engines, I have had plenty of successes and failures; both have led me to where I am today. I’ve always used the common-sense approach to engine building rather than what was the common and/or accepted current practice. When competing at a national level, you are running against people who are all talented, or else they would not be there.
Given that, this type of competition forces you to do your best and make enough horsepower with your engine combination to compete. Racing against Fords and Chevrolets with an Oldsmobile forces you to design and fabricate nearly the entire engine, rather than just going out and buying the latest performance cylinder head or intake manifold and bolting it on.
I hope you enjoy what I’ve learned from those years of hard work, hard thinking, and great mentors!
Oldsmobile engine blocks, as far as most Olds enthusiasts are concerned, started in 1965. The first V-8s in this new-style Oldsmobile engine line started as the 330-ci models for the small-block family and the 400- and 425-ci models for the big-block family. The engine-block designs remained virtually unchanged right up to the last Olds engine that came off the assembly line in the late 1980s. Olds blocks are very easy to identify. A letter cast into the front of the engine block under the intake manifold sealing rail identifies a small-block. A letter in the same place identifies a big-block.
Oldsmobile small-block V-8s consisted of a variety of cubic-inch models, including the 260-, 307-, 330-, 350-, and 403-ci engines. For high-performance use, the 350- and 403-ci versions are the most popular with Olds enthusiasts.
Oldsmobile small-blocks share many features. The bore spacing (defined as the center-to-center distance between the cylinders) was set at 4.625 inches, which is shared with the Olds big-block. The main bearing bores are set at 2.687 inches on all small-blocks with the exception of the diesels. The diesel block main-bearing bores are the same diameter as the big-block Olds, which are measured at 3.189 inches. The deck height, measured from the crankshaft centerline to the cylinder head mounting surface, is set at 9.330 inches on all small-block Oldsmobile V-8s. The lower portion of the block has only subtle differences, with the exception of the main-bearing webs. The lifter-valley areas in all the small-blocks are very similar and are not considered to be a weak spot. I have never seen a failure in this area. The lifter bores on all small-blocks are .842 inch in diameter and were all designed for use with flat-tappet hydraulic lifters. The exceptions are the later diesel blocks and late-model 307 gas engines, which had .921-inch-diameter lifter bores for use with hydraulic-roller lifters.
The Sunnen CK 10 cylinder hone does a great job of honing cylinders round and with little distortion. Newer, more advanced honing machines are available, but many top engine builders prefer this machine.
The letter at the end of the casting number indicates which big-block engine it is. An “A” casting is the 1965 425-ci engine, a “D” casting is the later 425-ci engine, a “G” casting is the long-stroke 400-ci engine, and an “F” casting is the 455 engine. Here, the little “A” next to the “F” means that this block has no provisions for a 4-speed.
The large numeral after the casting number indicates which small-block engine it is. A “2” is a 350-ci engine, a “4A” or “4B” is a 403-ci engine. Blocks with these numbers are about the only ones you want use for serious high-performance.
The main webbing on a gasoline small-block engine is even thinner than on the Olds big-block. How much stress can it handle? Who really knows? It depends on the weight of the rotating assembly, RPM, crank stiffness, detonation, static compression, and numerous other factors. Oldsmobile racers have pushed these blocks pretty far, but it’s only a matter of time before you run over the crankshaft when pushed to the limit.
As mentioned, the first of the Olds small-blocks was the 330-ci model, which was produced from 1965 to 1967. These engines have a 3.937-inch bore and a stroke of 3.385 inches. I have sonic tested a few of these blocks and you can safely overbore the cylinders .060 inch (or 4.000 inches) at best. The 330 blocks are, in my opinion, only good for restoration pieces. Although they appear to be as strong as any of the newer 350 blocks, the small bore size makes it an undesirable choice for high-performance use.
In 1968, the introduction of the 350-ci Olds engine stepped up the performance of the small-block by increasing the bore size to 4.057 inches. I have always wondered why Olds engineers chose such an oddball bore size. The 350 blocks share most of the features of the 330-ci block, with this notable exception. I have sonic tested some of the 1968–1974 blocks and have not found any whose cylinders couldn’t be safely overbored .060 inch. Most of the high-performance 350 engines that I build use a 4.125-inch bore with a custom-made piston. The 4.125-inch size is the best you can use to obtain the most horsepower in these engines. The cylinder wall thickness at this bore on these blocks is enough to allow the bore to remain round at all but the highest horsepower levels. I estimate that to be at 650 hp or less.
The main webs in this 403 block don’t have a whole lot of material to hold the crankshaft in the block. Some have had success in 12-second quarter-mile cars for a while, but I have seen them fail also. My opinion: Why go there? An engine failure is too costly. It’s too bad that Olds engineers lightened the structure; there would be an awful lot of these out there racing.
I have seen numerous performance Olds engines over the years and have yet to see a lifter bore failure. There is no need for modifications here.
In 1975, Oldsmobile introduced the 403-ci engine. This block shared a redesign with the 350 block to include a weight savings. One of these changes was the removal of the already thin main-bearing webs that tie into the main-bearing bores from the oil pan rails. Do not use one of these later-model 350 blocks for any high-performance build because the beefier pre-1975 blocks are still readily available.
The 403-ci small-block has the largest bore of any Olds engine, set at 4.350 inches. The cylinder walls are not that thick, however, and the best method of keeping a round bore is to keep the walls as thick as possible by honing the existing bore until it is perfectly straight and round, and then using a custom piston with file-to-fit rings. The maximum amount I would ever consider for an overbore on a 403 is .030-inch oversize. Forget any more than that. This block has siamesed cylinders that strengthen the area by tying the bores together side to side, but the rest of the cylinder thickness is borderline too thin due to
