The engine block is the foundation of the engine. All parts of the block must be of the correct size and they must be aligned. The parts must also have the proper finishes if the engine is to function dependably for a normal service life. Engine blueprinting is the reconditioning of all the critical surfaces and dimensions so that the block is actually like new.
After a thorough cleaning, the block should be inspected for cracks or other flaws before machine work begins. After the block has been cleaned and cracked checked, the block should be prepared in the following sequence.
Operation 1 Main bearing housing bore alignment, often called “align boring” (or honing)
Operation 2 Machining of the block deck surface parallel to the crankshaft
Operation 3 Cylinder boring and honing
Main Bearing Housing Bore Alignment
The main bearing journals of a straight crankshaft are in alignment. If the main bearing housing bores in the block are not in alignment, the crankshaft will bend as it rotates. This condition increases rotational friction of the crankshaft and will lead to premature bearing failure or a broken crankshaft. The original stress in the block casting is gradually relieved as the block is used.
Fig.1 The main bearing bores of a warped block usually bend into a bowed shape. The greatest distortion is in the center bores.
Some slight war page may occur as the stress is relieved. In addition, the continued pounding caused by combustion will usually cause some stretch in the main bearing caps. ● SEE FIGURE 1. The main bearing bores gradually bow upward and elongate vertically. This means that the bearing bore becomes smaller at the centerline as the block distorts, pinching the bore inward at the sides. ● SEE FIGURE 2.
Fig.2 When the main bearing caps bow downward; they also pinch in at the parting line.
The procedure includes the following steps.
The first step in determining the condition of the main bearing bores is to determine if the bore alignment in the block is straight. These bores are called the saddles. A precision ground straightedge and a feeler gauge is used to determine the amount of warpage.
The amount of variation along the entire length of the block should not exceed 0.0015 inch (0.038 mm).
Caution: When performing this measurement, be sure that the block is resting on a flat surface. If the engine is mounted to an engine stand, the weight of the block on the unsupported end can cause an error in the measurement of the main bearing bores and saddle alignment.
If the block saddles exceed one-and-a-half thousandth of an inch distortion, then align honing is required to restore the block. If the block saddles are straight, the bores should be measured to be sure that the bearing caps are not distorted. ● SEE FIGURE 3.
Fig.3 The main bearing bore alignment can be checked using a precision straightedge and a feeler gauge.
The bearing caps should be installed and the retaining bolts tightened to the specified torque before measuring the main bearing bores. Using a telescoping gauge, measure each bore in at least two directions.
Check the service information for the specified main bearing bore diameter. The bearing bore should not vary by more than one-half of a thousandth of an inch or 0.0005 inch (0.0127 mm). A dial bore gauge is often used to measure the main bearing bore. Set up the dial bore gauge in the fixture with the necessary extensions to achieve the nominal main bearing bore diameter. Check the service information for the specified main bearing bore diameter and determine the exact middle of the range.
Arbor Check Method
The arbor is installed, then all main caps are tightened to specifications. After tightening, the arbor is checked to make sure it rotates freely, indicating a true centerline. However, because of all the different diameters required, it is an expensive method. ● SEE FIGURE 4.
Fig.4 (a) A precision arbor can be used to check the main bearing bore alignment. (b) If the sleeve can be inserted into all of the main bearing bores, then they are aligned.
Machining the Deck Surface of the Block
An engine should have the same combustion chamber size in each cylinder. For this to occur, each piston must come up an equal distance from the block deck. The connecting rods are attached to the rod bearing journals of the crankshaft. Pistons are attached to the connecting rods. As the crankshaft rotates, the pistons come to the top of the stroke. When all parts are sized equally, all the pistons will come up to the same level. This can only happen if the block deck is parallel to the main bearing bores. Therefore, the flatness of the block deck should be checked. ● SEE FIGURE 5.
Fig.5 (a) Checking the flatness of the block deck surface using a straightedge and a feeler gauge. (b) To be sure that the top of the block is flat, check the block in six locations as shown.
The block deck must be resurfaced in a surfacing machine that can control the amount of metal removed when it is necessary to match the size of the combustion chambers. This procedure is called decking the block. The block is set up on a bar located in the main bearing saddles, or set up on the oil pan rails of the block. The bar is parallel to the direction of cutting head movement. The block is leveled sideways, and then the deck is resurfaced in the same manner as the head is resurfaced. ● FIGURE 6 shows a block deck being resurfaced by grinding.
Fig.6 Grinding the deck surface of the block.
Deck Surface Finish
The surface finish of the block deck should be:
■ 60 to 100 Ra (65 to 110 RMS) for cast iron
■ 50 to 60 Ra (55 to 65 RMS) for aluminum block decks to be assured of a proper head gasket surface The surface finish is determined by the type of grinding stone used, as well as the speed and coolant used in the finishing operation. The higher the surface finish number the rougher the surface.
Cylinders should be measured across the engine (perpendicular to the crankshaft), where the greatest wear occurs. In other words, measure the bores at 90 degrees to the piston pin. Most wear will be found just below the ridge, and the least amount of wear will occur below the lowest ring travel. ● SEE FIGURE 7.
