# Poncho 428 iron D port heads, spark plug question



## danford1 (Nov 15, 2016)

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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## PontiacJim (Dec 29, 2012)

danford1 said:


> 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.
> ...



"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. :thumbsup:

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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## 64GTOConvertible (Aug 28, 2016)

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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## danford1 (Nov 15, 2016)

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


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## PontiacJim (Dec 29, 2012)

danford1 said:


> 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. :yesnod:


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## danford1 (Nov 15, 2016)

PontiacJim said:


> 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. :yesnod:



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

Danford1


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## Pinion head (Jan 3, 2015)

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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## 1968gto421 (Mar 28, 2014)

Thanks you two, with this respiratory infection plaguing all of us here, I needed a laugh.


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## 1968gto421 (Mar 28, 2014)

Pinion head said:


> 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.:surprise:

(Apologies to Danford for butting in his post)


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## gtoguy389 (Jun 3, 2012)

> 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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## danford1 (Nov 15, 2016)

Sorry for the late response guys. I've been very busy lately. This is one of those projects that every day and every step of the way is a fight. No matter how small the task it will be some sort of a fight.

I just looked quickly by the distributor for the block code. I saw a D 15 there. It had a D with a triangle next to it and under that was 15. I did see there were some smaller numbers on the block to the left and close to the back of the block but I had on the wrong glasses to read those. Plus, I'm short, this car is partially jacked up and I can't see back there easily.
The heads have a 6X on them and it has an 8 bolt water pump.

I should be back at it today. I have a couple coolant leaks to fix, broken and leaking trans fluid TV cable connection and some other fights to fight. On this car even the little things turn to a crap fight, Murphy likes me  The New front freeze plug in the head is leaking coolant but to get at it I have to remove that 467 lb R12 A/C compressor - again. I'm Real excited about doing that. I'm not only short but old and weak too...

The owner of the car bought a very nice brand new Be Cool radiator with electric fan set up for me to install (Thanks Doug). I need to run some more wires where I already have them bundled and tie wrapped nicely. I just love doing things over and over. I get a lot of opportunities to do that on this car.

Danford1


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## 1968gto421 (Mar 28, 2014)

gtoguy389 said:


> 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.


Thanks, gtoguy389, I appreciate it. My other thought on the 8 bolt timing cover/water pump is that someone who did the engine swap used the timing cover/water pump from the '68 400 that was originally in the car.


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## 1968gto421 (Mar 28, 2014)

danford1 said:


> Sorry for the late response guys. I've been very busy lately. This is one of those projects that every day and every step of the way is a fight. No matter how small the task it will be some sort of a fight.
> 
> I just looked quickly by the distributor for the block code. I saw a D 15 there. It had a D with a triangle next to it and under that was 15. I did see there were some smaller numbers on the block to the left and close to the back of the block but I had on the wrong glasses to read those. Plus, I'm short, this car is partially jacked up and I can't see back there easily.
> The heads have a 6X on them and it has an 8 bolt water pump.
> ...


Sounds like great fun in a Michigan winter:surprise: Danford, I wish you the best of luck with this mess. As PJ said earlier, you will be a Pontiac guru!!


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