Beginning in the early '70s, it seems as if Pontiac history changed daily. For true hobbyists, the mid-to-late '70s are the most challenging to research since they were clouded by obscurity and unannounced changes were commonplace. We can only speculate how often novice enthusiasts passed off the 4X head as "just another smog head" with no worthwhile performance value that's not worth the effort to uncover what application it was for. Rarely will you find anyone who knows that the 4X was originally intended for both 400 and 455ci engines, had large and small intake valves, threaded and pressed-in rocker studs, paired and individual stud bosses, and even a secondary application stamp. Furthermore, very few are aware that variations of the 4X head even exist! Because the vast number of original applications, the variations that evolved, and a general lack of attention, the 4X head is one of the most controversial castings for Pontiac people to identify.
The 4X head was the 49-state casting used on all '73-74 400 and 455ci engines. Since Pontiac offered the 400 2-barrel, the 400 4-barrel with both manual and automatic transmissions, and the 455 4-barrel for 1973 and 1974, separate applications of the 4X head were required to meet such broad usage. To achieve an 8.2:1 compression ratio on both the 400 and 455 engines, machined combustion chamber volumes were different for each application. Chamber volume for the 400 head was approximately 98 cc, while the 455 head held approximately 114 cc. Furthermore, intake valve diameter was reduced from 2.11 inches, which was used on all 4-barrel applications, to 1.96 inches for the 400 2-barrel.
The 4X head; used on all '73-74...
The 4X head; used on all '73-74 400/455s not originally equipped with A.I.R. injection.
To externally identify heads for assembly purposes, Pontiac stamped each one with a secondary application code on the vertical accessory boss between the number 1 and 3 (or number 6 and 8) exhaust ports. More commonly known today as the means of identifying 6X heads, the 4X head was among the first Pontiac heads to receive an external application stamping. The stamped number was the last digit of the complete part number for the assembled head for each respective application. By October 1972, the stamping changed from a single digit (ex. "7") to a single digit followed by the letter "H". This would carry on through the end of tenure for the 4X head. Up to this point, reasoning for the change has yet to be determined though it is speculated that the "H" could stand for "head" or a possible change in the foundry casting process.
A running change, which occured midyear 1973, helped to fuel much of the controversy that surrounds the 4X head. Many have reported 4Xs as having pressed-in rocker arm studs, while others with the same original application were found with threaded rocker arm studs. Through much effort and careful note taking, a few hard core researchers had narrowed down a specific window for a possible changeover date and then theorized that most 4X heads up to that time frame had pressed-in studs, while all 4X heads after that had threaded studs. It was soon found that the theory not only applied to the 4X series, but the entire 1973 head casting line-up, the only exception being the SD-455 #16, which had threaded studs throughout.
With the discovery of issue #4 of the 1973 Craftsman Service News set, all theories were validated with a simple, one-paragraph statement on page 54 entitled, "Threaded Rocker Arm Studs". The paragraph clearly states that all Pontiac V-8 engines would receive threaded rocker arm studs beginning with engine #709986 which was assembled on or about May 7, 1973 and would carry throughout the remainder of 1973 engine production and on to 1974. What has been proven as an attempt to save material costs and a step in the machining process as well as to eliminate the need to stock a specific yet redundant part, Pontiac engineering streamlined production by discontinuing the pressed-in stud installation process and machined all heads for threaded studs.
Here's the location of secondary...
Here's the location of secondary application stamp found on all '73-up cylinder heads. Early cast heads were stamped with a single digit, while later heads were stamped with single digit followed by the letter "H". The stampings identified different applications of heads with the same casting number.
Because heads were cast and assembled much earlier than the engine assembly, it is not uncommon to see 4X heads with casting dates as early as mid-March 1973 machined for threaded studs. A pressed-in stud head with a casting date past mid-April has yet to surface, leading researchers to believe that head assembly had completely changed to threaded stud by mid-April 1973.
Pushrod guide plates were fastened to pressed-in stud heads by a bolt in the "flashing" area between stud bosses giving the appearance of paired stud bosses. However, threaded stud head guide plates were retained in place by the rocker arm stud itself. Since the newly implemented threading process was used on all heads, the flashing material between the stud bosses was eliminated in an attempt to further reduce material costs. Individual stud bosses first appeared in 1974 along with a new series of full production part numbers slightly different from their 1973 counterparts though the "4X" casting and secondary stampings remained the same.