Fig.7 Cylinders wear in a taper, with most of the wear occurring at the top of the cylinder where the greatest amount of heat and pressure are created. The ridge is formed because the very top part of the cylinder is not contacted by the rings.
Fig.8 Using a dial bore gauge to measure the bore diameter at the top just below the ridge (maximum wear section) and at the bottom below the ring travel (minimum wear section). The difference between these two measurements is the amount of cylinder taper. Take the measurements in line with the crankshaft and then repeat the measurements at right angles to the centerline of the block in each cylinder to determine out-of-round.
The cylinder should be checked for out-of-round and taper. ● SEE FIGURE 8.
Most cylinders are serviceable if they:
■ Are a maximum of 0.003 inch (0.076 mm) out-of-round
■ Have no more than a 0.005 inch (0.127 mm) taper
■ Have no deep scratches in the cylinder wall
Note: Always check the specifications for the engine being serviced. For example, the General Motors 4.8, 5.3, 5.7, 6, and 6.2 liter LS series V-8s have a maximum out-of-round of only 0.0003 inch (3/10 of one thousandths of an inch). This specification is about one-third of the normal dimension of about 0.001 inch.
The most effective way to correct excessive cylinder out-of-round, taper, or scoring is to re bore the cylinder. The re bored cylinder requires the use of a new, oversize piston.
The maximum bore oversize is determined by two factors.
1. Cylinder wall thickness—at least 0.17 inch for street engines and 0.2 inch for high-performance or racing applications.
2. Size of the available oversize pistons.
If in doubt as to the amount of overbore that is possible without causing structural weakness, an ultrasonic test should be performed on the block to determine the thickness of the cylinder walls. An ultrasonic tester can measure the thickness of the cylinder walls and is used to determine if a cylinder can be
bored oversize and, if so, by how much. All cylinders should be tested. Variation in cylinder wall thickness occurs because of core shifting (moving) during the casting of the block. For best results, cylinders should be re bored to the smallest size possible.
Note: The pistons that will be used should always be in hand before the cylinders are re bored. The cylinders are then bored and honed to match the exact size of the pistons.
The cylinder must be perpendicular to the crankshaft for normal bearing and piston life. If the block deck has been aligned with the crankshaft, it can be used to align the cylinders.
Portable cylinder boring bars are clamped to the block deck. Heavy-duty production boring machines support the block on the main bearing bores. Main bearing caps should be torqued in place when cylinders are being re bored. In precision boring, a torque plate is also bolted on in place of the cylinder head while boring cylinders.
In this way, distortion is kept to a minimum. The general procedure used for re boring cylinders includes the following steps.
STEP 1 Set the boring bar up so that it is perpendicular to the crankshaft. It must be located over the center of the cylinder.
STEP 2 The cylinder center is found by installing centering pins in the bar.
STEP 3 The bar is lowered so that the centering pins are located near the bottom of the cylinder, where the least wear has occurred. This locates the boring bar over the original cylinder center. Once the boring bar is centered, the boring machine is clamped in place to hold it securely. This will allow the cylinder to be re bored on the original centerline, regardless of the amount of cylinder wear.
STEP 4 A sharp, properly ground cutting tool is installed and adjusted to the desired dimension. Rough cuts remove a great deal of metal on each pass of the cutting tool. The rough cut is followed by a fine cut that produces a much smoother and more accurate finish. Different shaped tool bits are used for rough and finish boring.
STEP 5 The last cut is made to produce a diameter that is at least 0.002 inch (0.05 mm) smaller than the required diameter. ● SEE FIGURE 9.
Fig.9 A cylinder boring machine is used to enlarge cylinder bore diameter so a replacement oversize piston can be used to restore a worn engine to useful service or to increase the displacement of the engine in an attempt to increase power output.
Sleeving the Cylinder
Sometimes cylinders have a gouge so deep that it will not clean up when the cylinder is re bored to the maximum size. This could happen if the piston pin moved endways and rubbed on the cylinder wall. Cylinder blocks with deep gouges may be able to be salvaged by sleeving the cylinder. The cylinder wall thickness has to be checked to see if sleeving is possible. Sleeving a cylinder is done by boring the cylinder to a dimension that is greatly oversize to almost match the outside diameter of the cylinder sleeve. The sleeve is pressed into the rebored block and then the center of the sleeve is bored to the diameter required by the piston. The cylinder can be sized to use a standard-size piston when it is sleeved. ● SEE FIGURE 10.
Fig.10 A dry cylinder sleeve can also be installed in a cast-iron block to repair a worn or cracked cylinder.
It is important to have the proper surface finish on the cylinder wall for the rings to seat against. Honing includes two basic operations depending on the application.
1. When installing new piston rings on a cylinder that is not being bored, some ring manufacturers recommend breaking the hard surface glaze on the cylinder wall with a hone before installing new piston rings. This process is often called “deglazing” the cylinder walls.
2. The cylinder wall should be honed to straighten the cylinder when the wall is wavy or scuffed. If honing is being done with the crankshaft remaining in the block, the crankshaft should be protected to keep honing chips from getting on the shaft.
Two types of hones are used for cylinder service.
