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 the weight-saving redesign.
With the large-bore design, this block sounds like a great deal. Unfortunately, another way the Olds engineers saved weight in this block was by removing material and effectively adding windows to the already-weak main bearing webs. When looking at one of these blocks on the engine stand, it is plain to see that there is not much material to hold the spinning crank assembly in the block. You’d be lucky if the crankshaft doesn’t fall on the floor when you turn the engine stand over to bolt the heads on, let alone spin some RPM with rods and pistons connected! I’m exaggerating here, of course, but the main bearing webs are one of the important areas of strength in an engine block design. The Olds designers missed the boat on this one. There are, however, some alternatives.
One way of reducing some bottom-end stress with these engine blocks is to reduce the weight of the rotating and reciprocating lower-end components. Another way to help these engines is to use a main-bearing girdle (like the part made by J&S). It has been debated whether the girdle helps or doesn’t help, but I can say that the 403 engines that I have built with the girdle in place have not failed. The main-bearing girdle cannot weaken the lower end, so I put it on.
One particular 403 engine that I built for a customer has used the girdle and lightweight components with great success. This particular street-and-strip engine has aluminum connecting rods and lightweight custom pistons. The street-driven 1987 Cutlass in which the engine was installed ran 12.30-second passes at the quarter-mile drag strip (at full street weight and naturally aspirated) and 11.50-second times with a small amount of nitrous oxide. Yes, I said nitrous oxide. This 403-ci “bomb” is still ticking some four years later.
The Mondello engine-block girdle is a welded-together unit. I have seen numerous high-horsepower Oldsmobile engines perform reliably with it.
In 1977, the Oldsmobile Division decided to produce a diesel engine for the full-size Delta 88s. These Oldsmobile diesel engines were not necessarily good when new, and had a variety of warranty issues. However, it gave Olds racing enthusiasts a good block to convert into a bulletproof gasoline-powered high-performance engine.
Three versions of these blocks were available: the “D” casting block with .842-inch-diameter lifter bores, or the “DX” block with either .842-inch or .921-inch-diameter lifter bores. These blocks had the same deck height (9.330 inches) as all the gasoline Olds small-blocks. The main bearing bore used big-block dimensions (3.189 inches), which accepted a 3-inch main bearing journal big-block crankshaft, or the standard cast diesel crankshaft. The cylinder wall is much thicker than the gasoline small-blocks, and a diesel block’s cylinders can be bored to as much as 4.250-inch, although there are some parts of the bore that start getting thin at that size. Half-inch-diameter head bolts were used on these engines for improved head gasket sealing and will not have to be upgraded. The oiling system remains unchanged on these blocks.