These photos show the difference between the rear main seal designs: a two-piece seal (left) and a one-piece-seal (right).
This is the one-piece seal and mounting fixture that was used on 1986-and-later production small-blocks.
The oil dipstick entry location moved around over the years. The dipstick tube entry was usually through the driver’s side of the block deck and then into the oil pan. However, there are blocks on which the dipstick tube enters the block on the passenger’s side and then runs into the pan. On a few engine/vehicle combinations, the hole in the block that was meant for the dipstick was plugged, and the dipstick tube entered the side of the oil pan in order to clear obstructions such as the exhaust manifolds. Most interchange situations don’t cause a dipstick problem as long as you use an oil pan that matches the block dipstick’s mounting position—driver’s side or passenger’s side. If there is an obstruction that prevents dipstick entry into the block, you can replace it with a pan that allows the dipstick tube to enter directly into the side of the pan. This usually clears up any problems with stock exhaust manifolds. A few late-model Gen I blocks have a dipstick entry position on both sides of the pan rails. Take the time before you choose the parts to determine which ones you need.
The 1986-and-later production blocks switched to a one-piece rear main seal, which required other changes to crankshafts, oil pans, flywheels, flexplates, and engine block sealing surfaces, affecting the interchangeability of parts between the two-piece oil seal and one-piece oil seal motors. All production small-blocks with one-piece rear main seals are externally balanced. Some GM engines’ replacement crate motors and some of the performance Bowtie blocks still use two-piece rear main seals.
PN 14088556 is a stock aluminum rear seal retainer for one-piece rear oil seals on 1986-or-later production blocks with matching oil pan, timing cover, and pan gaskets. It also uses an adapter-to-block gasket (PN 12337823) and rear crank seal (PN 10088158).
PN 10051118 is a seal adapter that allows the use of a two-piece-style crank in a one-piece seal block with a one-piece-style oil pan and pan gasket. It requires a two-piece seal, adapter gasket (PN 12337823), and one-piece oil pan gasket (PN 14088505). The Moroso (PN 38315) seal adapter allows the use of a two-piece-style crank with a one-piece oil seal–style block, and it allows the use of an early two-piece-style oil pan, timing chain cover, and pan gasket.
The 1986-on model blocks machined for roller lifters and camshafts have taller machined lifter bores and three bolt bosses in the middle of the lifter valley to hold the lifter retainer fixture that was used with production Chevy hydraulic roller lifters.
Some blocks, such as this 1961 283, have dual-starter bolt patterns. Most blocks only have one pattern—either straight or offset.
When you have your block bored, make sure the final honing is done with a torque plate bolted in place. The plate distorts the cylinder walls in a way that is similar to the block distortion when the heads are torqued down on the block. The cylinders are round when honed and this helps improve ring seal.
Walk into your friendly Chevrolet dealer and order PN 10105123 and you’ll receive a bare, cast-iron 350 block. This is the 1986-on block with four-bolt mains, a one-piece rear seal, and clearanced for a 3.80-inch-stroker crankshaft. (Photo Courtesy Chevrolet Performance)
The 1987-and-later Gen I blocks were produced with taller lifter bores for use with factory-installed hydraulic roller cams and roller lifters with bolt bosses in the lifter valley for a roller lifter retainer spider. Flat tappet cams and lifters can be used in these blocks. The late-model hydraulic roller lifters are taller than a hydraulic flat tappet lifter and should be used with shorter factory pushrods for production roller lifters. All Gen II production engines use hydraulic roller cams and roller lifters.
Aluminum blocks weigh approximately 90 pounds bare. Cast-iron production blocks weigh about 150 pounds. The heavier Bowtie cast-iron bare blocks weigh approximately 185 pounds due to their thicker cylinder walls and decks and reinforced block webbing.
There are significant differences between the Gen I, Gen II (introduced in 1992), and Vortec small-block V-8s (introduced in 1996).
The large-diameter flywheels and flexplates (14 inches) that were used with one-piece oil pans may not fit some early engine and vehicle applications. The crossmembers and small-diameter bellhousings were originally made to be used with a 12¾-inch flywheel or flexplate. To fit the larger flywheels and flexplates, change the bellhousing or change to the smaller-diameter ones. Keep in mind that most blocks are drilled for two different starter bolt patterns, while some blocks have only been drilled for one starter mounting position.
Small-Block General Specifications
Configuration:
90-degree ohv V-8, with cast-iron or aluminum blocks and heads
Cylinders (Front to Rear):
Driver’s side: 1-3-5-7
Passenger’s side: 2-4-6-8
Firing Order (Clockwise on Distributor):
1-8-4-3-6-5-7-2 (Gen I, Gen II, and Vortec)
Computer controlled, no distributor
Crankshaft:
Gen I, II, and Vortec: Forged or cast, five main bearings, thrust on rear bearing
Crankshaft Journal Diameter:
1955–1968 small journal: Mains 2.30 inches, rods 2.00 inches
1967–1996 medium journal: Mains 2.45 inches, rods 2.10 inches
1970–1980 400 Crank: Mains 2.65 inches, rods 2.10 inches
1989-up LT5: Mains 2.76 inches, rods 2.10 inches
Crank Strokes:
Small journal: 3.0 inches and 3.25 inches
Medium journal: 3.0, 3.10, 3.25, and 3.48 inches
400 Cranks: 3.75 inches
LT5 350: 3.66 inches
Rod Length:
Forged steel 5.7 inches center to center
5.565-inch 400 rod
5.740-inch LT5 rod
5.940-inch Gen II 265-ci L99 V-8
Pistons:
Cast, hypereutectic, or forged aluminum slipper skirt design
Lubrication:
Full pressure oiling, wet sump
Some Bowtie blocks offered for dry sump
Block Bores:
3.50 to 4.126 inches
Bore Centerlines:
4.040 inches
Cubic-Inch Displacements:
262, 265, 267, 283, 302, 305, 307, 327, 350, 400