By Rocky Rotella
The traditional Pontiac V-8 engine's short block has some unique characteristics. If you're planning to rebuild an engine you've already got, or you're considering a Pontiac V-8 purchase in the near future, there are some important engine block, crankshaft, and connecting rod information you should be aware of.
The Pontiac V-8 block is constructed of cast iron, and though its displacement increased from 287 to 455 ci by enlarging bore and lengthening stroke over the years, the external dimensions didn’t waiver far from the original design. A number of changes occurred during the course of its production run, however, and some limit component interchange. Included among them are revised engine mounting points, transmission bellhousing bolt pattern, starter location, main bearing diameter, and cooling system differences.
The block casting number contains several digits and, depending on the year, is sometimes found on the passenger’s side of the block or at the rear of the block on a ledge near the number-8 cylinder. This 9790079 block is a 1968–1969 350 casting. Depending upon the application, Pontiac sometimes ground off several or all of these numbers and stamped others into place.
The basic block features a relatively tall deck height of 10.24 inches, which is measured from the crankshaft centerline to the deck. The deck surface is very thick and rigid. Cylinder head bolt holes are drilled and tapped through the deck surface and into individual bosses within the water jacket walls. Unlike many other makes, Pontiac V-8 bolt holes do not extend into the water jacket, so no thread sealer is required during installation.
The symmetrical 90-degree design features large main bearing saddles, which adds overall rigidity and allows for using long connecting rods for good rod-to-stroke ratio. The main bearing caps are fastened to the block by two large bolts in most cases and by four bolts in certain high- performance applications. In either instance, the main caps are located by dowel pins, which are intended to prevent the caps from wandering during high-speed operation.
The block’s casting date is generally located on the distributor pad, and “L186” found on this unit translates to December 18, 1966 (L = Dec, 18 = 18th day, 6 = last digit of 1966). The block casting number is also found in this area on most 1964–1967 castings. The visible digits in this example are what remain of the original “9786133” after machining. It indicates that this block is a 1967 400.
When searching for a block to use in a project, one from the model year of the vehicle is ideal since the engine and transmission mounts should correspond with those of the chassis. If that’s of no real concern, then any 1965 to 1979 block generally makes a suitable choice since these engines are the most supported by OE and aftermarket parts suppliers. The engine mounts may be different for certain years, but most engine builders and restoration parts suppliers offer specific adapter kits for such situations.
Pontiac blocks featured two freeze plugs per side through 1966. Another was added in 1967, bringing the total to three. This is a quick and easy way to narrow down the vintage of a particular block when hunting for possible options at salvage yards or swap meets.
The motor mounts were moved to the side of the block in 1959, where they remained throughout the duration of Pontiac V-8 production. Some blocks contain two, three, or five motor-mount bosses depending upon the year and chassis application. In some instances they are not drilled and tapped. Most quality Pontiac engine builders and restoration parts suppliers offer adapter kits if one is required.
To identify the block, the 2-digit engine code, “XU” in this instance, denotes original application. It is located on the front of the block, just below the deck surface of the passenger side. The six- or seven-digit numeric stamp just above it is the engine serial number (ESN), which is the engine’s sequential build number. It does not directly correlate to the vehicle identification number (VIN) of the vehicle in which it was originally installed.
The oil pan is on the same plane as the crankshaft, and it contains a large sump at the rear. Oil drawn from the sump is pressurized by the oil pump and filtered before being dispersed throughout the engine. Oil travels across the rear of the block to a galley that runs adjacent to the crankshaft on the left side. It feeds the camshaft, crankshaft, and left-side lifter bores as it travels toward the front of the block. It then crosses to the right side, feeding those lifter bores.
A relatively large filter is used to keep the oil as clean as possible. It mounts to a cast-aluminum adapter that’s bolted to the right side of the block toward the rear. Depending upon the model year and application, a small variety of oil filter adapters were used throughout the course of V-8 production. It’s best to use the filter and adapter that was originally installed on your engine, unless a suitable replacement is available.
The Pontiac V-8 uses a bolt-on oil filter adapter, and the type most hobbyists are familiar with comes in two distinct versions. One places the filter at about a 70-degree angle from the block (left) while the other places the filter at a 90-degree angle. The original application depends upon the type used to assemble a particular Pontiac. The 75-degree unit seems most common and tends to provide the best exhaust system and chassis clearance.
The stock Pontiac crankshaft is a durable and well-balanced unit. It features large counterweights and relatively large 2.25-inch-diameter connecting-rod journals. As stroke was added to increase displacement, main bearing diameter was increased to maintain sufficient crank pin overlap for adequate strength.
