Spark Plug Protrusion into Head?

According to Champion, they are 13/16" reach, but they fit fine. After looking at the charts, I'll admit those numbers are confusing. I'll go with "hands-on" experience every time on this old stuff. Someone posted earlier in this thread than Offenhauser recommends "L" series plugs for their heads, which is obviously wrong. If you don't believe me, spend the $4.20 with Amazon and check for yourself. It beats "calling Eric".
 
GOSFAST said:
Just a heads-up, but to the best of my knowledge and as far back as I can recall all the "N" series Champions were 750" reach.

Believe I ran that series in my 13:1 468" years ago!

Thanks, Gary in N.Y.

P.S. Simple to find out, give a call out to federal-Mogul with the part number, they'll give you all the particulars. You can reach them at: (800) 325-8886, I spoke to Eric already this morning to confirm!
Click to expand...

According to the NGK cross reference - it is a .750 reach plug. Hmmmmm, the plot thickens!
 
That plug is the number I used in my drag car (G.M. aluminum heads), it is a reasonably "cold" plug, worked well with the 13:1 C.R.

Here's a "cut & paste" from O'Reilly Auto parts:

https://www.oreillyauto.com/
Copper Plus
Champion Copper Plus Spark Plug

(Quote)Part #120 (N5C)

Line:CHP

Check Vehicle Fit
Details
Product Information

Warranty: 1 Year Limited Warranty
Unit of Measure: Each
UPC: # 37551000869
Material Grade: Copper
Hex Size (in): 13/16 Inch
Hex Size (mm): 20mm
Thread Size: 14mm
Center Electrode Design: Standard
Thread Reach (in): 0.75 Inch
Thread Reach (mm): 19mm (.749")
Number Of Ground Electrodes: 1
Preset Gap (in): 0.023 To 0.028 Inch
Preset Gap (mm): 0.58mm To 0.71mm
Short Style Spark Plug: No
Resistor Plug: No
Ground Electrode Tip Material: Nickel-Copper Alloy
Heat Range: 5
Center Electrode Core Material: Nickel-Copper Alloy
Seat: Gasket
Ribbed Core Nose: No (End quote)

(Add) Eric was correct, after all F-M makes these Champion plugs!

Thanks, Gary in N.Y.

P.S. I believe the "13/16" being mentioned above is most like the wrench size, just a hunch here though??
 

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I got 8 coming in a couple. I’ll let all know, looks good in tubman’s pic! Great thing about Amazon as a Prime member u can return no cost��
 
O K, i gotta get back in here after look'n at some older posts where several guys are runnung 216 & 437 Autolites, NGK BL6's & 4's and non of them indicated any problems with pre-ign. spark knock, heating issues etc.and seem quite happy with performance.
I'm just not so concerned now...???
I sure do admire & appreciate you fella's with so much experience & knowlege though. Thanks 4 keeping guys like me advised. mike
 
"AnthonyG" (and "Mart") - I am not in a position to do some actual clearance testing with these plugs. I did however take a picture of a set of "N" series plug in my unmolested Offenhauser heads as well as a regular plug in an 8BA iron head. As you can see, it looks like the stock setup is a bit more recessed than the modified setup. Given this, I would follow "Mart"'s advice and clay this set up upon assembly. I don't think there would be a problem, but you can never be sure. I hope everything works out.

"AnthonyG" - I am glad that you brought this up, as it appears to have been a continuing problem through the years. I have been reconstructing in my mind what I went through. I think I came to the right conclusion in 1961 after trying many different types of Champion plugs and seemed to have forgotten it by 1994 when I used the outboard motor "surface gap" plugs (although they worked fine). Please get back to us after you get the engine together and running so we can determine the definitive answer to what plugs to run in Offenhauser heads (it seems that Offenhauser doesn't have a clue). I don't know that the heat range (the "5" in "N5") is correct; I just used it because it is in the middle of the heat range. Also, while I believe the right size plug is mandatory, the brand is not, so you may want to try some others. In addition, from Gary's "GOSFAST" comments, I think that some plugs said to be compatible aren't.

