[gto4evr]

I came across some threads awhile back that threw into question whether a #48 head off a 350 was the same as a RA3 400 #48 head. It had to do with the CC size possibly being different.

This auction seems to confirm that or is this common knowledge and I'm just uninformed?

http://cgi.ebay.com/ebaymotors/ws/eB...m=250152718802

[BLKBRD]

I have also heard of this rumor (mostly on ebay) and have wondered if there was any validity to it. I would suspect that the "48" heads were the same casting regardless of the application. Pontiac was very good at identifying head applications. I'd find it very hard to believe that Pontiac would use the same casting number for heads of a different chamber volume unless there was some other SIGNIFICANT differentiator for each of the castings. From my calculations, the "48" heads used on the 400 C.I. block rated at a 10.75:1 compression ratio would produce a 9.4:1 compression ratio when used on the 350 c.i. block. This compression ratio would certainly be within the realm of possibility in those days.

[north]

Pontiac adjusted compression ratios on different displacements with piston changes not heads. the 48 heads were used on 350, 400 and 428 engines. 428's had an extra dish in their pistons to adjust and 350's have less dish than a 400 to compensate in the opposite direction. If you look in the parts book there is only one part number for those 48 heads for all three displacements.

The eBay listing mentions 66cc but this is probably due to head shaving done for performance or warp correction purposes sometime in the life of those particular heads

[RicksGTO]

this same topic was discussed on another post too:

http://forums.maxperformanceinc.com/...=48+heads+66cc

[John V.]

north, that is a broad statement that is not quite accurate.

In the late '50's, Pontiac used their regular head casting on the 10.5 cr hi-po engine. In doing so, they stamped an X in the dime sized circle on the left side center exhaust port. It is reported that they milled the head to boost compression on this engine, the X signified the milling. In the circle on the right side center exhaust port, an O was stamped to indicate HD valve Springs were installed.

In '63, Pontiac introduced the 421HO, using the 9770716 Head Casting. This head was carried over for use on the '64 421HO as well. For '64, the same head casting was also used for the big car Tri-Power 389 and all '64 GTO engines.

For the 1st 3+ months or so of Engine Production, the GTO heads were not assembled with the same valve springs as used for the 421HO (a cookie for anyone who knows the p/n for the early '64 GTO head assembly). With Motor Unit No. 190810 or so, the valves in the GTO heads were changed to the same HD Valve Springs as used in the 421HO heads.

The MPC ultimately lists the same head assembly p/n for the 421HO, big car Tri-Power 389, & the GTO engines, p/n 9770981.

Were they all really the same? I do not know for certain.

But I have a 9770716 casting that was cast in Oct. '63. It is stamped on the center exhaust port bosses "X X". I also have one cast in May '64. It is stamped "X O". I cannot be positive, but I believe both were originally installed in GTO engines, the first one with 99% certainty.

I do not believe Pontiac had forgotten the earlier practice of identifying same castings with these secondary code stampings. So I speculate that the 389 applications were milled and id'd by the LH side "X" stamp. Early year, when the GTO got the lighter Valve Springs, I speculate that the RH side "X" stamp indicated that. Then later, when they revised the GTO heads to the HD Valve Springs, the RH side stamp became "O". Chances are, the '64 big car Tri-Power 389 heads always got the "X O" stamping. It is virtually impossible to prove, but I would think that the 421HO heads did not get stamped, as they were machined according to the expected usage for them.

These marks are pretty faintly stamped, though fairly clear on my heads. The heads were in pretty nice shape, the stampings were discovered after chemical bath cleaning.

I am not the only one who has ever seen these stampings on the 9770716 castings.

Granted, Pontiac did later learn to use larger dished pistons to allow usage of the same heads on a 400 and 428. I am less certain that this was the case with the 389 and 421, at least not until the revised 421HO piston was released for '65.

The case for the cast code 48 is not so clear. The head itself was almost undoubtedly released for the 350 application. The machining specs likely were suited for that usage.