Applications were specific and each head was given a unique secondary stamping. The 4H version of the 4X head used the small 1.96-inch intake valves and appeared on all 400 2-barrel non-A.I.R. engines. The 4H along with all other 1973 small intake valve heads (4C, 46) would receive pressed-in rocker studs until the changeover point. Large intake valve heads were specific to 4-barrel applications yet most pre-May 7, 1973 large valve heads used pressed rocker studs. For 1973 and 1974, the 1H version of the 4X head was used on all 455 4-barrel engines, regardless of transmission, while the 7H was used on all 400 4-barrel applications with automatic transmission. Both the 1H and 7H had 2.11/1.66 valves and featured pressed-in studs prior to the May 7 changeover date but a few early castings with threaded studs exist. This only proves that nothing is written in stone and these types of examples should be considered the exception and not the rule.
However, the 3H head with 2.11/1.66 valves, used on all 73-74 400 4-barrel engines with manual transmission, appears to have received threaded studs all along, which was thought to have been reasoning for the independent secondary stamping. Since, the 3H head did not disappear in mid-1973 when all studs became threaded speculation arose about the 3H having different valve springs than those used on the 7H. This speculation is refuted with some very key Pontiac engineering specification documents. These documents show the only difference being the 3H version received intake valve oil seals where as the 7H did not with all other parts being identical. The following chart is for further clarification of the secondary stampings and their original applications.
Paired stud bosses and pressed-in...
Paired stud bosses and pressed-in rocker studs were found on nearly all pre-mid-'73 heads. Material between the stud bosses was drilled and tapped and bolts were used to retain pushrod guide plates. Extra material was eliminated in 1974 to reduce material costs.
From a performance standpoint, only minimal port differences separate the 4X from the 6X making them excellent candidates for any performance application where the 6X head is being considered. Given the popularity of 6X heads over the past few years, supplies are dwindling and prices are rising. The 4Xs offer similar performance and flow in stock form and with little effort they, too, can produce excellent flow numbers. While the 1H has 114cc combustion chamber volume and 4H's small valves, the 3H and 7H are excellent alternatives for a performance engine. With the variety and stigma surrounding them, many times unknowing owners will literally give them away not knowing the potential they are passing off. Because of this, large valve, small chambered 4Xs can often be bought for less than $100 per pair making them a bargain for performance builds.
Since 1973 proved to be the best selling year for the Pontiac Motor Division and 1974 was only slightly off pace, engine production was at its highest and the 4X could rival the 6X as the most abundant single casting Pontiac head ever produced. They were never considered rare or desirable so many were scrapped or tossed aside for a more popular casting number. Even though not all 4X variations are as suitable for performance as some of the late '60s and early '70s D-port performance heads, like the 6X though, the 4X should not be discounted.
With a better understanding of the differences between casting applications and how the same casting application can have a different appearance, perhaps you should make an attempt to recognize and identify the next 4X heads you see. You may find that they have the potential to make your Pontiac faster at a price that's easy on the wallet.
||400 4-bbl Manual
||400 4-bbl Auto
|*All rocker arms are threaded beginning with Motor Unit Number 709986 assembled on or about May 7, 1973.
Drilled and tapped rocker...
Drilled and tapped rocker stud bosses appeared on all engines beginning with Motor Unit Number 709986 assembled on or about May 7, 1973. Note the individual stud bosses, which first appeared on the '74 heads and continued on through with 6Xs.
Pushrod guide plates are shown...
Pushrod guide plates are shown here installed on the pressed-in stud heads. Note the bolts retaining them.
Threaded rocker arm studs...
Threaded rocker arm studs held pushrod guide plates in place by sandwiching them between the boss and the stud.
This is a secondary application...
This is a secondary application stamp as it appears on most 4X heads. The "1H" denotes an original 455 4-barrel application and large intake valves.
Here is an early cast head...
Here is an early cast head stamped with a single digit "4". This head has a casting date of September 18, 1972 and was originally installed on a 400 2-barrel engine. It has pressed-in rocker studs and small intake valves.
This is how the casting date...
This is how the casting date code appears on all 4X heads. K212 shows this head being cast November 21, 1972. Heads with pressed-in rocker studs started being converted to threaded studs in March 1973 and by mid-April, all heads had been converted.