[PontiacJim1959]

Originally Posted by Kenth

I have these AMA specs too.
Those differences in spring loads on #48 heads are minor and not enough to indicate different springs IMHO, more like an indication of usage of different camshafts, #744 M/T and #068 A/T, with different cam lift at the time of measurement for the AMA specs.

The other heads #62, #47, #46 and #45 have other springs due to usage of #067 or milder camshafts.

OK, that may be a good way to look at it. Not sure how they were measured, but I would think that using a spring that fits a number of heads/applications would be what the factory was after just in saving money not having to buy several different type spring rates.

[John Eiseman]

Following. Interesting fodder here!
Has anyone come.up with any PMD special bulletins that might shed some light on the differences with cc sizes or the additional X above the cast #4 ?

Like others have said , putting a 65 cc head on one side and a 72 cc on the other would.make for. an uneven combination.

[steve25]

Here's factor info showing the casting #13 head of 1970 at some 75 CCs.

Weather this is completely right or not I can't say, as the strange thing here is the chart showing that a 66 CC casting # 12 would only provide a quarter of a point gain in compression over the 10.25 comp of the 13 casting .

So I don't know, maybe Pontiac was still trying to give the racers a edge and NHRA the shaft for all the pro Chevy crap they tossed on Pontiac in the 60s!

Also would anyone like to venture some info as to what it was that gave the 455 in the GP a 10 hp gain over the 455 in the GTO?

Could not the 455 in the GTO be had with a cold air package which the GP could not?

[PontiacJim1959]

Originally Posted by steve25

Here's factor info showing the casting #13 head of 1970 at some 75 CCs.

Weather this is completely right or not I can't say, as the strange thing here is the chart showing that a 66 CC casting # 12 would only provide a quarter of a point gain in compression over the 10.25 comp of the 13 casting .

So I don't know, maybe Pontiac was still trying to give the racers a edge and NHRA the shaft for all the pro Chevy crap they tossed on Pontiac in the 60s!

Also would anyone like to venture some info as to what it was that gave the 455 in the GP a 10 hp gain over the 455 in the GTO?

Could not the 455 in the GTO be had with a cold air package which the GP could not?

With regards to the GP 455 numbers, my guess might be because the GP was heavier? Pontiac had their weight-to-HP rule, so checking the weight of a GP may provide the answer on that one.

My understandings are that Pontiac made the combustion chambers 2-3 cc's larger because they took into account the carbon build up that would take place on the chambers, valves, and pistons when leaded gas was being used. This carbon build up did equate to a small amount of cc's. Anyone who used to live in the day of leaded gas, probably has experienced all that carbon black smoke pouring out the exhaust after the car has been driven easily, then put the pedal to the floor. If the carb was running rich, it was worse.

And, my guess is also that head chambers vary simply because of the machining processes back then. They were mass produced and machines were not the accurate computer controlled equipment used today to hold exacting & tight tolerances head after head. So these #48 heads can be measured all day long and I don't think any definitive conclusions will ever turn up other than the cc chambers fall within a certain tolerance range. The advertised compression was always higher than actual compression. It was also PR advertising to get buyers interested in their products, so trying to keep the engine sizes/spec's compared to other makes similar had the buyer looking at the other features the car offered.

[shermanator2]

Getting back to my #48 head that started this whole very interesting discussion, I have now cleaned it up and made some measurements and below is what I have found.

Maybe the back story first. Around 1980 an acquaintance of mine tore gown his ’69 GTO engine. After everything was hot tanked (like we did back then), someone stole his rotating assembly and apparently one head. Everything else sat outside for long enough to get rusty, which did not take long because it had all been well degreased. Then he gave what was left to me. I have had it all sitting around for about 40 years.

The block is standard bore. It has an A149 date code and is stamped and is 0454946 WT. The #48 head date code is A139, leading me to conclude that in all likelihood, this head is original with the block. There is not X or anything else stamped on the pads above the 4 or 8.

After cleaning this head up and studying it, I believe that this head is completely untouched. The seats all measure about .090 width and there is no top cut leading me to believe that it has never had a valve job. The valves also all look to be the same thickness and I see no evidence that they have been faced. The surface “looks” factory from all can tell.

I cc’d all four chambers. I measured 66.0 cc for the center two chambers and 66.6 for the outer two chambers. The difference could be my measurement accuracy, but the point is that they are in the ballpark of 66cc and this head came off of a 400, not a 350. Using a Felpro 8518PT to scribe a line, the bottom of the chambers all measure 0.270” form the gasket.

I also wanted to get some measurements of the depth of the chambers. Using a depth micrometer and compensating for the diameter of the rod, I measure the deepest part of the chamber to be 0.722” and the opposite side between the valves to be 0.148”. The latter is what I think steve25 was calling shallow side depth.

