Machine Shop Basics - The Engine Block

The block needs to be fully disassembled prior to any machining. This can be done at home, or by the machine shop. After a quick inspection for obvious damage, the first step is to thoroughly clean the block. If the block is obviously very rusty, the shop may place it in a shot-blast cabinet. This machine uses steel balls (instead of sand) to strip away corrosion and grime. The shop may opt to treat the block to a hot bath in a tank containing a caustic solution heated to 200 degrees Fahrenheit. This serves to clean away all of the grease and grime in addition to the rust, and makes inspecting for cracks and flaws much easier. The most-common procedure is the "jet-clean" system shown in the photo. It uses hot water and a cleaning solution sprayed on the block, while it revolves around on a turntable. During the rebuild procedure the block will usually be cleaned, at least twice, in the "jet wash" machine.

The next step is to inspect the block thoroughly for cracks in the casting. These may be caused by overheating, freezing, or stress from severe-duty usage. The prescribed method is called Magnafluxing. The Magnaflux machine uses an application-specific, high-powered electromagnet placed on the engine block. With a strong magnetic field induced, a colored iron powder is sprinkled around the area between the poles of the magnet. If a crack is present, the iron powder will enter it and identify its outline. A crack was found in this block during the Magnaflux process. It is represented as the long line seen just above the lifter valley in the photo.

Another way to locate and identify cracks is to use a liquid that can be applied in suspension and viewed under a black light. Permanent dyes from spray cans work well too. Shown is a multi-step crack detection kit form the Goodson Company.

If the main bearing bores are not aligned properly, the crank cannot run true, and the bearing loads will be uneven. Main bearing cap distortion can be the result of many cycles of heating and cooling over the lifetime of an engine. Depending upon the amount of correction required to bring the alignment true, either align-boring or align-honing may be necessary. We feel at least align honing should be part of every rebuild on a used or "seasoned" block. If new or different main bearing caps are being installed, align boring is required to achieve both straightness and proper sizing along the main bearing caps.

The machinist can adjust the cutting speed during the decking procedure to achieve the desired finish. A profile meter is used to confirm the surface texture for proper head gasket sealing. This instrument reads in root mean square, or RMS. An RMS of 60 is the norm for cast iron; on aluminum it's normally 30.

Cylinder boring is a procedure where material is removed from the circumference of the bore cylinder with the use of a cutting bit. This operation can be achieved using either a boring center or a boring bar. When a block is overbored, new pistons are required to match the new, larger diameter of the cylinder. Common overbores are usually .030, .040 or .060-inch.

The deck of the cylinder block is identified as the top portion of the bore where the cylinder heads bolt on. It is common for this surface to develop irregularities over time and heat cycling. These irregularities can lead to a loss of compression, coolant leaks, and head gasket failure. The term "decking" refers to the cutting and leveling of this critical surface. This is done to create an absolutely flat surface for the head gasket and cylinder head to mount upon. The flat surface must also be parallel to the centerline of the crankshaft, and at the correct angle relative to the bores. This procedure is also used to ensure the surface of the deck is the same distance from the crankshaft centerline on each bore. It may also be a method of adjusting compression ratio or decreasing the squish area. When material is removed from the block on a "V" design engine, both cylinder heads will be closer to the crankshaft, which can cause intake manifold fit problems. This can lead to other forms of corrective machining that will be covered later in the series.

Most machine shops will bore the cylinder to within a few thousandths of the final target dimension, and then achieve the target diameter through honing. This is accomplished on a dedicated machine. If the shop chooses to use a drill motor with a hone attached, get your stuff and run out the door. The quality of the hone being used will affect the ring seal, ring life, and the oil consumption of the engine. The two acceptable procedures for block honing are both hand-operated manual stroke and full-automatic stroke machines.

A quality shop will probably choose to use torque plates that bolt to the block and simulate a cylinder head. If the plates are in place and torqued to the same specification as the cylinder heads, the distortion caused by the tightened fasteners will be duplicated while the block is being machined. Should an engine be honed without a torque plate in place, the bore distortion could affect ring seal once the heads are torqued on.

The hone creates the necessary crosshatch pattern on the cylinder walls through the use of dedicated, application-specific stones. The bores are rough-honed with a harsher grit, then the stones are changed to a finer and finer grade until the desired crosshatch pattern is achieved. The stone in the far right portion of the photo is called a "soft hone", and resembles a brush. It produces a finish called a plateau hone. Auto manufacturers have used this type of finish for the past ten years, this is one of the reasons modern engines have such long life spans relative to their classic predecessors.

A proper crosshatch surface finish will resemble the angles in this drawing.

After the bore is honed, a special fixture is used to chamfer the top of the deck and break the sharp edges left by the boring procedures. This also allows piston ring installation to be accomplished without the ring being tempted to catch on the edge of the bore.

After all of the machine work is completed, the block will be washed. The final step is the installation of the cam bearings. A specially designed tool is used to locate and install the cam bearings within the block, so the bearings will not be damaged during installation. The same tool is also used to remove the cam bearings.