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Old 11-05-2019, 06:22 PM   #41
AnthonyG
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Default Re: Spark Plug Protrusion into Head?

Tubman, I kinda thought the same thing but being a relative novice on the inside of flatty’s & my 7/16” reach seem really short w 3.5 internal threads exposed I’m trying to learn? Even w 1/2” reach that’s only another 1/16” reach into my thread will still leave 3/16” of internal thread exposed. Interested to see what u see on ur Offy heads
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Old 11-05-2019, 07:03 PM   #42
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Default Re: Spark Plug Protrusion into Head?

Thanks Denny [tubman]. Will be anxious to see those pics. I read on the H A M B, i think about guys using NGK B6HS- 7354 1/2 ' reach in Offys & like'n them. mike
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Old 11-05-2019, 08:16 PM   #43
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Default Re: Spark Plug Protrusion into Head?

I'd run an NGK plug any time before I would a Champion . . . just sayin!
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Old 11-05-2019, 08:17 PM   #44
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Default Re: Spark Plug Protrusion into Head?

I can add this tonight, the total thickness of the threads on these particular Offy heads is .570", when you add the spark plug gasket thickness it's another .065", this gives you a grand total of .635". A plug with a .500" depth is still not really correct, it will run, it's still not the right way to do things.

You'll generally find this spark plug issue only with the Offy's?

Thanks, Gary in N.Y.

P.S. This is why I stated earlier we prefer to use the Edelbrock castings, they are set up for the .750" long reach plugs, this is how it should be on ALL aluminum castings, and they have the lengths right! Sometimes we fire-slot the spark plug holes or flycut the valve pockets but we always compensate for this machining.
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Old 11-05-2019, 08:19 PM   #45
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Default Re: Spark Plug Protrusion into Head?

Just a note - Gary is referring to NEW Edelbrock castings - none of their vintage heads had long reach plugs - so make sure you know what you're buying or using.

Truth be told, I like the cast chambers in their original designs a lot better than there new CNCd'd chambers. The new ones have a lot more room over the valves - which I don't always need or want.
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Old 11-06-2019, 12:27 PM   #46
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Default Re: Spark Plug Protrusion into Head?

Gary, not sure u can see scale but it shows 1/4” deeper than head surface to face of thread dia.
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Old 11-06-2019, 02:50 PM   #47
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Default Re: Spark Plug Protrusion into Head?

Well, wrong again. I tried some "L" series Champion 1/2" reach plugs in an unmolested Offenhauser head and it came up a full 3 threads short of filling the hole. The attached pictures show the head and the plug, both by itself and screwed into the head. The only plug I had was used, so it's kind of hard to see from the combustion chamber side, but it's definitely short.

This whole thing is staring to bring back memories of the early sixties when I drag raced a '36 Ford in "D/Altered". My buddy's dad at the time could get us any plugs we wanted (as long as they were Champions). I vaguely remember fitting various plugs into our heads until we found something where we liked the fit. I seem to remember something about a Champion "N" series which have a 13/16" reach, but that's even longer. Just for shits and giggles, and because it's so easy, I just ordered a Champion N5C from Amazon.

I cannot believe that the original engineers at Offenhauser designed a high performance head that no commercial plugs fit, but it's beginning to look that way.

I'll be looking for another set of Edmunds heads after this.
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File Type: jpg IMG_1668.jpg (34.0 KB, 36 views)
File Type: jpg IMG_1669.jpg (42.4 KB, 26 views)
File Type: jpg IMG_1672.jpg (51.4 KB, 26 views)
File Type: jpg IMG_1676.jpg (56.1 KB, 27 views)
File Type: jpg IMG_1679.jpg (52.8 KB, 33 views)

Last edited by tubman; 11-06-2019 at 03:24 PM.
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Old 11-06-2019, 04:24 PM   #48
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Default Re: Spark Plug Protrusion into Head?

Hi Dennis, thought I was about to get proved "wrong" here, like I said, it's only really an issue with the Offy heads, believe me!

While we're doing the builds here, not that often with the Offy's, we do take the time to make certain a conventional, over-the-counter, plug will fit with no effort!

If you go back and "collect" all the dimensions I posted up here you should find all are "on the money" for the most part!

On a side note with respect to Anthony's photo above it appears to me to be less than .250", maybe more like .180" or so? hard to tell from the photo.

Thanks, Gary in N.Y.

P.S. Here's a customers OEM C8CM Canadian casting we tried to repair (no luck so far) with an Autolite 216 plug, fits perfect, exactly like it should!
Attached Images
File Type: jpg Flathead C8CM Spark Plug Fit.JPG (76.2 KB, 64 views)
File Type: jpg Flathead C8CM- Autolite 216 Spark Plugs.JPG (83.6 KB, 49 views)
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Old 11-06-2019, 04:50 PM   #49
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Default Re: Spark Plug Protrusion into Head?