■ A deglazing hone removes the hard surface glaze remaining in the cylinder. It is a flexible hone that follows the shape of the cylinder wall, even when the wall is wavy. It cannot be used to straighten the cylinder. A brush-type (ball-type) deglazing hone is shown in ● FIGURE 11.
Fig.11 An assortment of ball-type deglazing hones. This type of hone does not straighten wavy cylinder walls.
■ A sizing hone can be used to straighten the cylinder and to provide a suitable surface for the piston rings. Honing the cylinder removes the fractured metal that is created by boring. The cylinders must be honed a minimum of 0.002 inch (0.05 mm) after boring to cut below the rough surface and provide an adequate finish. Honing leaves a plateau surface that can support the oil film for the rings and piston skirt. This plateau surface is achieved by first using a coarse stone followed by a smooth stone to achieve the desired surface. The process of using a course and fine stone is called plateau honing. ● SEE FIGURE 12.
Fig.12 After boring, the cylinder surface is rough, pitted, and fractured to a depth of about 0.001 inch.
Its honing stones are held in a rigid fixture with an expanding mechanism to control the size of the hone. The sizing hone can be used to straighten the cylinder taper by honing the lower cylinder diameter more than the upper diameter.
As it rotates, the sizing hone only cuts the high spots so that cylinder out of round is also reduced. The cylinder wall surface finish is about the same when the cylinder is refinished with either type of hone. ● SEE FIGURE 13.
Fig.13 Honing enlarges the cylinder bore to the final size and leaves a plateau surface finish that retains oil.
The hone is stroked up and down in the cylinder as it rotates to produce a crosshatch finish on the cylinder, wall which aides in proper ring break-in. The speed that the operator moves the hone up and down controls the angle. Always check service information for the specified crosshatch angle.
The angle of the crosshatch should be between 20 and 60 degrees. Higher angles are produced when the hone is stroked more rapidly in the cylinder. A typical honed cylinder is pictured in ● FIGURE 14.
Fig.14 The crosshatched pattern holds oil to keep the rings from wearing excessively, and also keeps the rings against the cylinder wall for a gas-tight fit.
Cylinder Surface Finish
The size of the abrasive particles in the grinding and honing stones controls the surface finish. The size of the abrasive is called the grit size. The abrasive is sifted through a screen mesh to sort out the grit size.
A coarse-mesh screen has few wires in each square inch, so large pieces can fall through the screen. A fine-mesh screen has many wires in each square inch so that only small pieces can fall through. The screen is used to separate the different grit sizes.
The grit size is the number of wires in each square inch of the mesh. A low-numbered grit has large pieces of abrasive material; a high-numbered grit has small pieces of abrasive material. The higher the grit number is, the smoother the surface finish will be. Refer below chart…
A given grit size will produce the same finish as long as the cutting pressure is constant. With the same grit size, light cutting pressure produces fine finishes, and heavy cutting pressure produces rough finishes.
The surface finish should match the surface required for the type of piston rings to be used. Typical grit and surface finish standards include the following:
■ Chrome rings: 180 grit (25 to 35 micro inches)
■ Cast-iron rings: 200 grit (20 to 30 micro inches)
■ Moly rings: 220 grit (18 to 25 micro inches)
Note: The correct honing oil and coolant are critical to proper operation of the honing equipment and to the quality of the finished cylinders.
Cylinder Honing Procedure
The procedure includes the following steps.
STEP 1 The hone is placed in the cylinder. Before the drive motor is turned on, the hone is moved up and down in the cylinder to get the feel of the stroke length needed. The end of the hone should just break out of the cylinder bore on each end. The hone must not be pulled from the top of the cylinder while it is rotating.
Also, it must not be pushed so low in the cylinder that it hits the main bearing web or crankshaft.
STEP 2 The sizing hone is adjusted to give a solid drag at the lower end of the stroke.
STEP 3 The hone drive motor is turned on and stroking begins immediately. Stroking continues until the sound of the drag is reduced.
STEP 4 The hone drive motor is turned off while it is still stroking. Stroking is stopped as the rotation of the hone stops. After rotation stops, the hone is collapsed and removed from the cylinder.
STEP 5 The cylinder is examined to check the bore size and finish of the wall. If more honing is needed, the cylinder is again coated with honing oil and is honed once again. The finished cylinder should be within 0.0005 inch (0.013 mm) on both out-of-round and taper measurements. ● SEE FIGURE 15 for an example of cylinder surface finish reading.
Fig.15 (a) The surface finish tool is being held against the cylinder wall. (b) The reading indicates the Ra roughness of the cylinder. More work is needed if moly piston rings are to be used.
Chamfering the Cylinder Bores
Whenever machining is performed on the block such as boring and decking, the top edge of the cylinder bores has sharp edges.
Fig.16 Using a tapered sanding cone to remove the sharp edges at the top of the cylinders created when the block was machined.
These sharp edges must be removed to allow the piston with rings to be installed. The slight chamfer allows the rings to enter the cylinder easily when the pistons are installed. A tapered rubber cone covered in sanding cloth is used to remove the sharp edges. ● SEE FIGURE 16.