39-degree 425 block used .921-inch lifter bores and had bronze lifter bore bushings installed that were finish machined to .842 diameter for use with more readily available parts.
The endmill machines a pocket that is 2.930 × .140 inches deep to allow the Cloyes roller thrust bearing to sit in the pocket without falling out. This DX block has main-bearing spacers. It is best to use a main cap that is the proper size so that the only spacer needed is in the block.
The timing cover on the left is the stock version found on every gasoline-powered Oldsmobile engine. The timing cover on the right is from a diesel engine. The harmonic-balancer seal protrudes from the front of the timing cover. It is best to check for interference before you glue this cover on. This hits some harmonic balancers, and does not hit others.
The siamesed-cylinder NASCAR blocks are the rarest, strongest, and most desirable of these high-performance castings. I have seen many of the special high-performance Olds blocks and there seems to be so many variations and inconsistencies, so it isn’t worth discussing them all. I have seen the PN 22528096 “non-siamesed” block have siamesed cylinders. I have seen blocks with 2.500-, 2.750-, and 3.000-inch main-bearing diameters. I have seen each of these blocks with .842-, .875-, and .921-inch-diameter lifter bores, etc. The only way to determine if the block has siamesed cylinder bores is to visually inspect them through the freeze-plug holes. Each block, regardless of part number, must be visually inspected to see exactly what features and dimensions it truly has.
Oldsmobile big-block engines also consisted of three different cubic-inch displacements. They were offered at 400, 425, and 455 ci. With regard to high-performance use, the 455 is the most popular and is used most by Olds enthusiasts.
Oldsmobile big-blocks have many shared features and dimensions. The bore spacing between the cylinders is the same as all Olds engines at 4.625 inches. The main-housing bores are all set at 3.189 inches on all big-block engines. The deck height (measured from the crankshaft centerline to the cylinder-head mounting surface) is set at 10.625 inches on all of the big Oldsmobile blocks. Most of the factory blocks measure right on that dimension too. The lifter-valley areas in all the big-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 the 400/425 engines could be .842 or .921 diameter, depending on the application. The 455 engines all retained the .842-diameter lifter bores, and all were designed for hydraulic flat-tappet lifters. The main webs are quite thin and are virtually the same on all models. Forget about four-bolt main-bearing caps; there is nothing to bolt to.
This number designates a siamesed NASCAR block.
Older 1968–1970 model-year engine blocks had a casting designation at the rear of the engine, near the bellhousing area, of 68F, F1, F2, or F3. It has been circulated that these blocks had higher nickel content, and are therefore more desirable. I consider this to be a myth. I have honed many blocks with high nickel content, and soft honing stones are always required to finish high-nickel content blocks. But I have never had to use a soft stone on an Olds block. I have never found much of a difference at all between later-model and earlier-model blocks. I have also sonic tested many Oldsmobile big-blocks and actually found some later blocks to have thicker cylinder walls than some older models. Some of the thickest cylinder walls I have seen on 455 blocks are the later F6 blocks. It’s easy to see if you look through the water jacket holes in the deck at the backside of the cylinders. If you look at one of these blocks next to any of the others, it’s easy to see the difference.
The main webs are considerably thicker on this NASCAR block, compared to a production diesel block. Other than the main bearing bore diameter, the bottom side of the blocks is the same.
With all that being stated, the best way to select a good Oldsmobile big-block core is to simply look over the entire casting. This includes inspecting inside the water jackets from the deck and freeze-plug area. Some blocks have thicker walls than others, and this can typically be seen by the naked eye. A block that has had antifreeze in it all its usable life ultimately has thicker cylinder walls, due to less deterioration from rust. Rust is most common on cooling systems serviced with water alone. The thinnest spots in the cylinder walls are in the area between the cylinders. Sonic testing, or measuring the space between the cylinder walls through the freeze-plug holes, can determine this thickness.
NASCAR blocks (shown) are essentially the same in the lifter valley as diesel blocks, other than the lack of the injector-pump boss.
If you know the amount of space between the cylinders, you can determine the approximate cylinder wall thickness at that point by subtracting the distance between the cylinder walls from the bore-spacing dimension (4.625 inches) and dividing by 2. The average big-block can be bored safely to 4.185 inches and maintain round cylinders with as much power as the block can handle. A 4.200-inch bore can work well on a good core with about 600 hp. Some builders go to 4.250-inch bore, but this leaves the cylinder wall thickness about .090-inch thick at the thinnest area. I do not recommend this. Good piston-ring seal and oil control far outweigh the small cubic-inch and potential cylinder-head flow gain the bigger bore brings with it.
The main webs on big-block Olds engines are very thin and need some sort of a girdle instead.
The latest long-stroke 400 G block can be bored and honed to 4.000 inches safely in limited horsepower applications. This bore generally leaves about .250-inch wall thickness everywhere at that size, which is acceptable in the 400- to 500-hp range, which is all you would ever ask for in one of these situations anyway.
I have half-filled Olds blocks with cement-type products (HardBlock or similar) that occupy the water jackets for the purpose of tying in the cylinder walls for strength. I have not found that it helps ring seal significantly; I don’t use it unless the customer requests it specifically. The negative effect of block filling is that there is less cooling of the crankcase oil, which can cause high oil temperatures while driving on the street or when making back-to-back passes during round-robin bracket racing.
I call this the poor man’s sonic test. The thinnest parts of the cylinder walls are in the 9-o’clock and 3-o’clock position as you are facing the block decks. So if you measure the space between the cylinders and subtract the bore spacing (4.625 inches) and divide by 2, you get the approximate cylinder wall thickness in the thinnest areas.
