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.
A typical cylinder wall thickness for a 4.185-inch-bore big-block in the 9-o’clock position is about .200 inch. This one is on the thick side.
This same “D” 425 block sonic tested in the 3-o’clock position is still plenty thick to hold a round bore.
Cylinder-wall thickness in the 6-o’clock and 12-o’clock positions is almost not even worth checking. I have always found them to be very thick.
If you must half-fill your Oldsmobile block, make sure that the fill is not so high that the water cannot transfer from cylinder to cylinder. Generally, if you fill to the bottom of the water pump holes, it is too high. Look through the water holes in the deck and plan the amount of fill before you pour. If your block is already filled to the base of the water pump holes, look through the deck at the water passages to verify that water is able to transfer from cylinder to cylinder. If the water cannot transfer, the only way to fix it is to fabricate an external manifold, or use a combination of pipe fittings and hoses to return or feed the water from each cylinder.
My 1,250-hp nitrous, siamese NASCAR small-block build is all-out max-effort build. I have completely filled the block; I use no water for cooling. I have found that this method seems to hold the cylinder walls in shape even with the abuse of detonation and burning pistons. This technique is good for builds such as mine but is also good for any Oldsmobile engine block. However, you have to pay attention to the way you race with this type of block and how much heat you put into the engine prior to the run. Forget bracket racing in round-robin situations; I don’t recommend it.
There are two lifter-bank angles in the Oldsmobile engine, commonly referred to as the 39-degree and the 45-degree blocks. The older 400- and 425-ci engines could be either, depending on their production year and model. All 330-ci Olds engines have a 45-degree bank angle, and all 1968-and-newer Olds engines have the 39-degree bank. What most people don’t know is that the actual bank angle on the 39-degree block is 42 degrees, meaning that the lifters are actually 42 degrees apart from each other.
The easiest way to determine which block you have is to install something (like a straight edge) into the lifter bore and determine if it is parallel to the cylinder wall or not. The cylinder bores are cast at a 45-degree angle, so if the lifter bores align with them, you have a 45-degree block. It should be plain to see one way or the other.
The camshaft cores are sometimes more difficult to find (and may be more expensive) for the 45-degree block. If you have a choice, the 39-degree block is more desirable. I have found that the 45-degree blocks cannot be converted to a 39-degree (42-degree) bank angle by boring and bushing due to the lack of sufficient material in the lifter-bore bosses. I did try.
The days of “hot tanking” a block are gone. Heating a large vat of some form of acid is just not feasible in business, due to