Crankshafts produced through 1958 were steel forgings, while castings were used exclusively in later years. Because of main journal diameter differences, the early production forged-steel units do not directly interchange with later units without significant modification. The heat-treated steel forgings used in specific high-performance applications during the late 1950s and early 1960s are some of the strongest crankshafts Pontiac ever produced and fit later blocks, but are quite rare.
Pontiac placed displacement callouts on each side of the block in 1968, and cast the last two digits of the displacement size into the center of the lifter valley. The “00” found on this block indicates that it’s a 400. Other examples include “50” or “55,” which indicate a 350 or 455, respectively. These features allow quick and easy determination of displacement.
The first production cast crankshafts, which appeared in the late 1950s, were constructed of a material referred to in Pontiac literature as ArmaSteel. ArmaSteel is a GM trade name for its Pearlitic Malleable Iron (PMI) alloy, which possesses some strength qualities similar to certain steels. The crankshafts were constructed of cast-nodular iron in later years. ArmaSteel cranks can be identified by the ArmaSteel name cast in them, while a large “N” was often used to signify nodular-iron composition in later years. A crankshaft of either type is quite durable and completely adequate for regular production engines and even high-performance applications.
In most instances, the cast crankshafts directly interchange if the main bearing diameter is the same. Depending upon the original application, there are slight differences in the length of the front snout, diameter of the rear flywheel register, and counterweight shapes. So, if at all possible, comparing a potential replacement to the original is highly suggested.
A connecting rod is one of most highly stressed components within an engine. The inertial force placed upon it during normal operation requires that it be constructed of a sturdy material that’s elastic enough to not fatigue during normal operation. Steel forgings are tough but the process is somewhat expensive. The rod must be properly heat treated to be able to withstand the additional force associated with greater power amounts or high engine speeds, which makes it even more costly.
Cast connecting rods are much cheaper to produce. Cast iron is generally very strong up to its elastic point, but tends to shatter once surpassed. Pontiac constructed its cast rods of ArmaSteel, a specific iron desirable for its steel-like strength qualities. If an engine is operated within its intended limit, a properly designed cast connecting rod should provide a long service life in a given application, which makes it ideal for typical production engines.
Pontiac’s basic I-beam connecting rod was introduced in 1955. The block’s tall deck height allows the use of a relatively long connecting rod with a center-to-center length of 6.625 inches, which produces a good rod-to-stroke ratio. This tends to lessen the amount of side loading placed on the cylinder wall, reducing cylinder wear and overall operational friction.
Pontiac introduced its common cast connecting rod in 1963 for 389 applications. The ArmaSteel unit was then used in all production engines from 1967 through 1979. It’s an excellent piece that is quite reliable when operated within its intended limit. Adding modern fasteners and proper preparation is an easy way to increase its durability during any rebuild.
Though a low-grade steel forging, Pontiac’s 1955 connecting rod is completely adequate for its intended application of stock street performance, but the lack of heat-treating limits its strength for high-performance applications. A floating-piston wrist pin, which consists of a slip-fit wrist pin retained in the piston by snap rings on either end, was used through 1957. The pin was pressed into a fixed position beginning in 1958. Forged-steel connecting rods were used in all applications through 1962 and in select 421 applications through 1966.
The Super Duty engines of the era used a moderate-grade steel forging that was heat treated to various standards, producing a noticeably stronger piece. These are quite rare today. The most common Pontiac connecting rod that hobbyists are familiar with is the cast unit introduced in 1963 for 389 applications. It was used in all production Pontiac V-8s from 1967 through 1979, except for the SD-455.
Pontiac developed a beautiful forged-steel connecting rod for the 1973 Super Duty 455. Retaining the stock 6.625-inch length, the quality steel forging was heat treated and blasted with metal shot to improve strength while being magnetic particle inspected for minute cracks several times during the manufacturing process. The SD-455 rod is an excellent piece that was the best available option for several years, and the price reflected it.
The 1973–1974 Super Duty 455 received a beautiful forged-steel connecting rod, which was a direct replacement for the cast unit. When hobbyists learned of its availability, orders poured into dealership parts departments. By mid 1974, more than 1,350 orders for complete sets were on backorder, yet Pontiac assembled fewer than 1,300 complete orders! A bulletin was issued stating that a valid VIN was required when placing any order and only four were supplied if the claim was accepted. Modern forgings have lessened their desirability today, but they remain fairly valuable.
The cast connecting rod includes a machined groove that serves to direct a jet of lubrication toward the camshaft and cylinder wall. Most likely because of emissions concerns, it was eliminated in September 1972. Factory literature states that the two rod types can be freely interchanged. Your rebuild can include either, but modern bearings do not include the required feedhole, which renders the feature useless.
