[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

[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.

[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.

[bill ryder]

Sorry, Haven't paid any attention for the weekend. I need to correct myself. Originally I thought the X and O was on the same boss, not one on each. So what I said before is not correct.
I've tried to upload a picture with no success so I will explain:
My X is on the right hand side of the LEFT dime sized boss. Not centered. The right hand side dime sized boss is almost empty with just a small arc on the top side about 2 o'clock position. Could be part of an O. I can't say for sure. The other head is completely void of marks. SORRY if I got you going in the wrong direction!!!!!!!!!!!! "Bill"!
Just found the head pics I took in 1990. Both heads date code is C254.

[John V.]

Follow up to my own earlier post with the Engine Spec pages, this WAS published as a News Flash in case anybody wants to hunt for it.

News Flash No. 64-95, dated 5-1-64.