[Dick Boneske]

I think you're right. It's hard to believe they used that flat 275 intake on the Tripower 421 HO.

[John V.]

Several years ago I tried to collect info about the X & O stampings on the '63/'64 9770716 Head castings.

Never was able to collect enough data to come to a conclusion.

I did speculate based on what I did find and have mostly forgotten if any data contradicted what I came to believe.

It starts by the knowledge that PMD used these Heads for the '64 421HO and also for various '64 389 applications, including the '64 GTO.

All had an advertised 10.75 cr. The advertised cr did not especially match reality, but it made no sense to me that the 421HO Heads could be used directly on a 389 and yield a higher cr than the 10.5 cr 9773345 "regular" 389 Heads without some milling.

b-man, you may be right about the valve relief accounting for the chamber volume difference.

But I was never very "satisfied" by the various explanations that suggested larger vol reliefs used for the 421 pistons and/or head gasket thickness differences.

Especially since the Heads exhibited the stampings.

Then there was the fact that the early '64 GTO Heads made use of the lighter duty Valve Springs, switching to the 421HO Valve Springs around Nov. '63.

I have a pre-Nov Head example which I believe was original to my own Nov. built GTO which was stamped X X and a later '64 Head that was stamped X O (not original to my GTO).

My speculation is that the early X X Head indicated a Head milled for 389 use with lighter Valve Springs.

And the later X O Head indicated a Head milled for 389 use with heavy duty (421HO) Valve Springs.

What I was never able to find were examples of 9770716 Heads known to be original to a '64 421HO to see how they may have been stamped.

Logic suggested that they wouldn't be stamped at all, only those reserved for 389 use would have needed to be specially milled and given secondary stamps to identify them.

Also, IIRC, the 9770716 Head was only used for the 421HO in '63, so no reason for any stamp codes on them (if they only came in one flavor), but finding examples from '63 proved impossible for me.

Just never could prove whether my speculation was valid.

The '65 77 and '66 093 Heads also had shared 389/421 usage too but I never heard of anybody claiming to have seen secondary stamped codes on any of them.

The '64 X and O stampings can be found on the dime sized bosses on the center exhaust ports.

The biggest problem is corrosion of the casting which makes detection of secondary stamp codes difficult.

[b-man]

I have a set of 421 HO 716 heads that are original my 1964 421 HO 45B block.

I will try to get a look at the heads this weekend to check for any stampings.

[b-man]

Originally Posted by johnta1

Mescabug, is that your head?IG car
If so, what engine did it come off of?

The head pictured at the bottom is mine, it was pulled off of a '64 389. The block had no stamping to identify it as it had been ground off by a previous owner, but it must have been either a GTO block or a Tri-Power big car block.

I will try to look for any stampings on them this weekend.

[bill ryder]

Well, after all these years, I thought I new my car pretty well, BUT. I just went to the garage, knowing I wouldn't find any markings but I did. One head has a distinct X on the right side of the dime. Left side of the dime is pretty uneven and I can't make out anything. By the X being on the right, guess I have XX head, which I think I should from this discussion. They are original April 64 heads. "Bill"!

[b-man]

1964 421 HO 9770716 head - X and X stamped:



421 HO head date code - I 30 3 September 30, 1963



1964 421 HO block date code - I 27 3 September 27, 1963



1964 421 HO block code - 45B 370 HP manual transmission



1964 389 GTO (or big Pontiac 389 Tri-Power) 9770716 head - X and O stamped:



389 head date code - B 14 4 February 14, 1964



1964 GTO 389 (or big Pontiac 389 Tri-Power) block date code - B 18 4 February 18, 1964

[tom s]

Bart,any chance you can CC the xx and the xo heads?Tom

[b-man]

Tom,

I won't be able to do it any time soon. I don't have the tools to do it here. Both sets appear to be untouched.

Bart

[tom s]

Thats why it would be a good pair to check.Im betting they are diff.Tom

[John V.]

b-man, that is excellent input. Just not sure what I can make of it.

I will also be curious if there is a chamber vol. difference.

I went back to check some of my notes from years ago. My heads aren't accessible at the moment, so relying somewhat on memory.

First off, the original engine in my '64 GTO was EUN around 101000.

The valve spring change was made at EUN 190810 (somewhat later than I was remembering the other day).

My Head marked XX has a Jxx3 date code as best I remember it. This is the one that I believe was original (at least by cast date, it seemed legit). The Head marked XO has a Exx4 date code, again from memory. The mismatch was consistent with some known history of the car, although not proof positive that the early Head was original to the original engine.