Good luck and keep us informed.

(I have included a couple of other pictures because the difference between the "L" series and "N" series is remarkable.)
 

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How about this scenario: reach of plug without gasket (total shank length)=13/16". Reach of plug with gasket installed=3/4" (assuming gasket is 1/16" thick). Anyway, the end result is what's important, and it looks like Tubman has it figured out!
 
With a high-lift cam, wonder if the valves will hit the spark plug ground strap? Sure looks like it. I would test fit on an engine where you know the exhaust valve is at full lift. Make sure you do it on the side where the valve is higher from the deck.

FYI: Given the valve angles/heights are different side to side, there was a time when Edelbrock actually made a left and right head - the cast chambers were different side to side. I bet the new NC machined ones are non side-specific.
 
Obviously my Offy heads depth of thread different than tubman’s. The Champion N5C’s R indeed 3/4 reach and extend into the head chamber almost 4 full threads. Oh well there going back to Amazon & guessing I’ll pull the other head & just Chamfer for the Autolite 437’s
 

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Boy, that's strange! I wonder if someone has modified your heads or they are from different production periods. Mine came straight from Speedway in the late '90's and have been sitting on the shelf ever since because of my involvement with the Edmunds heads, and I know they haven't been messed with.

Anthony, would it be possible to get a close up photo of the top of the spark plug hole? Except for the carbon, the bottom looks just like mine.
 
Heads don’t show any evidence of additional machining on either side of spark plug threaded holes. Light spot-faces on outside of head where plugs wire connect end is. No chamfer or counter bore on inside, threads enter as-cast chamber area. Guess it could be a different model design? I had an 8BA built by Jerry Livingston, owner of United Engine Specialists in Wichita Ka. Only marking on the head is the Offenhauser cast in & a 400 stamped on top side near temp gauge threaded hole see in pic.
 

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Not sure why but can’t get photos to load for some reason? Sorry? As I said though the view from the top shows spot-faces on each to same depth clean machining the casting. Most casting removed is shy of 1/16” the rest r about 1/32”. I’ll try to load photos again later.
 
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Anthony, Going on the assumption (which may not be correct) that your heads were modified to fit some particular type of spark plug in the past, I think it may be more productive to try to figure out what that plug was than to further modify your heads. Any modification would probably reduce the number of threads holding the plug, which is not a good thing with aluminum heads. In one of my posts I posted a link to a site that has the parameters for most brands of plugs, including reach. I think that some time going through those tables to see if there is something out there that may fit the heads as they are now would be more productive than cutting the heads again.

Here is that link again : https://www.sparkplugs.com/learning-center/article/173/manufacturers-numbering-systems.

I find the Champion table to be the most comprehensive and usually use it. If you find what you want, but don't like Champions, you can always find another brand that will cross-reference to the Champion plug.
 
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Thx tubman, I have the site & will check again. Really not being stubborn but as a retired Tool & Die Guy I can tell the heads not modified! U previously indicated it may be a different model # & that may be it. Thx & I’m working on figuring out how to reduce the file size on Pics so I can post.
 
From the picture, it sure looks like the heads weren't modified. On the plug hole on the right, you can see the complete set of threads. I'll try to take a picture of my head from the same angle and see if I can get a comparable view. BTW, my heads are stamped "425". As I said, my heads were procured from Speedway in the late nineties; do you have any idea of the age of your heads?
 
Hard to say age on mine, as said procured engine w heads on. United Engines Specialists has a Flathead only dept so assuming they have heads they’ve bought in bulk on the shelf who knows how old. I’d think ur’s marked 425 indicate sequentially a more recent run? Scale the reach on the N5C’s u have. If they’re actually 3/4” as spec indicates it’s the reason they’re not fitting mine. Calculating mine require 5/8” reach which Don’t seem to exist?
 
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