminum heads

From the dark recesses of my mind, I seem to remember that these heads take a Champion "N" series plug or equivalent (3/4" reach). Back in the early sixties, I raced a '36 Ford with a '41 Merc in "D"-Altered class at the drag strip. I found a pair of used Edelbrock heads for use on the engine. I bought a set of stock (Champion H-10) plugs, but when I screwed them in, they were very short of engaging all of the threads in the head. Even I could see that plugs shrouded that bad could not be good for optimum performsnce. Plus, the aluminum being weaker than cast iron, needed more threads to hold the plug.

A friend of mine's father was a rep for International Harvester back then and had a complete set of parts books. They had a listing for, of all things, an Allard that used Champion "N" series plugs. (Back in those days a lot of Allards were powered by Mercury flatheads using Edelbrock heads.) I ordered a set of N" series Champions and they fit like a glove.

Make sure that you do a test fit first, and be sure to clean the carbon out of the lower threads in the head (they get all carboned up from running H-10's).
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Old 07-08-2023, 03:15 PM   #4
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Default Re: Question for the guys running aluminum heads

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Originally Posted by tubman View Post
From the dark recesses of my mind, I seem to remember that these heads take a Champion "N" series plug or equivalent (3/4" reach). Back in the early sixties, I raced a '36 Ford with a '41 Merc in "D"-Altered class at the drag strip. I found a pair of used Edelbrock heads for use on the engine. I bought a set of stock (Champion H-10) plugs, but when I screwed them in, they were very short of engaging all of the threads in the head. Even I could see that plugs shrouded that bad could not be good for optimum performsnce. Plus, the aluminum being weaker than cast iron, needed more threads to hold the plug.

A friend of mine's father was a rep for International Harvester back then and had a complete set of parts books. They had a listing for, of all things, an Allard that used Champion "N" series plugs. (Back in those days a lot of Allards were powered by Mercury flatheads using Edelbrock heads.) I ordered a set of N" series Champions and they fit like a glove.

Make sure that you do a test fit first, and be sure to clean the carbon out of the lower threads in the head (they get all carboned up from running H-10's).
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Old 07-08-2023, 03:51 PM   #5
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Default Re: Question for the guys running aluminum heads

The Edelbrock heads of vintage late 40's and 50's used 1/2 in reach plugs like La10 or L63R.
Later vintage use 3/4 reach.

Measure the holes like 512 suggested. That will tell you for sure what you have.
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Old 07-08-2023, 06:25 PM   #6
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Default Re: Question for the guys running aluminum heads

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My memory is returning. I think we ended up using Champion N-5's. A lot of people don't like Champions any more, but I'm sure NGK has an equivalent.

Pete, if the used heads I bought in 1961 were 3/4" reach, the change from 1/2" reach must have happened a looong time ago.
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Old 07-08-2023, 06:37 PM   #7
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Default Re: Question for the guys running aluminum heads

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Pete, if the used heads I bought in 1961 were 3/4" reach, the change from 1/2" reach must have happened a looong time ago.
Probably in the mid 50's or so. The last set I bought was in 54. We used to put 1/2 inch Helicoils in new heads so we would never encounter a pulled thread when changing hot plugs at the track.
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Old 07-08-2023, 07:32 PM   #8
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Default Re: Question for the guys running aluminum heads

Actually I had done a measurement exactly as described above but forgot to include it. I came up with .60” .0625 being 5/8” that would be my guess. I found some information indicating that back in the day Edlebrock recommended a Champion plug (I’ll find the number somewhere and post it later) with .75” of threads. I bought a set of those plugs. I may be dumb but I’m not stupid so I screwed the plugs in 1/2 turn at a time and then rolled the motor over. The plug gap started to close before the plug was all the way in, so those plugs are a no-go.
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Old 07-08-2023, 07:37 PM   #9
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Default Re: Question for the guys running aluminum heads

What got me on this rabbit trail is that when I had the plugs out I could see that there was some soot/carbon/gunk in the bottom of the holes indicating that the plugs were too short. And as mentioned above, not only is the plug shrouding an issue but aluminum heads need every bit of thread used.
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Old 07-08-2023, 07:48 PM   #10
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Default Re: Question for the guys running aluminum heads

People have been known to do some strange things when it comes to making plugs fit. One of these is to "spot face" the outside of the head so the plug sits deeper in the head. If something like this has been done, common knowledge is out the window. Since the consensus here is a 3/4" reach plug is most likely proper, the fact that there is insufficient clearnace for the elctrode is damning. It looks to me like someone has been messing around with those heads. There was a thread on the H.A.M.B. several years ago where a couple of guys espoused the "spot-facing" method so they could run (get this) H-10's. Unless you bought the heads new and did the initial installation yourself, you can never be sure of what you have. There were even a few suggestions to chamfer the bottom of the plug hole to unshroud the plug.
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Old 07-08-2023, 10:09 PM   #11
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Default Re: Question for the guys running aluminum heads

I've seen a lot of heads, but never an early Edelbrock with 3/4" reach. My guess is that they take a 7/16 to 1/2" reach plug - but you'll have to measure the thread depth. Truth be told, I bet I could just look at the plug threads and I'd know what plug to run.
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Old 07-08-2023, 10:52 PM   #12
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Default Re: Question for the guys running aluminum heads

Champion RC12YC is the plug that comes up as being what Edelbrock recommended. This is the plug that went too far in. It is absolutely correct that I do not know for certain what may have been or not been done to these heads prior to me acquiring them in 1976. They do not look like the exterior surface where the plug seats has been altered but who knows? The plugs that we had previously were NGK B-4L and those were too short. Before that it had Bosch WR8EC and they were even shorter. Interestingly the Champions have a 5/8” hex on them which seems very strange for a plug used in the 1950’s? These champions are .75” BTW.
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Old 07-08-2023, 11:44 PM   #13
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Default Re: Question for the guys running aluminum heads

Spark plugs are something very few people give much thought to. They buy a set of heads, a box of plugs recommended by the head manufacturer and assemble them. If the plugs fit the threads in the heads and they screw in, they must be right.
For a stock or mild street engine, that will usually get by.
Some times a heat range adjustment is needed and that is it.

