Machine Shop Procedures and Tools
Machine Shop Procedures and Tools
The following is an overview of machine shop procedures at H&H. If you have an engine that needs to be rebuilt, you might like to know a little more about the process. Or, if you are a true gearhead and understand the process, but would like to know a little more about our specific equipment and tooling at H&H, please see below.
Decking the Block
An engine’s cylinder head rests on the deck of the cylinder block. This surface is prone to developing irregularities that can cause leaks in compression and coolant. Straight-edges and feeler gauges can easily be used to check the flatness of the deck. Decks should be resurfaced if the maximum deformation at any point exceeds 0.002 inches, according to a standard rebuilder’s specification. This specification should be cut in half or preferably omitted when building a high-performance engine. An automated rotating cutter is used for the procedure. At H&H, we use a COMEC RP1200 that is equipped with cast iron cutter or aluminum cutter.
The term ‘decking’ refers to the cutting of the surface that the cylinder head gasket will sit on. By doing this, a flat face is created that is parallel to the crankshaft centerline and at the correct angle to the cylinder bores. During the assembly of a performance engine, this procedure is also applied to ensure that the deck is always the same height from the centerline of the crankshaft.
In addition to changing the deck height, other conditions may also affect this dimension. Among them are piston profile height (pin location), connecting rod center-to-center length, crankshaft position, and crankpin index.
In a V-shaped engine, the cylinder head is positioned closer to the crankshaft when metal is removed from the engine block. As a result, the intake manifold may fit differently and may need to be adjusted.
A specific surface finish should be achieved when decking the block. A very smooth surface is not desirable for good head gasket sealing, contrary to popular belief. Micro-inches are used to measure surface roughness. A microinch is one millionth of an inch. It is impossible to have a perfectly smooth surface. Machined surfaces have thousands of minute grooves of various depths that cannot be seen with the naked eye.
Boring and Honing
The procedure of cylinder boring involves removing material from the circumference of the bore using a cutting bit. The purpose of honing is to achieve the final dimension of the cylinder bore, while preparing the piston ring seal and oil control surfaces. Honing is performed with a stone that comes in varying grits. H&H utilizes two Kwik-Way boring bars to accommodate the various sizes of blocks and jugs we service and a Petersen Power Hone.
The boring operation is accomplished by either a boring machine or a boring bar. While boring bars are just as precise as boring machines, they require more operator labor than automated boring centers.
The piston will wear the cylinder unevenly as it goes up and down in the bore. When the crankshaft swings in its arc of rotation and the connecting rod changes direction, it places a load on the piston and cylinder wall. This is called thrust-side bore wear. Additionally, the heating and cooling cycles and distortions caused by tightening the cylinder head contribute to tapering and out-of-round bores. In addition to measuring the bore, we check if the taper and concentricity are correct. The decision will be made based on these findings as to whether to bore and hone the engine block or just hone the cylinders.
Every time a block is bored, new over-size pistons are needed. There are three common overbore sizes: 0.030, 0.040, and 0.060 inches. The size of custom pistons can usually be made to almost any requirement.
Boring is performed using a tool bit attached to a rotating cutter head. It is common for boring machines to attach to and reference from the surface of the block deck. That’s why it’s so critical to have the deck true before boring. A small error there will impact the bore’s concentricity.
The cylinder is typically bored to a few thousandths of its final dimension and then honed to its desired size. A piston’s piston-to-wall dimension refers to the clearance between the piston and the cylinder wall when it is fitted to the engine. Manufacturers determine this specification based on the piston’s expansion rate when heated. Cast pistons expand less and typically have piston-to-wall clearances of 0.002 inches. In some cases, forged race pistons require 0.006 inch of clearance. As you can see, the piston needs to be identified before the boring service is performed.
Honing requires a separate machine and is a separate process. An engine’s ring seal, ring life, and oil consumption will be determined by the quality of the hone. For honing, two types of machines are acceptable: a hand-operated manual stroke machine and an automatic stroke machine. Automatic strokes (such as H&H’s) are most desirable since they are consistent in hone rate with each pass.
By honing the cylinder wall, a crosshatch will be created. Usually, this occurs when multiple stones are used. A machinist may rough hone a bore to within 0.005 inch of its final dimensions using a 180-grit stone, then switch to a finer 280 or 320-grit stone for finishing. Quality control and identifying crosshatches are often accomplished with the help of a profilometer. RMS finish of 20 can be achieved when a stone with 280 grit is used properly. With 320-grit stone, you will get a finish closer to 15 RMS.
In order to achieve the best results, it is best to use a torque or stress plate during honing. By attaching the fixture to the block deck, it simulates distortion of the bore caused by the installed cylinder head. Using a torque plate will prevent the bore from distorting once the cylinder heads are attached, but it will not be able to measure within specifications in an unstressed state. In addition to being relatively expensive, torque plates are application-specific to each engine family.
It is common for the machinist to wash the block between procedures to get rid of metal shavings and cutting oil. Following the final wash, the block is covered in a plastic bag to keep dirt and moisture out. H&H has two pressure washers, one for aluminum and one for cast iron. Surfaces that have just been machined are highly susceptible to rust. Due to this, H&H has a climate-controlled machine shop.