This is another thin-cylinder-wall Gen I casting, which should only be bored a maximum of 0.030 inch over. This block has two-bolt caps and uses a 2.45-inch main journal crank. It had a 3.50-inch standard bore and 3.48-inch stroke and were made from 1979 through 1982.
283: Blocks originally having this displacement are a mixed bag. The blocks have a 3.875-inch standard bore and use a 3-inch stroke. The 1957 block castings can be bored a maximum of 0.060 inch over. The 1958 to 1962 blocks had thicker cylinder walls and can be bored 0.125 inch over to a maximum diameter of 4 inches. The 1963 to 1967 blocks went back to thinner cylinder barrel walls and should be limited to a 0.060-inch overbore. The 283 has two-bolt main caps and uses the small main journal 2.30-inch crankshafts.
302: These 4-inch standard-bore blocks can be bored 0.040 inch over. Many can go as large as 0.060 inch over, but this is cutting it thin and you may have problems. In 1967, the 302 used a small journal 2.30-inch crank with a 3-inch stroke and the block had two-bolt main caps. In 1968 and 1969, the Z28 302 used a 2.45-inch main journal crank and the block used four-bolt main caps.
305: These 5.0-liter Gen I motors are all medium-journal 2.45-inch blocks, and production blocks were only made with two-bolt main caps. They can be safely bored 0.040 inch, with some blocks going to 0.060 inch over. These Gen I blocks were introduced in 1976 with a 3.75-inch bore and 3.48-inch stroke. Blocks made from 1986 to 2003 have a one-piece rear main seal. A Vortec 5000 305 was introduced in 1996. Later-model blocks have been cast for use with roller cams.
307: Most of these 3.875-inch standard bores can be opened up 0.060 inch. I have heard of some being bored 0.125 inch over to 4 inches, but this is risky. If you need a 4-inch-bore medium-journal block, go find a good 350 core. These two-bolt blocks use a 2.45-inch main journal crank with a 3.25-inch stroke (the same stroke length as a 327). They were factory installed from 1968 until 1973.
327: These blocks were factory installed from 1962 through 1968 and have a 4-inch bore with a 3.25-inch stroke. They were all two-bolt blocks and the 1962 to 1967 blocks used small journal 2.30-inch crankshafts. The 1968 327 used a medium journal crank with 2.45-inch-diameter main journals. It can be safely bored 0.040 inch, with some blocks able to go up to 0.060 inch over.
350: The Gen I 350 first made its appearance in the 1967 Camaros. The Gen I and II cast-iron 4-inch blocks can be bored 0.040 inch over to 4.040 inches, and some blocks can go to 4.060 inches. All Gen I and II 350s used medium-journal 2.45-inch cranks with 3.48-inch strokes, and the blocks came in two- or four-bolt main cap varieties. Production blocks made from 1986-on have one-piece rear main seals. Many of the blocks made from 1987-on used roller cams.
400: The Gen I 400 is unique in that it has siamesed cylinder barrels. The standard bore of 4.125 inches can be increased safely to 0.040 inch over, with some blocks going to 0.060 inch over. This block came with either two- or four-bolt main caps from 1970 to 1980, and it was the only production motor to use a crankshaft with 2.65-inch main journal diameters and 3.76-inch strokes. The connecting rod length was shortened to 5.565 inches in order to fit everything in. These 400 engines were all externally balanced.
Cast-Iron Bowtie Blocks
PN 12480174 (casting number 10051184) has siamesed cylinders, 9.025-inch deck, with a 3.980- to 4.030-inch bore range. It has four-bolt mains, 2.45-inch crank journals, wet-sump oiling and a one-piece oil seal, and tall lifter bores. This bare block is CNC-machined and weighs 196 pounds.
PN 12480047 (casting number 100511184) is a Bowtie Sportsman Block with the same specs as the above block but it features a two-piece oil seal and weighs about 8 pounds more.
If you’re looking for a 350 main with a 400 bore size, Chevrolet Performance offers two Bowtie blocks to consider. Both feature siamesed cylinders with deck heights of 9.025 inches with a bore range of 4.117 to 4.755 inches. The mains are drilled at 20 degrees and accept nodular caps. The difference is in the rear main seals: PN 12480175 has a one-piece seal, while PN 12480157 features a two-piece seal. (Both blocks are based on casting number 100511184.)
PN 12480049 is a four-bolt block with 400 main and 350 bore size that has been CNC-machined to within +/-0.005-inch tolerances. It has splayed caps on the center three mains to accept four-bolts. The finished bore is 3.980 inches with a range up to 4.155 inches. The max stroke is 3.750 inches and has a wet oiling system. Another block is offered (PN 24502503) that includes steel caps rather than nodular like its sister block.
PN 24502650 was treated to a reduction in deck height (8.325 inches), so a short stroke/short rod, big bore motor was easier to build with improved high-RPM power, more direct intake port runner angles, compact size, weight reduction, and a lower center of gravity. The bore range is slightly larger at 3.980 to 4.185 inches and accepts a 283 crank journal for a max stroke of 3.250 inches. This block weighs in at a svelte 167 pounds.
Aluminum Bowtie Blocks
As with most things cast in aluminum versus iron, the major benefit is less weight. Chevrolet Performance offers several cast-aluminum race blocks with the same features as the cast-iron blocks, but they tip the scales at 100 pounds or even less. T-356 aluminum is used and CNC machined and fit with centrifugal spun cast-iron cylinder sleeves. Each of the aluminum blocks uses a two-piece rear main seal and has enlarged cam bosses allowing for larger cam bearings if desired. The wet oiling system is designed with priority on the mains, though the block cab be converted to a dry sump system with plugs.
PN 10134400 (casting number 10134996) offers siamesed cylinders with a 9.025-inch deck height. The block is set up for steel main caps with bolts at a 20-degree angle with the crank journal size at 400. The rough finished bore is at 4.117 inches with a max of 4.135 inches. There is also a similar block offered (PN 10185075), but it has main journal sizes of a 350 with a smaller bore starting point of 3.986 inches. The bare block weighs in at 90 pounds.
PN 24502495 is a tall-deck aluminum Bowtie case with a deck height of 9.525 inches and it can be decked to 9.0 inches. Deck thickness is 1.125 inches. Four-bolt splayed 8620 alloy steel caps are used on the center three caps, while the front and rear caps are straight four-bolt caps. Use 2.65-inch main journal cranks and a 400-style two-piece rear main seal. It has sleeves that are semi-finished at 4.117 inches, and it can be bored to 4.135 inches. The pan rails are spread 0.800 inch and take strokes of 4.00 inches without modification, strokes of 4.125 inches with minor grinding, and a 4.25-inch stroke with major grinding and modifications.
Additional tin or nickel is added to the cast iron in some blocks. These high-tin and -nickel blocks are very desirable. In this photo, the extra tin is noted on the rear face of this 350 block by the “010” signifying that 1 percent more tin was used to help the molten metal flow into the casting mold. The “020” indicates that 2 percent more nickel was used in this block, which helps make the cylinder barrels harder and more durable. These casting identifiers are also found on the front face of these blocks under the timing cover.
Buying a used small-block can be a gamble. Do you know the history of the engine? Has it been stored inside or in the open? Can you tell if a block or head is cracked? Also, you’ll need to research what the engine was used in to help make sure your new replacement parts work!
Bowtie Casting Numbers
Casting numbers generally can be used to identify most Chevrolet parts. However, I must caution you about two casting numbers that have been used on one Bowtie block and another casting number that has been used