I also reviewed a discussion I had several years ago with Keith Seymore about an early mag article he shared with me. It was a puff piece about the '64 GTO in Hot Rod.

In it, it claimed the Heads were the same as used in the 421HO, and claimed the difference in valve relief vol enabled the 389 to achieve the same cr as the 421, the chamber vol was claimed to be 67 cc in either case. So the claim was being made in '64. No mention of any factory milling for the GTO heads. So whoever provided the info for that article was unaware that the Head was being milled by PMD for 389 use or it truly wasn't being done.

Another article from Popular Hot Rodding that Keith shared with me stated that Royal was typically seeing these heads come in 5-6 ccs over the 67 cc spec, so Royal was milling them for increased cr.

Assuming my early Head is in fact original to my 389 GTO engine and is marked the same as your early 421HO Head example, it would seem that there is some other reason for the change from the XX marking to the XO marking.

It occurs to me that the known change was to the Valve Springs for the GTO Heads starting with EUN 190810 and perhaps the XO came into use at that point. However, to my knowledge, the 421HO did not use the lighter springs at any time, so it would make no sense that later 389 Heads would be marked differently than early 421HO Heads UNLESS there is some difference between them.

Possibly my XX Head is ALSO not original to my GTO. Bill didn't get a clear read of the marking on his late Heads, but if I understand what he wrote, he has an X on the right side boss. So presumably his are marked XX. And they are April Heads, long after EUN 190810.

Your known Feb 389 Head and my May Head (unknown origin) are both marked XO.

So you can see how the data for the stamped markings gets to be pretty confounding.

[tom s]

I'm sure the 64 GTO and the 421 HO used the same cam and we know the same heads so would assume had the same springs.Tom

[b-man]

1964 GTO - 524009 cam.

1964 421 Tri-Power with standard exhaust manifolds - 524009 cam.

1964 421 HO Tri-Power with long-branch HO exhaust manifolds - 9770543 cam.

The 524009 cam has the same profile as the 9779067 cam.

The 9770543 cam has the same profile as the 9779068 cam.

Looking at the valve spring specs listed in the John Angeles/Pete McCarthy Racer's & High Performance Handbook it appears all the above listed engines used the same exact valve springs.

No piston valve notch/relief volumes were listed in the 1964 engine tables, but in the 1965 tables all 389 engines had 4.0cc valve notches and all 421 engines had 10.0cc valve notches.

[johnta1]

1964 Pontiac Engine Chart:





1964 Tempest Engine Chart:



The HO used a 'HD Dual Spring' instead of the dual spring which is different than the normal single spring used on most engines.

[John V.]

Originally Posted by tom s

I'm sure the 64 GTO and the 421 HO used the same cam and we know the same heads so would assume had the same springs.Tom

Tom, this does NOT appear to have been true at the start of the year until EUN 190810, which would have been assembled some time around late Dec. '63.

Pontiac documentation exists which details the valve spring change for the GTO Head usage, no mention that it affected the 421HO Head usage.

If anybody wants, I can scan the document. It is also referred to in the '66 MPC for Valve Springs with the EUN break point noted.

b-man, would the '65 piston valve reliefs be same as '64? That 6 cc difference is a lot more than I would have guessed, but it seems to confirm that the cr was "controlled" by the valve relief difference between the 389 and 421.

But that still leaves the XX and XO stamp markings to be explained.

[tom s]

I have owned a lot of 716s over the years and all have been dual springs and I never put them on a spring checker.I just thought they were the same.I never payed any attention to the XX or XO codes on them either.I would be interested in the CCs of both untouched heads just for snicks.Tom

[johnta1]

Here are a couple pics from McCarthy's book:







I still can't find the bulletin or whatever it was that had the info on how/why these were stamped this way.


Someday, maybe.

Anyway, I think this was kind of a standard practice doing it this way, just not sure when they ended it.
(probably when they quit putting that flat boss on the center exhaust ports?)

Later years like from 73 on they stamped the ID's on the upright boss like the 6X - 4, etc.

[MescaBug]

Originally Posted by johnta1

Mescabug, is that your head?
If so, what engine did it come off of?

Didnt saw your posts.

Found them on my computer. I probably got them from the forum here.

Like b-man said, I think its his.

[John V.]

Originally Posted by tom s

I have owned a lot of 716s over the years and all have been dual springs and I never put them on a spring checker.I just thought they were the same.I never payed any attention to the XX or XO codes on them either.I would be interested in the CCs of both untouched heads just for snicks.Tom

I also am curious to know about the ccs, although I'm leaning toward the likelihood that they were machined the same.

