Quote:
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
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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.