Surely they didn't plan to machine them for 400 usage and then mill them for the 350 usage. The scheme as I believe was used in '64 might not be realistic for this case.

I don't know if anybody has ever thought to compare the two, but to my thinking, I would expect the 350 version of the 48 head (assuming there was such a thing) would very likely have virtually the same chamber volume (if not shape) as the '68 18 head. The '68 4 bbl 350 was advertised at the same 10.5 cr as for '69.

The earlier post depicts two 48 heads with a modest apparent difference in the machining. From time to time, other evidence has pointed to two different chamber volumes. The MPC lists a common assembly p/n and no secondary stamped markings have ever been noted.

Several years later, Pontiac used the smaller chamber 350 version of the 6X head to give a bit of a compression boost to the W72 400.

Maybe Pontiac used that same trick and employed the smaller chamber 48 heads on some '69 400 engines to give them a bit of a compression boost.

On occasion, guys have reported finding a somewhat larger chamber 48 head. So many years later, it is impossible to know if they cc'd larger because they were machined wrong at the factory, poorly cc'd today, suffered valve recision thru the years, or even misidentified (how many times has somebody thought they found a 48 head when in fact it is a '69 46 head) or if there really is a larger chamber version. Most of the time, guys who own these heads have no idea what engine they were original to anyway.

This thread started based on an ebay listing. I think the answer to that is very simple. The seller was stating that these were 48 heads with 66 cc chambers. He did not intend to compare them to another version of the 48 head, I believe he was comparing them to more common 400 D port heads which typically are claimed as 72 cc.

The premise that there may have been two versions of the 48 head remains unresolved in my mind. Personally, I think that reference to published info on the subject is worthless. I doubt that indisputable evidence to prove it one way or the other will ever be found.

It is confusing enough to pin down the head usage by Engine Code without worrying about the correct chamber vol. for a given casting. Just my opinion.

[north]

What would be indicative is to talk to a Pontiac rebuilder and look at the amount of combustion chamber volume present in the block (not including the head or gasket) of both a 350 and 400 with a stock piston at TDC. Does the difference in indents and deck height relationship in the pistons compensate aproximately for the difference in displacement (13%). This would have to be about 8cc give or take a few CC.

I find it hard to believe that Pontiac would make two different heads with different combustion chamber sizes and give them not only the same two number code but even the same part number. So if someone had to change one head on an engine he could have a 10-12% engineered variance of compression (plus or minus further machining tolerences) from one side of the engine to the other! This makes no sense when we see how many different heads Pontiac went to the trouble of producing that had much much smaller differences from each other than that between the supposed "two" 48's.

I think John V's comment that a decision might have been made to combine the heads of the 350 HO and 400 GTO manual trans into one might be logical, it would have resulted in two advantages:
1..get rid of a specific head for the slow selling 350HO (especially since unlike RA engines it had to be cheap to justify itself)
2..to take advantage of the hotter cam (068 vs. 067) in MT GTO's that permitted a bit more actual compression ratio on 98 octane (the industry threshold at the time for premium fuel designation) while still meeting GM's very strict rules about pinging potential in regular production cars.

Just a theory.

[Robert Williams]

The pair of #48 heads i have here i picked up years ago, i had them cleaned up and i checked the chamber cc's myself and i got 64 cc's so it look's like these have been milled .010", 1 cc for every .005" milled.

Checking some of my older High Performance Pontiac Mags last night, in the Feb 1998 issue they have an article with engine builder Jim Taylor on raising compression ratio with a head swap. They have a chart on page #63 showing casting numbers, year, and chamber volume.

They show the 1969 casting #48 head has a 66 cc chamber, this #48 head with the 66 cc chamber was used on the 350" and the 400" H/O and Ram Air 3 engines. It look's like there is only one chamber size for the #48 heads and that is 66 cc's.

[baron_]

i thought it was confirmed that the 093's were similar


389 093's and 326 HO 093's were quite different in CC (326 lower, of course)

so, 48's = similar?