I do not recall how the attached pages were published, I'm thinking something like a News Flash, maybe somebody here knows for sure.

The purpose was to provide the necessary info for the unique features of the GTO engine, and it is here where they comment about the change in springs for the GTO engine.

As you can see, they were all dual springs. But the early production GTO Heads used the same standard springs used for the 10.5 cr 389 and the 326 Tempest Heads.

Later production was switched so that the GTO Heads used the same Heavy Duty Springs that were specified for the 421HO application in '63 and '64.

I have also relied on what McCarthy wrote about the meaning of the X and O stampings on pre-'64 Heads.

But unless the known original Heads b-man has from a '64 421HO were incorrectly selected at Engine Assembly, I would NOT have expected them to show XX as if to indicate standard springs (or at least, NOT HD springs).

And if Bill Ryder's April Heads are really stamped XX, long after the GTO valve springs were switched to the HD, that also makes no sense.

And I have a May Head stamped XO. Certainly by that date, there should have only been one flavor of the 9770716 Head.

It should be obvious that the original p/n for the GTO Head Assembly was NOT 9770981.

Maybe somebody has an early unrevised '64 MPC that lists the original GTO Head Assembly p/n.

My guess is that it would have been p/n 9774767.

The '64 10.5 cr 389 & 421 Head and Tempest 2 bbl 326 Head (8.6 cr) was p/n 9774766.

The '64 10.5 cr Tempest 4 bbl 326 Head was p/n 9774768.

Head Assembly p/ns were often issued in consecutive p/n sequence so the "missing" p/n 9774767 would logically have been the GTO Head with standard Valve Springs.

And then obsoleted when the GTO was switched to the HD Valve Springs.

As an aside, according to the MPC, there was no special Head Gasket used for the GTO or 421HO. All of them specified p/n 542973 for service, same as for all '63-'66 V8 applications.

It occurs to me that when the early GTO Head shared the same casting as the 421HO Head, but specified different Valve Springs, there may have been some Engine Assembly confusion.

That could be the reason b-man's 421HO example shows the XX code. Perhaps that Head was intended for an early GTO engine but mistakenly assembled on a 421HO engine?

Bill Ryder or anybody else that has a set of known original GTO Heads, it would be useful to see if they are stamped XX or XO and what dates are on them.

Same for any later 421HO Heads.

And for that matter, would be interesting to know if the '63 421HO Heads have any stamped markings.

Unfortunately, I think I am right back where I wound up several years ago. Contradictory evidence even given the fact that the early '64 GTO Head was known to be different in some respect from the '63/'64 421HO Head and later the '64 GTO used the same Head as the '63/'64 421HO. Just can't establish what the XX and XO was supposed to signify.

[johnta1]

Cool docs, John.



If the 'Eng No. 190810' is the Motor Unit Number/Engine Unit Number, what would the approximate date be for them?
(from the valve spring info on last pic)

Dec. 63/Jan.64?

[John V.]

John, see pg. 2, EUN 190810 was assembled 12-19-63.

That break point date is consistent with my '64 engine log;

EUN 163xxx has a block cast date of K283
EUN 203xxx has a block cast date of L233
EUN 225xxx has a block cast date of A64

Cast dates can vary but generally, I would say that 9770716 Heads cast after about L153 should generally all have gotten the HD valve springs at Head assembly assuming they stopped using the standard valve springs on or about 12-19-63 and on that date most of the Head castings would have been cast L153 or later.

I imagine some earlier castings would have been assembled after the break point, but not a lot. Every now and then, I found a block cast much, much earlier than the engine assembly date (established by the EUN). So it is possible that a rare Sept, Oct, or Nov Head casting wasn't assembled until after the change was made. But for sure, you should never find a Head cast after 12-19-63 assembled with the standard springs.

If the stamped codes indicate the springs used for the Head assembly, you would not find the standard spring stamp code on a later dated Head casting.

My assumption is that the XX would indicate the standard springs. So I would not expect XX stamped on later dated Head castings. If Bill's April Heads are stamped XX, that would blow that assumption up.

I never recorded Head cast dates for my log (since it is under the valve cover), just the block cast date. But you can see that the block cast dates were usually within a few days or less of the date the engine was assembled. From what I understand, the Heads were typically cast very close to the date of the block in many/most cases. So that is why I suggest L153 would be a pretty good guess of the break point.