Chevrolet 1955 – 1998 Small Block number-3

Three of the most famous small-block Chevrolet engines ever built, the 302, the 327, and the 350, share the same 4.0-inch bore, the same block casting, and in many cases the same casting numbers. Only the stroke tells them apart. This third installment in our small-block series follows the 4.0-inch bore family from the 1962 327 through the 1968 journal change, the Trans-Am 302, the Duntov cam, and the 350 that eventually powered everything from Corvettes to delivery vans. Read on to find out how one casting did so much.

The 4.0 inch Bore Family 1962 to 1998

Pull the heads off a 302, a 327, and a 350 and set the bare blocks side by side, and you will struggle to tell them apart. Same bore, same casting, and in many cases the very same casting number stamped into the iron. One of them was built to win road races under a strict rulebook, one became the performance heart of the mid-1960s Corvette, and one ended up in family sedans and delivery vans for three decades. The only thing separating them is how far the piston travels. So how did Chevrolet squeeze three very different personalities out of one casting, and why does the 4.0-inch bore family still confuse so many people who think they know what is on their engine stand?

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One Block, Three Displacements

There are three engines in this family: the 302, the 327, and the 350. Every one of them uses the same block casting with the same 4.0-inch bore, and many share identical casting numbers, which confirms that stroke length alone determines the displacement. A 3.00-inch stroke produces the 302, a 3.25-inch stroke gives you the 327, and the 3.48-inch stroke yields the 350. That shared architecture is why the small-block earned its reputation for interchangeability: crankshafts, rods, and pistons could be mixed and matched across the family in ways that would have been unthinkable with most engines of the era.

Where the 327 Came From

The original 1962 engine in this family is the small-journal 327, and it quickly became the small-block that performance buyers wanted. By the mid-1960s the 327 was available in a range of states of tune, topping out in Corvette form with the carbureted 327/365 and the fuel-injected 327/375, the latter figure referring to the advertised horsepower. Those high-output versions relied on solid-lifter camshafts and free-breathing heads, and they proved that a small-block could hang with far larger engines at the track. The 327 set the pattern that the 302 and 350 would follow in different directions.

The 302: Built to Satisfy a Rulebook

For 1967, specifically to meet the needs of the Sports Car Club of America, Chevrolet produced a special 302-cubic-inch V8 for the new Z/28 Camaro. The Trans-Am road racing rules of the 1967 through 1969 seasons capped displacement at five liters, so engineers dropped the 283’s 3.00-inch stroke crankshaft into a 327 block and called the result a five-liter. For 1967 the 302 used a nodular cast crank like the 283, though a forged steel crankshaft was also produced. It was an engine created by a rulebook, and it turned out to be one of the highest-winding small-blocks Chevrolet ever sold to the public.

The 1968 Journal Change

The 1968 model year brought the revision that splits this family into small-journal and medium-journal eras. The 302, 327, and 350 all received larger crank bearings, with the rod journals increased to 2.10 inches and the main bearing journals growing from 2.30 to 2.45 inches. The new connecting rods were heavier, so a larger 3/8-inch cap bolt was used to hold them together. Blocks for 1968 were produced in both two-bolt and four-bolt main designs; the four-bolt version fastens the three center main caps with two added bolts each and supports them with thicker bulkheads to resist cracking under high loads.

Windage Trays and Balancers: Living at High RPM

The journal change came with a set of supporting upgrades aimed squarely at sustained high-rpm use. With the journals standardized at a larger size, the steel used for the crankshaft was of higher quality, as were the connecting rods, and an eight-inch harmonic balancer was added to keep the crank stable at speed. Inside the oil pan, a three-quarter-length windage tray was fitted to help keep oil on the vital parts when the engine is spinning hard, because at high rpm the oil tends to splatter into droplets rather than staying where it belongs. These are the details that separate a race-ready small-block from a grocery-getter.

The Duntov 30-30 Cam and the Double-Hump Heads

The high-performance versions of this family leaned on a specific recipe. The 30-30 Duntov solid-lifter camshaft, named for Zora Arkus-Duntov, the engineer known as the father of the Corvette, was used in the carbureted 327/365 and fuel-injected 327/375 Corvette engines of 1964 and 1965, then lived on in the Z/28’s 302 from 1967 through 1969. It was paired with the 327 high-performance heads, the double-hump 461 castings with 2.02-inch intake and 1.60-inch exhaust valves, plus pushrod guide plates, blue-stripe pushrods, and stiff valve springs. Special edge-orifice lifters were used to limit the oil sent up to the valvetrain, ensuring adequate oil stayed in the crankcase at high rpm.

1969: Forged Pistons and Better Rings

For 1969 the pistons were upgraded to forged aluminum and fitted with better-sealing single-moly rings, another step toward durability at sustained high engine speeds. Forged pistons tolerate the heat and detonation stresses of a hard-running engine far better than cast pieces, and the improved ring package helped control blow-by, which in turn kept oil cleaner and compression more consistent. Combined with the earlier journal upgrade, the four-bolt main blocks, the windage tray, and the eight-inch balancer, the 1969 engines represent the fully developed form of the 4.0-inch bore family as a performance package.

The 350 Takes Over Everything

The first introduction of the 302 was meant to be a high-performance engine, but when the 350 came into production it became something else entirely: an all-purpose engine. The 350 first appeared in the Camaro and then spread across the lineup, ending up in Corvette sports cars, family sedans, and delivery vans alike. Its extra stroke gave it the low-end torque that everyday drivers wanted, while the same block and head architecture meant it could be built into a serious performance engine with the right parts. That flexibility is why the 350 became the default small-block for a generation of Chevrolet products.

Why the Casting Numbers Confuse Everyone

Because so many engines in this family share the same block casting and even the same casting numbers, identifying what you actually have takes more than reading the side of the block. A block cast for a 327 could just as easily have left the factory as a 350, and a 302 is essentially a 327 block with a short-stroke crank inside it. Sorting them out means checking the crankshaft, the stroke, the journal sizes, and the stamped engine code on the pad at the front of the block. It is one reason so many surviving small-blocks are misidentified, and one reason a careful teardown is worth the effort.

From Gen I to the LT1

The medium-journal 350 carried the family through its final Corvette years, with the L98 running through 1991 before the design evolved into the Generation II LT1 350 for 1992. The LT1 kept the small-block’s basic dimensions but reworked the cooling system and induction, and it marked the point where the classic 1955-era architecture gave way to something new. Even so, the 4.0-inch bore family never really went away: the 350 remained in production for trucks and crate-engine duty long after it left the Corvette, and it remains the most rebuilt, most swapped, and most recognizable American V8 ever made.

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