Check the engine for cracks. Perform a visual inspection on all of the components. Some reliable methods for inspecting for cracks include Magnaflux® a magnetic process or Zyglo®, a dye penetrant. Magnaflux is used only on ferrous metal (cast iron). Zyglo uses a spray on fluorescent mixture along with a black light to reveal the cracks and is suitable for aluminum engine components.
Inspect all the main bearing saddles and caps for damage, burrs or high spots. If damage is found, and was caused by a spun main bearing, the block will need to be align-bored or, if severe enough, replaced. Any burrs or high spots should be carefully removed with a de-burring tool.
Place a machinist’s straight edge along the bearing saddles in the engine block. If any clearance exists between the machinist’s straight edge and the saddles, the block must be align-bored.
Align-boring consists of machining the main bearing saddles and caps by means of a fly cutter that runs through the bearing saddles.
Ensure that the deck surface is clear of dirt, carbon deposits and old gasket material. Place a machinist’s straight edge across the surface of the deck along its centerline. Check the clearance along several points using feeler gauges. . Move the machinist’s straight edge diagonally across the deck surface and check clearances along the diagonal line. If the reading exceeds 0.003 in. (0.076 mm) within a 6.0 inch (15.2 cm) span, or 0.006 in. (0.152 mm) over the total length of the deck, it must be machined.
The cylinder bores are slightly larger than the pistons themselves. A common piston-to-bore clearance is 0.0015 - 0.0025 in. (0.0381 - 0.0635 mm). Inspect and measure the cylinder bores. All bores should be checked for out-of-roundness, taper and size. The results of this inspection will determine whether the cylinder can be used in its existing size and condition, or a re-bore to the next oversize is required (or in the case of removable sleeves, have replacements installed).
| Measuring cylinder bore diameter
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Measurements are taken at three locations in each cylinder: at the top, middle and bottom. Two measurements are taken at each location. One that is perpendicular (ninety degrees) from the crankshaft centerline and the other that is parallel to the crankshaft centerline. The measurements are made with either a dial indicator or a telescopic gauge and micrometer.
Start measurements at the top of the cylinder bore, and proceed to the middle and finally at the bottom. A total of six measurements will be taken for each cylinder bore.
The difference between these measurements will indicate the cylinder wear. The measurements taken perpendicular (90 degrees) to the crankshaft centerline will typically reflect the most wear. This is where the greatest force is exerted on the cylinder as the piston is forced down during combustion. This is known as thrust wear. Take the top, 90 degree measurement and compare it to the bottom, 90 degree measurement. The difference between them is the cylinder taper. The amount of cylinder wall wear is always greater at the top of the cylinder than at the bottom. Any cylinder that has a taper of 0.0012 in. (0.305 mm) or more must be re-bored.
Compare the cylinder bore measurements to the size of the pistons to determine piston-to-wall clearance.
Inspect the crankshaft for visible signs of wear or damage. All of the journals (piston and main bearing) should be perfectly round and smooth. Slight scores are normal for a used crankshaft, but should hardly be felt with a fingernail. Make the crankshaft measurements with a micrometer at the front and rear of each journal (x). Turn the micrometer 90 degrees and take two more readings (y), front and rear. The difference between the x and y reading is the out-of-round measurement. Generally, there should be no taper or out-of-roundness found. A difference up to 0.0005 in. (0.0127mm) for taper or out-of roundness is acceptable. All measurements should be within the factory specifications for journal diameters.
If the crankshaft journals are within specifications, it is recommended that they be polished before being reusing the crankshaft. Polishing the crankshaft ensures that any minor burrs or scoring are smoothed, thereby reducing the chance of scoring the new crankshaft bearings.
The piston should be visually inspected for any signs of cracking or burning (caused by hot spots or detonation), scuffing or excessive wear on the skirts. The piston should move freely on the wrist pin, It should slide and pivot freely. Grasp the connecting rod securely, or mount it in a vise, and try to rock the piston back and forth along the centerline of the wrist pin. There should not be any excessive play evident between the piston and the wrist pin. If C-clips retain the pin in the piston then the piston may be equipped with wrist pin bushings in the connecting rod. There should not be any excessive play between the wrist pin and the rod bushing. Normal wrist pin clearance is approx. 0.001 - 0.002 in. (0.025-0.051 mm). Check the manufacturer’s specifications for the exact values.
| Measuring the piston’s outer diameter
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Use a micrometer and measure the outside diameter of the piston perpendicular to the wrist pin at the skirt. Compare this measurement to the cylinder measurement obtained earlier. The difference between the two readings is the piston-to-wall clearance. If the clearance is within specifications, the piston may be used as is. If the piston is out of specification and the bore is not, you will need a new piston. If both the piston and the bore are out of specification, you will need the cylinder re-bored and an oversized piston. Generally, if two or more pistons or bores are out of specification, it is best to re-bore the entire block and install oversize pistons. If the pistons are worn, but the cylinders are acceptable, one alternative is to use +0.005 inch oversize pistons and hone the cylinders to the correct size.
Check the connecting rod for straightness. If the connecting rod is bent, it will unevenly wear the bearing and piston, as well as place greater stress on these components. Any connecting rods that are bent or twisted must be replaced. If the rods are straight and the wrist pin clearance is within specifications, then only the bearing end of the rod need be checked. Place the connecting rod into a vice, with the bearing inserts in place, install the cap to the rod and torque the fasteners to specifications. Use a telescoping gauge and carefully measure the inside diameter of the bearings. Compare this reading to the rods original crankshaft journal diameter measurement. The difference is the oil clearance. If the oil clearance is not within specifications, install new bearings in the rod and take another measurement. If the clearance is still out of specifications, and the crankshaft is not, the rod will need to be reconditioned or replaced.
NOTE: You can also use Plastigage® to check the bearing clearances.
All of the engine bearings including full circle ones used on most camshafts, auxiliary shafts and balance shafts should be visually inspected for wear and/or damage. The bearing should look evenly worn all around with no deep scores or pits. If the bearing is severely worn, scored, pitted or heat blued, then the bearing, and the components that use it, should be inspected.
NOTE: The oil pump is responsible for providing constant lubrication to the entire engine. Always replace the oil pump when rebuilding the engine.