Aw shucks, looks just like a 216 fit to me..........maybe using Go Fast's spacers and somp'n
longer......otherwise i'll keep the 216s. cause it sure runs good.
Thanks 4 your time Denny.
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Old 11-06-2019, 05:25 PM   #50
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Default Re: Spark Plug Protrusion into Head?

Picture is tough to get clear. I’m a Tool & Die Maker Machinist, believe me it’s within .01” of .25”. The angle w the phone came distorts.
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Old 11-06-2019, 07:21 PM   #51
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Default Re: Spark Plug Protrusion into Head?

Anthony, if you're referring to my last picture, you can just barely make out the side electrode at about 10 o'clock in the plug hole.
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Old 11-06-2019, 07:34 PM   #52
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Default Re: Spark Plug Protrusion into Head?

Anthony, as a machinist, you know that where a thread extends threw something, it needs to have the edge broke, for one, to as many as two threads. 1.5 is the standard I use. Besides getting the reach sorted out, I would be sure to do that.

Those sharp, partial threads can cause pre-ignition. I'd recommend that to all, especially with cast iron heads, as the iron can stay hot longer.
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Old 11-06-2019, 07:35 PM   #53
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Default Re: Spark Plug Protrusion into Head?

Me - if I had a set of the Offy heads, I would spot-face the gasket surface down whatever is required so that I could run 1/2" reach plugs. Depending on the type of electrode, the other approach might be to put a 45 degree chamfer on the combustion chamber side - which would allow a more projected nose plug.

Lots of ways to skin this cat . . . and most of them quite doable on a good drill-press. Then just chase the threads when finished.

No biggie boys, no biggie . . . LOL
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Old 11-06-2019, 07:57 PM   #54
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Default Re: Spark Plug Protrusion into Head?

Everything you are saying is true, Dale, But I am still troubled by the fact that the original designers at Offenhauser would design a high performance head that had no matching plug available. There has to be a solution out there.
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Old 11-06-2019, 08:13 PM   #55
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Default Re: Spark Plug Protrusion into Head?

Quote:
Originally Posted by tubman View Post
Everything you are saying is true, Dale, But I am still troubled by the fact that the original designers at Offenhauser would design a high performance head that had no matching plug available. There has to be a solution out there.
If they designed it for a 5/8" reach plug . . . I've never seen or used anything like that (but I've never ran Offy heads). The best situation is to set it up for 3/4" reach plugs - then you have a world of options. Obviously this requires mods to the heads, or something like Gary has done (thick washers).
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Old 11-06-2019, 08:21 PM   #56
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Default Re: Spark Plug Protrusion into Head?

No wonder these have been sitting on a shelf in my shop for the last 25 years!
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Old 11-06-2019, 08:39 PM   #57
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Default Re: Spark Plug Protrusion into Head?

Quote:
Originally Posted by tubman View Post
Everything you are saying is true, Dale, But I am still troubled by the fact that the original designers at Offenhauser would design a high performance head that had no matching plug available. There has to be a solution out there.
It comes down to the same reason that those "new" Navarro heads we checked out recently got "shot-down", by us anyway, some of the mfr's just don't care! It's sad but true.

With just about any of the heads that have been mentioned here the issues are 100% fixable if you know what you're doing, there's not one issue here that could't be made to work. With the cost of those Navarro's in the 900.00 area it probably would have taken the same amount of money to fix EVERY issue we found??

This is just my take on the subject, but we find it all to often lately that this is now almost the "norm", it was very different years ago when many took great pride in their products, it has all but disappeared in today's marketplace for the most part!

Thanks, Gary in N.Y.

P.S. In my own opinion here to do a really "nice" 100% fix in this spark plug area I would most likely NOT be machining the spark plug lands any deeper to accept a shorter plug, to the contrary, I'd most likely do a modification to get the .750" long plugs to go. Again, not that big of a deal (for a decent shop anyway), I would also like to see threaded inserts in place for the plugs. On the short plug aluminum heads this would part of my modification. They would end up having .750" plugs when finished!
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Old 11-06-2019, 08:56 PM   #58
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Default Re: Spark Plug Protrusion into Head?

Tubman, no I tried to take a picture w s scale next to plug threads showing the 3/4” reach mounted in mine. Fully inserted the thread face protrudes .25” past the inside surface. Bad pic but it’s post #46
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Old 11-06-2019, 09:23 PM   #59
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Default Re: Spark Plug Protrusion into Head?

Gary , does the fire slot make any difference on a road engine? is it worth doing.
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Old 11-07-2019, 12:19 AM   #60
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Default Re: Spark Plug Protrusion into Head?

Those are a nice set of heads, but the plug would be better off in the center of the transfer area, This would provide a much shorter flame front and with a powertip plug stay nice and clean.
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