Some discussion of valve length and spring installed height has come up. The overall length of the intake valves I measured to be 5.090 to 5.093, at the exhausts were 5.078 to 5.083. My standard spring height micrometer would not collapse enough for me to get keepers in, but I used some test springs and digital caliper to estimate the installed height to be right near 1.600. I do not have a spring scale currently so I cant get the spring pressures right now.

I would like to get more measurements documenting the surface of this head, but I am not sure where there is a good reference. It is not obvious to me what points the tooling picked up to machine the surface and the chambers. I wonder if measuring the depths of all of the bolt holes would be a good reference.

If anyone would like any other measurements, let me know.

I have another head with a A099 date code coming from John Eiseman after the holidays. I will measure it too and hopefully it is a good match to this one.

I have attached pictures of each chamber

[steve25]

Thanks for your time input for getting these facts about your head, and the one you will be getting and putting time into also!

I find it extremely interesting that Overall lenght wise your Exh valves are shorter then other factory high comp D port valves and also any aftermarket replacement I have ever seen in hand or listed in a catalog!

I wonder if the next head that on its way to you will check the same lenght wise?

[shermanator2]

Interesting observation about the exhaust valves. That could add a twist to this trivia puzzle. The head of those exhaust valves looked thinner to me than typical. I did not measure it, but I will. If 5.110 is normal, and mine are 5.081, and the .029 difference is in the thickness of the head, then the thinner valves would add 1cc to the chamber volume. Normal valves would make my head 65cc. We can wonder if the factory did this intentionally for some time and then later maybe said F'it and just opened up the chambers. I know 1 cc is not much, but they may have been trying to meet some internal spec that they were just a little off of, and thinning the valve was for some reason the method the junior engineer in charge of the #48 chambers chose.

[JKrull66]

Not to turn the attention of this thread. But I have a G309 400 ys block w a cast 70 by the Dist hole. Wondering the latest (69) head Earliest (70) block and if the left overs or line change could have bleed over to the next production yr with extras till used up. This block is not related to these heads just using as a date ref. With the seeming 12 like chamber and the x... I know this is common in the tractor world. Im afraid my brother knows no history of this head. just that hes had it since the 70s
Thanks,
John

[PontiacJim1959]

shermanator2 - "The overall length of the intake valves I measured to be 5.090 to 5.093, at the exhausts were 5.078 to 5.083."

The AMA specs for the 350HO and RA head are - Intake length - 5.093" Stem Dia. - .3419 - .3412 Exhaust length - 5.082" Stem Dia. .3414 - .3407

[steve25]

In responce to post 88, so called extras would have gone into the factory replacement parts inventory.

[shermanator2]

As I mentioned earlier, I was getting another #48 head from John Eiseman. That head has a date code of A099 and an unknown origin or history. As a reminder, my original one is a date code A139 and came to me with a WT code 400 block numbered 0454946 and date code A149 about 40 years ago. I think that it is only reasonable to assume that this head came from the factory on that block. It came out of a GTO if that matters. The A099 head was cast on a Thursday and the A139 head on the following Monday, so there was only two production days between them.

The A099 head, like the A139, appears to be unmolested. It does not look like it has ever had a valve job or been surfaced. There is no X stamped on either head. Other than more rust pitting it appears to be just like the A139. It came stripped, so I have no more valve or spring data to report.

I put the valves out of the A139 into the A099 and cc’d all of the chambers. I got 67.0, 66.2, 67.0, and 66.4. With the A139 I got 66.6, 66.0, 66.0, 66.6. For all 8 chambers combined I get an average of 66.5cc and a range of 66 to 67cc.

For the shallow side depth I measured .143 to .148 on the A099 head and previously .147 to .152 with the A139 head. This is a hard measurement to make, so that variance could be measurement error.

For both heads I measure .270 from a line scribed with a Felpro 8518PT gasket to the edge of the chamber on the spark plug side.

So I conclude that they were making at least some #48 heads in January of 1969 with 66 to 67cc chambers and at least one of those came on a 400. The very late 1969 head that Jay S had does seem to be different. Although the short side depth seems to be the same as mine, it is more “open” on the spark plug side by about .040 and seems to have 3 or 4cc more chamber volume. Too bad we we don’t have them side by side so we could make sure we were using the same measuring techniques and look for other subtle differences.

[steve25]

Thank you for all of your efforts and time input on getting this info for us!
It surly helps to put more of the puzzle together

I wonder if what we are seing with the bigger chambered later cast 69 heads is a response by the factory to dropping the 744 cam and using the 068 cam across the board?

The 068 cams far less aggressive overlap would require less compression and the 3 to 4 greater CCs could have been there response! ?

[shermanator2]

Originally Posted by steve25

Thank you for all of your efforts and time input on getting I wonder if what we are seing with the bigger chambered later cast 69 heads is a response by the factory to dropping the 744 cam and using the 068 cam across the board?

The 068 cams far less aggressive overlap would require less compression and the 3 to 4 greater CCs could have been there response! ?