On a race or high performance street flathead engine many things come into play long before it is time to install plugs and fire it up.

First off, solid copper washers should be used, not the cheap thing that comes with the plug.

As a side note, 35% of the heat through the plug is dissipated through the washer, 30 % through the threads and the rest elsewhere. Increasing the heat conduction through the plug seat by using solid copper washers will speed heat dissipation and that increases efficiency.

The thickness will depend on the depth of the washer seat to the last thread in the chamber. When torqued to 18 ft.lb. there should be no threads showing in the chamber, either plug or head. This is critical.

The next thing to determine is will the valve clear the plug ground electrode. This is determined by how deep the chamber is and cam lift. Most head manufacturers offer several different depths. If the engine will be using side electrode plugs, closing the ground electrode will not be an issue.

Always use an anti-sieze compound on plug threads. Mainly it aids heat dissipation beside preventing thread damage.

Use wires that have a sealed rubber end that completely covers the porcelain on the plug. Dirty insulators cause misfiring.
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Old 07-09-2023, 08:49 AM   #14
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Default Re: Question for the guys running aluminum heads

Hey Pete . . . really good technical information!

The modern Edelbrock heads all take .75" reach plugs (which is a good thing), but I've not seen older Edelbrock heads with .75 reach plugs (but they may have made them).

I currently have 3 sets of vintage Edelbrock heads (59x - center outlets) - none are designed for .75" plugs.

In the end, just measure the thread depth . . .

Last edited by Bored&Stroked; 07-09-2023 at 08:54 AM.
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Old 07-09-2023, 11:58 AM   #15
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Default Re: Question for the guys running aluminum heads

Good advice all around. I do always use anti seize but copper washers is a new one for me (and it’s a good one too). I have done my best to measure the thread depth (and come up with .60 so probably 1/2” or 5/8”. In a perfect world I would pull the heads (or at least one of them) and sort this out but at this point, time is not on my side and I only discovered this issue on what was supposed to be the last day before “hittin the road”. Truth be told, I have the old beast running pretty good (there’s always room for improvement) (we won’t be racing it after all) and I would not be horribly concerned about shrouding for now and deal with it next winter but I am concerned about lack of threads in the aluminum heads. So, everybody says “just measure the threads” which I had done before I ever posted my original question on here (.60”) the current plugs are 7/16”, too short. The new plugs are .75”, too long. I would blame the valve/electrode contact on the bigger cam but they are clearly longer than the hole is deep. Another question is about the business end of the plug itself, the length of the electrode is significantly longer on the champions, indicating (to me) that they are a hotter plug, which concerns me as well. I am going to call Edlebrock themselves in the am and see what they have to say. Mr. Bored, what plug would you recommend for the heads you have? FWIW these are front outlet heads. Should I just go with the Champions and add washers for clearance (I would still be concerned about the heat range)? Assuming that I do need a plug with a .50”-.625”, where the heck to I go to source that? And of course, I still need to include heat range etc. in the equation, not just thread length.
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Old 07-09-2023, 12:17 PM   #16
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Default Re: Question for the guys running aluminum heads

If it were mine I'd lathe cut some .150" spacers from copper or brass and use NGK B6ES -B8ES depending on desired heat range. In NGK, lower numbers are hotter.


Terry
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Old 07-09-2023, 12:29 PM   #17
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Default Re: Question for the guys running aluminum heads

It sounds to me that Terry has the best advice. Since it appears that the heads have been compromised in the past, its probably the best way to go. It seems that nobody makes a 5/8" reach 14 mm plug. You may have to experiment to find the proper heat range, but that's just another aspect of the hobby.
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Old 07-09-2023, 12:55 PM   #18
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Default Re: Question for the guys running aluminum heads

I stumbled on Auto lite 3924 which has .60” threads and a total reach of .75” (it comes up on Speeday’s site). Thoughts?
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Old 07-09-2023, 02:08 PM   #19
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Default Re: Question for the guys running aluminum heads

I think the 3924's actually have 3/4" of threads, and maybe a projected tip that will almost certainly hit the valve. If I remember correctly that is....
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Old 07-09-2023, 04:37 PM   #20
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Default Re: Question for the guys running aluminum heads

If your heads need 1/2" or 5/8" plugs, then one method would be to make some thick copper or brass washers as Terry mentioned and tune the height of some .750 reach plugs to fit your requirements. If you can get away with a .500 reach plug, then there might be an option there as well (see below) and you may not need special spacers.

Also, I don't know what your ignition and wires are. I tend to run solid core wires and magnetos (or high-energy ignitions) and do not use resistor plugs (I want as much spark energy as possible). I never run resistor wires/plugs with ignitions like a Harman-Collins dual-point as the yellow mid-plate will tend to leak secondary voltage all over the place when you put any resistance into the secondary circuit.

19 mm Reach (.750): I'd probably look at an NGK B6ES-11. This is a non-projected tip, heat-range of 6

12.7 mm Reach (.500): I'd probably look at an NGK B6HS. This is a non-projected tip, heat-range of 6

Links:

B6ES-11: NGK 7910

https://www.summitracing.com/parts/ngk-7910

B6HS: NGK 4510

https://www.amazon.com/NGK-Spark-Plu.../dp/B0053EZ6MK
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