Poncho 428 iron D port heads, spark plug question - Pontiac GTO Forum
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post #1 of 13 (permalink) Old 01-03-2017, 06:02 PM Thread Starter
 
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Poncho 428 iron D port heads, spark plug question

I'm installing a 428 Pontiac and 200-4R trans into a 66 Lemans for a guy. I pulled out a 326 and power glide.
The 428 is out of a 68 full size car. (that is what I was told). It has already been rebuilt and is "ready to go". (again what I was told).

Here is the problem. The spark plugs won't thread in... I went to O'Reilley and got plugs for a 68 Catalina with a 428. The computer said AC Delco R43S. The engine has stock iron D port heads. It is like the thread diameter is to big for the holes. I took a R45S plug out of the 326 I just pulled from the car. It won't screw in either but the new R43S screws right into the 326.
Did Pontiac use a special plug thread is some 428 heads? This seems crazy. I tried multiple holes and none of them will accept a plug. All the holes look fine thread wise. Nothing is stripped.
Have you guys heard of this before?
I've been working on cars for 50 years and only know of two thread sizes for spark plugs, 18mm and 14mm.
Is there another size I'm, not aware of?

I have a thread chaser but wanted to check here first before I thread that is and really mess things up...


Danford1
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post #2 of 13 (permalink) Old 01-03-2017, 06:53 PM
 
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I'm installing a 428 Pontiac and 200-4R trans into a 66 Lemans for a guy. I pulled out a 326 and power glide.
The 428 is out of a 68 full size car. (that is what I was told). It has already been rebuilt and is "ready to go". (again what I was told).

Here is the problem. The spark plugs won't thread in... I went to O'Reilley and got plugs for a 68 Catalina with a 428. The computer said AC Delco R43S. The engine has stock iron D port heads. It is like the thread diameter is to big for the holes. I took a R45S plug out of the 326 I just pulled from the car. It won't screw in either but the new R43S screws right into the 326.
Did Pontiac use a special plug thread is some 428 heads? This seems crazy. I tried multiple holes and none of them will accept a plug. All the holes look fine thread wise. Nothing is stripped.
Have you guys heard of this before?
I've been working on cars for 50 years and only know of two thread sizes for spark plugs, 18mm and 14mm.
Is there another size I'm, not aware of?

I have a thread chaser but wanted to check here first before I thread that is and really mess things up...


Danford1

"I've been working on cars for 50 years and only know of two thread sizes for spark plugs, 18mm and 14mm."

Welcome to the world of Pontiac cars, unlike Fords & Chevies.

Sounds like the heads are later 1970's heads with the "peanut" plugs (which would be done to lower compression). The center exhaust port typically as two numbers that will identify the heads. There is also a date code that typically is found on the head outside below the valve cover, should be 4 numbers. The last number is typically the year.
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post #3 of 13 (permalink) Old 01-05-2017, 06:51 AM
 
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What Jim said. You have to look at the seats as well, but my guess is you have later heads. My 68 428 came with #16s and uses the larger plugs. Since just about any Pontiac head will bolt on, yours could be anything. Send us the codes!!!
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post #4 of 13 (permalink) Old 01-05-2017, 09:02 AM Thread Starter
 
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It turns out the engine is a 1968 but the heads are 1978. They take the tapered seat plugs. I bought some R44T plugs and they fit right in.
Thanks guys.
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post #5 of 13 (permalink) Old 01-05-2017, 08:03 PM
 
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Solved.
It turns out the engine is a 1968 but the heads are 1978. They take the tapered seat plugs. I bought some R44T plugs and they fit right in.
Thanks guys.
Danford1

Ah, probably the 6X's which are very popular swap. Eventually this deal will make you a Pontiac guru whether you wanted to be or not.
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post #6 of 13 (permalink) Old 01-05-2017, 09:42 PM Thread Starter
 
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Ah, probably the 6X's which are very popular swap. Eventually this deal will make you a Pontiac guru whether you wanted to be or not.

Gee, Thanks, just what I always wanted to be ...

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post #7 of 13 (permalink) Old 01-06-2017, 12:53 PM
 
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Tapered seat spark plugs began first Pontiac V8 use in late '71 production. Have never ran across a set of tapered seat plugs in a set of '71 model "94's or "96" cylinder heads, but have pulled many many '71 455's with "66" heads. One late '71 YC engine actually had an original "66" casting head with std seat plugs on one cyl bank, & a week later cast "66" head with tapered seat plugs on the other side. At the time, my thoughts were it must have been fun to change plugs, once AC spark plugs quit coming in the thin cardboard boxes & went to coming in plastic skin packs of 8.

Danford, did you ever note the block casting date on the 428 block back by the distributor? Actual '68 usage 428's are fairly uncommon, yet nearly a 100K '69 428's engine code YH were built, & many have gained 455 cranks during the rebuild process.
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post #8 of 13 (permalink) Old 01-06-2017, 12:54 PM
 
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Thanks you two, with this respiratory infection plaguing all of us here, I needed a laugh.
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post #9 of 13 (permalink) Old 01-06-2017, 01:10 PM
 
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Originally Posted by Pinion head View Post
Danford, did you ever note the block casting date on the 428 block back by the distributor? Actual '68 usage 428's are fairly uncommon, yet nearly a 100K '69 428's engine code YH were built, & many have gained 455 cranks during the rebuild process.
That's real interesting, Pinion Head. My '68 GTO has a '69 428, 9792968, XF. The engine feels to me like a very stock 428 (or 400) but it has the 6X4 heads and the 8 bolt timing cover, Performer intake and carb. The young couple I bought it from were sure it was a 455 because of super burnouts (they wanted a more fun iconic car, a '60's Mustang convertible!!!) but when I found the 9792968 engine number, I thought "sure" to the 455 notion. Your info has me wondering now what I actually have.

(Apologies to Danford for butting in his post)
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post #10 of 13 (permalink) Old 01-07-2017, 08:00 PM
 
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My '68 GTO has a '69 428, 9792968, XF. The engine feels to me like a very stock 428 (or 400) but it has the 6X4 heads and the 8 bolt timing cover, Performer intake and carb. The young couple I bought it from were sure it was a 455 because of super burnouts (they wanted a more fun iconic car, a '60's Mustang convertible!!!) but when I found the 9792968 engine number, I thought "sure" to the 455 notion. Your info has me wondering now what I actually have.
The 1969 428, 9792968, XF code block you have is from a 1969 Grand Prix rated at 370 HP. Definitely a good strong motor. The 8 bolt timing cover I'm guessing is for the water pump? The 8 bolt water pump was last used in 1968, 1969 was the first year for the 11 bolt water pump. The part number 9792968 was used in both '68 and '69, The date code would verify which year you have, however, they XF code was not used in 1968. Possibly a very early '69 production run block.
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