Align hone the block. This creates the centerline that everything else references.
Cut the decks to obtain a square block, where both decks are the same height from the main bore centerline and parallel to the main bore centerline.
Measure and correct cylinder bore spacing and angle. When correction is necessary, perform an overbore or overbore correction sleeve installation. The sleeves are resized to original or planned bore diameter.
Measure and correct camshaft bore centerline. If correction is required, overbore the cam tunnel and use the necessary oversize-OD cam bearings.
Check and correct all lifter bores for centerline location and angle. Correction may require overboring, installation of bronze liners, and resizing for proper lifter clearance.
Utilizing specialty main bore and cam bore inserts (various diameters are available for specific blocks) and a dedicated micrometer, it’s easy to accurately measure the distance between main bore and camshaft-bore centerline distance (measuring from the outside of each bore adapter and subtracting one-half of each guide’s nose diameter). (Photo Courtesy BHJ Products)
Production engine block decks may be off specification by a few thousandths of an inch; in some, by as much as hundreds of thousandths. This means that both decks might be too high or too low or one deck may be higher than the opposite deck. Here’s a case in point: A recent Pontiac 455 build started with an aged 1973 455 block. Although the OEM spec for deck height is listed as 10.210 inches, this block’s left deck measured 10.2545 inches, which is .0445 inch taller than spec. The right deck measured 10.2456 inches, which is .0356 inch taller than spec. Luckily, the excess material allowed correction precisely to the desired 10.210-inch spec. In this case, both decks were too high, and had a different deck height at each bank. As is, the OEM excess in deck height decreases compression ratio. Also, the uneven heights from left to right and front to rear results in different compression ratios per cylinder and from bank to bank. Yes, the engine would run, but it wouldn’t be running at optimum efficiency.
Keep this important point in mind: The OEM spec for block deck height should not be considered set in stone unless all other OEM engine dimensions and materials are used. If so, you must use the exact same crank stroke, the same connecting rod length and material, the same piston compression height and volume, the same thickness and material of head gasket, and the same volume and shape of cylinder head combustion chamber. Be sure to use the same intake and exhaust valve diameter and length and the same rocker arm ratio as specified in the OEM engine. You also mock the short block together (crank, rods, and pistons) to measure the pistons’ top deck location at top dead center (TDC) relative to the block deck. If the deck height is too short, the pistons can hit the heads. If the deck height is too tall, you decrease compression.
When test fitting, be sure to consider the installed (crushed) thickness of the head gaskets. Be sure to use exactly the same brand and type of head gasket for test fitting that you plan to use during final assembly.
You want to be as close as possible to zero clearance between the piston dome and cylinder head during engine operation. However, obtain static clearances, because the clearances decrease as the engine operates because of heat expansion and metallurgical stress. The minimum clearances you should generally have in static state (with the engine not running) between the piston and combustion chamber at TDC are:
• Performance street application with steel connecting rods: about .040 inch
• Racing engine with steel rods: about .045 inch
• Racing engine with aluminum rods: about .060 inch
Remember that these approximated clearances are minimum clearances.
With this in mind, it’s important to understand how block deck height relates to your crank, rod, and piston dimensions. In order to achieve a zero deck height (so the piston lies flush with the deck at TDC) consider crankshaft stroke, connecting rod center-to-center length, and piston compression distance.
A crankshaft’s stroke refers to its total stroke from a rod journal at TDC all the way to that journal’s bottom dead center (BDC). When determining where the top of the piston deck will be at TDC, consider one half of the crank stroke because you’re only concerned here with how far the crank is pushing the piston upward. Connecting rod length refers to the distance from the big-end centerline to the small-end centerline, not the overall rod length. Piston compression distance (CD) refers to the distance from the centerline of the piston pin bore to the piston’s flat deck surface.
A CNC cutter spot faces lifter bore roofs in preparation of lifter bore centerline correction. With a programmed CNC machine, no indexing fixtures are needed for lifter bore correction.
Here’s the formula for finding block deck height at zero deck:
1/2 crank stroke + rod length + piston CD
For example, the crank stroke is 3.500 inches, the rod length is 6.000 inches, and piston CD is 1.500 inches. When you plug the numbers into the formula, you get a 9.25-inch deck height.
(3.500 ÷ 2) + 6.000 + 1.500
1.750 + 6.000 + 1.500
9.25
If you want the piston top flat to be, say, .015 inch below deck, you add .015 inch. In the above example, the finished block deck height needs to be 9.265 inches. The decks may also be out of plane. You could have decks that are low at the front and high at the rear, high at the front and low at the rear, low inboard or low outboard, etc. In other words, decks may be flat, but they might be “crooked.” For corrective machining, you need to index from the crankshaft main bore centerline to make both decks the same height (and the proper height) and 90 degrees to the main bore centerline in both front/rear and inboard/outboard planes.
A blueprint illustration reveals all of the block dimensions according to the engineering design. Some points of reference may or may not be correctible, but a factory spec drawing allows you to double-check many critical dimensions to verify various areas of your OEM block. (Illustration Courtesy General Motors)
Obviously, the cylinder bores must be round and the correct diameter for the intended pistons and rings. Also the centerline placement of each bore and the angle of each bore must be accurate. If not, they must be corrected in order to blueprint the block. When OEM blocks are made, mass production line tolerances may allow for the bore centerline to be placed slightly off-center, and the angle of the bores may be slightly offset from