With the assistance of everyone involved, I believe we are starting to unravel this mystery a bit.
I took the chance this weekend to pore over my May 1968 and March 1969 parts manuals to compare everything connected to the 1967 670 casting. I found some very interesting information that may point us in a direction where we can eventually piece this puzzle together. Keep in mind, Ram Air-spec 670s are excluded below.
Background
There were other 400 applications in 1967, but it’s generally well understood that the 670 was used on Grand Prix’s 400 4bbl, the various 400 4bbls available in the Firebird and GTO, the base 428 4bbl (360 hp), and the 428 H.O. (376 hp). It’s these latter applications that may hold the key to piecing this together.
Cylinder Heads
The parts manuals show that two different 670 castings were used during the 1967 model year. Both were application dependent.
• 9788066- Cylinder Head was specific to Grand Prix’s 400 4bbl and the base 428 4bbl (360 hp).
• 9788067- Cylinder Head was specific to GTO and Firebird 400, and the 428 H.O.
Intake and Exhaust Valves
The parts manuals show that a single exhaust valve was used in the 670. The 1.77-inch number-9787584 Exhaust Valve was common to all applications. However, two intake valves were used.
• 9787587- Intake was specific to Grand Prix’s 400 4bbl and base 428 4bbl.
• 9789725- Intake was specific to GTO and Firebird 400, and 428 H.O.
Obviously both intake valves are 2.11-inch diameter. I believe both were constructed of SAE-1041 alloy with aluminum coated faces, but that 9789725 was further treated to a chrome-plated stem for improved performance.
Valve Springs
Pontiac used the same outer and inner valves springs on the intake and exhaust valves to arrive at the required spring pressure for a given combination. The parts manual shows that 9779009-Outer spring was common to all 670 applications. The Inner valve springs, however, were application dependent.
• 9779008- Inner was common to Grand Prix’s 400 4bbl and the base 428 4bbl regardless of transmission. It was also common to GTO and Firebird 400, and 428 H.O. with AUTO trans.
• 9781476- Inner (with increased rate) was specified to GTO and Firebird 400, and 428 H.O. with MANUAL trans.
Cylinder Head Gaskets
The parts manuals show two different cylinder head gaskets depending upon the application.
• 9787642 has a nominal thickness of approximately .052-inch. Usage was specific to Grand Prix’s 400 4bbl and base 428 4bbl.
• 9785654 has a nominal thickness of approximately .042-inch. Usage was specific to GTO and Firebird 400, and 428 H.O.
So what does all this tell us? It suggests a few different scenarios.
1) Since two different 670 cylinder head castings (9788066 vs 9788067), we can surmise that there had so be some type of special machining that differentiated the two cylinder heads that ultimately required a different part number. Since two intake valves (9787587 vs 9789725) were also used, based stated application usage it’s quite likely that intake valve guide clearance on 9788066 was compatible with the basic stem finish of the 9787587 valve, and that 9788067 included additional intake guide clearance (or a slightly larger overall guide diameter) to accommodate the chrome-plated stem of 9789725. There absolutely had to be a way for Pontiac to identify this in service.
2) The varying cylinder head gasket thickness was a way to give 428 H.O. a compression ratio boost when compared to the base 428 4bbl. I believe the advertised compression ratio of 10.75:1 and 10.5:1, respectively, was overstated. The actual compression ratio of either should roughly .5 less. The .052-inch gasket in Grand Prix’s 400 4bbl and base 428 4bbl applications gave buyers engines that were less sensitive to fuel octane, and possibly more user-friendly on the street.
3) With the help of a 1967 A-car Engine Assembly chart that lists the components comprising every A-car engine produced that year, I can make a few accurate statements. All GTO 400s included the 400 Ram Air utilized cylinder head gasket 9785654 and actual compression ratio was nearer to 10.2:1 than the advertised ratio of 10.75:1. The chart also clearly lists two different COMPLETE ASSEMBLY part numbers for the 670. 9787603 is specific to GTO 400’s with AUTO trans while 9786708 is specific to GTO 400’s with MANUAL trans. Undoubtedly, the differences were directly related to the valve springs. (We can surmise that the 428 H.O. followed suit.) Pontiac had to have a way to visually identify these.
4) Pontiac believed that buyers opting for non-H.O. 421 (in 1966) and 428 4bbl (1967-1969) backed by a MANUAL trans (as well as Grand Prix buyers opting for standard 400 4-speed) were purposely selecting those combinations for price and low RPM operation. With high RPM operation an unlikely concern, these particular applications used the same valve springs as those specified with the automatic.
With all this said and what’s been contributed so far, we may be closer to understanding the 670 differences that might require such stampings as “O”, “Z”, and Ram Air’s “X” on the 670 castings. Is the following possible?
• “O” denotes Grand Prix’s 400 4bbl and base 428 4bbl specific 670 assembly.
• “(blank)” denotes 9787603-specific 670 assembly for AUTO trans GTO and Firebird 400s and 428 H.O.
• “Z” denotes 9787608-specific 670 assembly for MANUAL trans GTO and Firebird 400 and 428 H.O.
• “X” denotes the Ram Air-specific 670 assembly for the 400 Ram Air option in GTO and Firebird.
This is simply my best guess at this point. I cannot claim any accuracy just yet!
Thoughts?
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