I was thinking the same thing and calculated the same 3 or 4cc to keep the same dynamic compression between a 744 and 068 cam. Being an engineer however, I would have to agree with those that have said that this would have to have a different part number for the bare head and any higher level assemblies that included it. Pontiac seemed to have so many different casting numbers for small differences, that I would think that they would give it a different casting number.

I did run into a SBC head casting years ago where Chevy used the same full long casting number for two versions with different size valves. I had a small valve set and cut them for the bigger valves and ran them in my truck for a few years. I presume that they must have had a different part number, and different application listings, but there was no distinguishing marking on the head itself that I knew of.

I did have another odd thought about how the chambers could have changed sizes without a part number change. What if the head was originally designed to have 70cc chambers, but they were incorrectly machining them on the factory floor? At some point this "defect" was discovered and corrected. The heads after that were built to print, so no new part number. I could even imagine that they discovered this on the test stand when they were testing the change to the 068 and had a detonation problem or maybe it just showed up in the field.

[PontiacJim1959]

Originally Posted by steve25

Thank you for all of your efforts and time input on getting this info for us!
It surly helps to put more of the puzzle together

I wonder if what we are seing with the bigger chambered later cast 69 heads is a response by the factory to dropping the 744 cam and using the 068 cam across the board?

The 068 cams far less aggressive overlap would require less compression and the 3 to 4 greater CCs could have been there response! ?

Just to throw this in as well. Emissions controls were really ramping up in 1968 and Federal mandates were being forced on the car manufacturers. I am thinking that there may have been an "early" production and "late" production head based on the CC's and the effects on emission numbers out the tail pipe.

The early heads would have been made in late 1968 for the 1969 models - so smaller combustion chambers. By 1969, emissions testing was on a role and Pontiac may have had to comply by Jan. 1st, so later heads were opened up to have more CC's, thus lower the emissions readings and adhering to Federal requirements.

Just a thought. It would take an assortment of heads having "early" and "late" casting dates is this theory were true.

[tooski]

Based on info from a couple of sources I have been using 72 cc for CR calculations. Since the heads are off I plan on measuring the chambers.

[shermanator2]

If it were a Jan 1st, 1969 emission thing, then the bigger chamber heads would for sure have a different part number so that if audited, Pontiac could demonstrate to the government that they were building compliant cars after Jan 1. They would also need a different service replacement part number so that non compliant heads did not end up on post jan 1 cars. The big and small chamber heads would not be functionally equivalent because the big chamber polluted less and the small chamber heads made more power. Also, both of my heads were cast after Jan 1st 1969 and are of the 66ish cc variety, not the 70ish cc variety exemplified by the head that Jay S. had.

[steve25]

Emissions where not anywhere as much of a factory concern by 69 with the open chambers be they large or small CCs since in late 68 they proved that fact by doing away with the air injection passage cast in below the exhaust side valve cover lip.

As always a much larger part of meeting the emissions Standards of the day was the overlap numbers of the Cam.

[Jay S]

Could the 69 Firebirds that where sold early in 70 because of the production delays of the new body style have had different engineering parameters than the other 69 cars?

There was probably a cut off date that a car had to be made by before it had to pass 70 regs’ instead of 69s????

It would be interesting to know if there are #48 heads that were mfg’d after Pontiac started making the 1970 #12 RA 3 heads (the first casting run of #12s appeared to start following week after the #48 heads that Krull and I had. Maybe for the late produced firebirds the lower power engines didn’t need any revisions, but the RA3 did. Curious if there where any RA 4 Firebird’s made in that same late time frame?

[Shiny]

shermanator2's comments reminded me of the ID question. I remain skeptical of 2 chamber volumes without unique identifiers but am also ignorant and surprised this is even a mystery.

Is it clear the 'x" isn't correlated with volume? If not, what is it?

2 sets of 48 heads on eBay right now. This pair appears to have the "x", date codes D299 and D309, but volume is not listed:

https://www.ebay.com/itm/PONTIAC-400...kAAOSwQihgBMqL





This pair doesn't have a great photo but the 'x' is not obvious. "Heads are 68cc's. Dates are K138 and L128":

https://www.ebay.com/itm/Pair-Pontia...sAAOSwj4Vf9h1h

[unruhjonny]

I just noticed this and figured I'd reply:

Originally Posted by JKrull66

Not to turn the attention of this thread. But I have a G309 400 ys block w a cast 70 by the Dist hole. Wondering the latest (69) head Earliest (70) block and if the left overs or line change could have bleed over to the next production yr with extras till used up. ...

If my numbering is correct, G309 would be July 30th 1969;
Since July was the last month of the production year, and I have understood that the last of the prior model years cars would be produced early in the month of July, it stands to reason that something cast on the second last day of July would probably be used for something produced for the following model year.

I think it is common to see items cast in August as being for the new model year, and if my memory serves me, it seems to be quite rare for anything cast after June to be used for that same model year.

I would guess that in this case, we could conclude that when that block was cast (on July 30th 1969), the "70" was there to indicate that it was being cast for the 1970 model year.
Was that block cast with the '914 casting number?