Sonic testing a block before boring can determine if there is a problem with core shift, porosity, or thin cylinder walls. Now, don’t get paranoid. These types of problems don’t happen often, but they do occur.
Blocks
262: The 262 has a standard bore of 3.671 inches and a stroke of 3.10 inches. These were thin block castings and should only be bored a maximum of 0.030 inch over, which would give a bore diameter of 3.701 inches. All of these Gen I blocks have two-bolt main caps and use 2.45-inch journal cast cranks. They were factory installed in 1975 to 1976 Novas and Monzas.
265: The Gen I blocks were made from 1955 to 1957 with a bore of 3.750 inches and stroke of 3 inches. They can be bored up to 0.125 inch over to a maximum bore diameter of 3.875 inches, which is the same as a standard-bore 283 block. All of these blocks have two-bolt main caps and use small journal 2.30-inch cranks. Blocks made in 1955 had no provision for an oil filter.
The Gen II 265, 4.3-liter V-8, introduced in 1994 in Caprice sedans and wagons can be safely bored 0.040 inch over. These are two-bolt blocks and use 2.45-inch main journal and 3-inch stroke cranks with a one-piece rear main seal. Don’t mix this block up with the Chevy 4.3-liter V-6 or the 5.7-liter Gen II LT1 motor that was used in the Impala SS.
The performance aftermarket offers a variety of small-block Chevy-based blocks. Dart’s Special High Performance (SHP) small-block offers some great features for performance enthusiasts. (Photo Courtesy Dart)
Another aftermarket source for blocks and engines is World Products. This is the Motown 350 cast-iron block, which is also offered in aluminum to save about 90 pounds.
The bottom end of the PN 10134400 aluminum race block is all business. The block is CNC machined with cast-iron sleeves and the crank is secured with stout splayed four-bolt steel main caps. (Photo Courtesy Chevrolet Performance)
267: 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