8BA Spark Plug thread length

[Steve in Denver]

So I had my 8BA rebuilt over the holidays. Had a few things done to it, 4 inch Merc Crank, mild cam, not sure how mild, Skip's water pumps, Columbia, etc.

I have Offie heads with 350 stamped at the top.

Everyone I talked to said to use 3/4 inch reach spark plugs.

I installed NGK BR6ES plugs but they only had a reach of 11/16 inch. Turns out that was 1/8 inch too long as, when I measured the thread depth with the heads off, it was only 9/16 inch.

Well, guess where the other 1/8 inch of material went. The motor comes out this weekend and goes back to the machine shop.

What kind of plugs are others with Offie heads on their 8BA using? And what is the thread reach?

[petehoovie]

[QUOTE=Steve in Denver;2141120] I installed NGK BR6ES plugs but they only had a reach of 11/16 inch. Turns out that was 1/8 inch too long as, when I measured the thread depth with the heads off, it was only 9/16 inch.
Well, guess where the other 1/8 inch of material went. The motor comes out this weekend and goes back to the machine shop.

[tubman]

The proper plugs for Offenhauser heads has been discussed here at great length. To make a long story short, it was determined that up until just recently, there was no such animal. They require about a 5/8" reach plug, which nobody makes. The condensus was to run 3/4" reach plugs with a spacer. Someone has reported that recently, Offenhauser has corrected that problem.

There are several threads on the subject on this forum. I'd find them for you but it's after midnight.

EDIT - Here's one : https://www.fordbarn.com/forum/showthread.php?t=272052.

[Tinker]

Guess that's why we discuss and re-discuss topics. Issues that still are an issue. A question from someone that may not be plugged into previous discussion or not common knowledge and that they are not familiar with, till they have to deal with it... Name of the game. Guess I don't know either as I can't tell you which plug.


If you clay a head for clearances you would like about .050 clearance. A 1/8" is .125.


Bummer about the motor.






.

[Tim Ayers]

I have Cyclone heads that have 5/8" reach as well. Using an ARP spacer (that needs to be drilled out to 14MM) and a copper washer from the Accel spark plug indexing kit, I was able to get the proper reach using a 3/4" reach plug.

I'll get the part #'s from my Amazon order later tonight.

[sshep]

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Circa 2002 Offy head castings running NGK B4L's for almost 20 years. MSD dizzy and MSD6AL box. 3-97's on an offy intake. 286-50 Merc Block.

[tubman]

Originally Posted by sshep

Circa 2002 Offy head castings running NGK B4L's for almost 20 years. MSD dizzy and MSD6AL box. 3-97's on an offy intake. 286-50 Merc Block.

That'll work as long as you don't mind shrouded plugs and a hit in performance.

One question : have you ever checked the spark plug fitment with the heads off? It should look like this.

[sshep]

Originally Posted by tubman

That'll work as long as you don't mind shrouded plugs and a hit in performance.

One question : have you ever checked the spark plug fitment with the heads off? It should look like this.

To be truthful I have not pulled the head to check plug fitment. I understand your argument, but the car runs way too good to worry about a slight decrease in performance. I would suspect that no one would be able to tell the difference in performance by the "seat of there pants" so to speak. If I was going to Bonneville,1/4 mile drag racing, or some other timed event then yes, of course I would want to do that. I am pretty familiar with reading plugs for fuel and timing. I drag raced an OT NOS BBC for many years so reading plugs was critical to know if the engine was happy. So again I understand the argument for the plug not being shrouded for maximum performance, but I guess I find it unnecessary in my particular application. YMMV

[deuce lover]

NGK B4L or B6L

[drolston]

Originally Posted by deuce lover

NGK B4L or B6L

I am running NGK 6BL in recent Offy 400 heads with good results. They are about one thread shy of coming flush. The reach is 11.2mm which is just a shade over 7/16".

[Steve in Denver]

Thanks, will post an update when the machine shop gets it taken apart later this week.