Chevrolet Small-block Engine - Major Changes

Major Changes

The original design of the small block remained remarkably unchanged for its production run, which began in 1955 and ended, in passenger vehicles, in 2003. The engine is still being built today for many aftermarket applications, both to replace worn-out older engines and also by many builders as high-performance applications. The principal changes to it over the years include:

• 1956 – Full flow oil filtration was introduced, using a paper element filter in a canister that was mounted to a feature that was added to the cylinder block casting and machined for this purpose.
• 1957 – The displacement of the base V8 continued at 265 cubic inches but optional V8s were introduced with 283 cubic inch displacement.
• 1958 – Bosses for side motor mounts were added to the block casting and utilized for production mounts for this and all future model years. However the features for front motor mounts as used in 1955–1957 remained part of the block casting in this and future years. The 265 cubic inch version of the engine was discontinued. Also the cylinder head valve cover mounting bolt holes were changed from the top row staggered to the straight across that remained the new standard way of identifying the early heads from the newer valve cover design that lasted until the 1987 center bolted style covers.
• 1962 – The block's cylinder wall casting was revised to allow four inch bores and the 327 cubic inch version of the engine using this bore diameter and increased crankshaft stroke was introduced.
• 1968 – The main journal diameter was increased to 2.45 in (medium) from 2.30 in (small) and the connecting rod journal diameter was increased to 2.10 in from 2.00 in. This allowed the use of cast iron crankshafts as the previous parts were made of forged steel. The rod bolts were changed from 11/32 in. diameter to 3/8 inch. Additionally, the canister/sock style oil filter was now converted to use spin on filters. The oil fill location was moved from a tube on the front of the intake manifold to a cap on either side valve cover.
• 1986 – The rear main seal was changed from a 2-piece rubber design to a 1-piece rubber design that used a mounting appliance to hold it in place. This necessitated a change in the flywheel/flexplate bolt pattern, as well as the need for an externally balanced flexplate/flywheel.
• 1987 – The valve cover surfaces were changed such that cylinder head mounting lip was raised and the bolt location was moved from 4 bolts on the perimeter, to 4 bolts down the centerline of the valve cover (this design debuted on the Corvette in 1986, and Chevrolet 4.3 L the year before). Also changed were the mounting angles of the center 2 bolts on each side of the intake manifold (from 90 degrees to 73 degrees) and the lifter bosses were increased in height to accept roller lifters. The alloy heads for use in the Corvette still retain the non-angled bolts (center 2 bolts attaching to the intake). Also all carburetors were done away with and replaced by TBI (throttle-body injection) fuel injection on the Camaro and Firebird.
• 1996 – This was the last change for the Generation I engine, and continued through the end of the production run in 2003; all 1997–2003 Generation I engines were Vortec truck engines. The cylinder heads were redesigned using improved ports and combustion chambers similar to those in the Generation II LT1, resulting in significant power increases. The intake manifold bolt pattern was also changed to four bolts per cylinder head instead of the traditional six.