Though Super Duty connecting rods remain quite valuable today, the common forged-steel units from the 1950s and 1960s are far less desirable. Typically selling at a reasonable price, some hobbyists have improved rigidity by having them heat treated. The cost of that process along with the cost of adding modern fasteners and having them correctly sized can make them an unreasonable choice.
Reusing your Pontiac’s original cast rods during your rebuild is certainly possible. A simple upgrade is the addition of modern fasteners, which tends to improve clamping force. However, the units are generally well over 35 years old and have endured countless miles and thousands of heating and cooling cycles during normal operation. There’s also no telling how close they’ve been pushed toward their elastic limit in the past. If there’s even a remote chance of a questionable past, consider upgrading to a modern forging. It’s cheap insurance.
Pontiac developed its own cast-aluminum pistons, which were cam ground for precise fit. Cast pistons were used in essentially all production Pontiac V-8s, except for some of the unique maximum-performance engines. These engines, such as the Super Duties and R/A-IV, received forged-aluminum units.
The factory cast piston is an excellent design. Its wrist pin is offset slightly toward the thrust side of the piston, which is intended to lessen thrust load on the cylinder wall and provide quiet operation. Depending upon the application, a valve relief was specifically positioned to provide maximum piston-to-valve clearance in most engines. A dish was machined into the crown to reduce compression in a limited number of others, such as the 428.
Mickey Thompson supplied Pontiac with forged-aluminum pistons for early Super Duty engines while TRW Automotive provided forgings for the R/A-IV and SD-455. The TRW pieces were precisely machined and quite reliable. TRW began offering a wide array of stock-replacement forged-aluminum pistons for Pontiac applications based on the R/A-IV design, and several of the high-volume offerings are presently available under Federal-Mogul’s Sealed Power line.
Most Pontiac engines were assembled with cast-aluminum pistons similar to this. Some applications have the valve relief in a different position while others feature a dish in the center or a noticeable lip around the edge to reduce compression. It was an excellent design that was relatively strong and was made to run with tight piston-to-wall clearance. The piston pin is offset approximately 1/16 inch toward the thrust side, to reduce side load and ensure quiet operation.
A harmonic balancer is located on the front snout of a crankshaft. It generally consists of a center hub and floating outer ring isolated by rubber. It’s designed to dampen the irregular torsional vibrations that occur during combustion. It also contains a top dead center (TDC) timing mark for conveniently setting spark timing and is flanged to accept a pulley that drives the engine accessories. For many other makes it was slightly imbalanced and can be adjusted to balance the reciprocating assembly, but it was neutral balanced in nearly all Pontiac applications.
Early Pontiac units bolted together and were somewhat complex. The design grew simpler over the years and eventually evolved into the unit that most hobbyists are familiar with today. Introduced in 1968, the common Pontiac harmonic balancer measures 6.75 inches in diameter and was used on select applications through 1979. It was quite reliable and rarely failed when installed properly. New old stock (NOS) units were still available from GM dealers until just a few years ago, so examples shouldn’t be very difficult to locate. If a used balancer in good condition isn’t readily available, there are several companies that can rebuild them for you.
The Pontiac harmonic balancer generally bolted together through 1967, and this two-piece unit was introduced in 1964. The inertia ring is pressed onto the stamped steel housing, which then gets bolted to the crankshaft hub. The accessory drive pulley is sandwiched between them.
The most common Pontiac harmonic balancer was introduced in 1968 and measures 6.75 inches across. An excellent design that operates reliably when properly installed, it was used in all applications through mid 1976 and then in select applications through 1979. Your machinist can verify its accuracy during your rebuild. It should be replaced if it shows any signs of inaccuracy or pending failure. New units were available through GM parts departments until just a few years ago, so NOS units are still available. Aftermarket units are also available from your favorite Pontiac vendor.
The same initiative aimed at reducing overall vehicle weight that resulted in material being removed from the block led to the introduction of a modified harmonic balancer. Pontiac knew that most of its mid-1970s engines wouldn’t rev quickly and that the internals wouldn’t be exposed to heavy inertial drag, so engineers determined that a traditional harmonic balancer wasn’t required for most production engines.
In mid 1976, Pontiac replaced the harmonic balancer on select applications with a crankshaft hub that offered no dampening ability whatsoever. It simply used drive engine accessories and contains a TDC timing mark. Engines unaffected by the running change include the long-stroke 455, any Pontiac engine backed by a manual transmission, and the T/A 6.6. These exceptions received a traditional balancer to ensure operational longevity. It’s highly recommended that a conventional harmonic balancer be used during any rebuild.