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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 cost and toxicity. As far as I’m concerned, the only proper way to clean cast-iron engine blocks is to use the bake/blast/wash process. In this process, the block is stripped of all the plug and cam bearings, placed in an oven, and baked at 700 degrees F for two to three hours. Whatever oil, grease, and dirt that is on the block or hiding in oil passages is turned to ash.
The next step is to place the block in the blast machine. The block is fastened inside a rotating fixture and steel shot is thrown around by a high-speed paddle wheel. The ash and rust is completely removed in about 10 minutes of run time. Jet washing with hot soapy water after that removes the rust residue and leftover shot. The engine block looks brand new at this point and the machining process can begin.
The first machining operation to do on the engine block is the main bearing bore work. If the block is machined properly, most of the block-machining operations were referenced off the main bearing bores. You can check main bearing bore size by torquing the main caps and measuring roundness and diameter by using a dial bore gauge. It is not quite as easy to check for alignment. Some old-school books show the use of a straightedge and feeler gauges, but I consider this method to be a waste of time. I have set some factory blocks on a mill and indicated the end main journals until they read zero on each, and run the indicator along the center three mains and found some to be within a few tenths, and some to be off by .002!
Some people wonder why they wipe out main bearings with a .0025 main bearing clearance. If you want to avoid a future headache, just have the mains align honed and be done with it. This process is one that requires some skill and experience; be sure to select the right shop to do the job. I have seen many blocks that were line honed and were far from “aligned.” If you are installing aftermarket caps, the block must be align bored and align honed anyway, so you are good to go.
Boring the cylinders is simply a method of sizing the cylinders for an oversize piston. The cylinders at my shop are bored .005 to .008 under final size. Some literature specifies that you can bore within .003 and then final hone, but depending on the finish after the boring operation, that amount may not be enough to remove tooling marks. It takes more time in the hone, but you are guaranteed the proper finish with the extra material. Engine blocks at my shop are bored on a very large Cincinnati CNC machine. A precision bar goes through the main journals and cam tunnel. When the block is set on the machine fixture, each bore is machined exactly 45 degrees from main/camshaft centerline, and parallel to them. The bore locations are referenced off the cylinder-head dowel pins, and bore spacing is set at 4.625 inches in the boring program. This ensures that all of the machine work of the block is “blueprinted.”
The blasting unit houses the previously thermal-cleaned, completely dry engine blocks or cylinder heads. They are fastened inside and rotate 360 degrees while a high-speed paddlewheel throws steel shot at the parts. Five minutes later, a previously rusty engine block looks like brand new and is ready for machining.
Block Decking
The next block machining operation is to deck the block for the purpose of making the surface flat for head-gasket sealing and machining a predetermined deck height. Deck height is referred to as the distance from the crankshaft centerline to the cylinder-head gasket surface of the engine block. I make the surface absolutely smooth and am convinced that a smooth surface seals the best on any gasket. Rough finishes to “bite” the gasket simply leave peaks and valleys. I’m sorry, but I don’t want any valleys in a sealing surface. The deck surfaces are cut immediately after the boring operation without the block being moved from the fixture; therefore, the deck surface has to be perpendicular to the bores and parallel to the main journals. The cutter is referenced (or zeroed) from the main-journal centerline. And if a deck height of 10.600 inches is desired, then the cutter is raised to 10.600 inches and the deck is cut.
A thermal cleaning oven does a great job of cleaning all of the oil and grease from an engine block, cylinder heads,
