Boring 1 . Before the honing or reboring operation is started, measure all new pistons with the micrometer contacting at points exactly 90° from the piston pin centerline. Some pistons must be measured at a specified distance from the piston crown. Refer to the proper Section for additional instructions. Then select the smallest piston for the first fitting. The slight variation usually found between pistons in a set may provide for correction in case the first piston is fitted too loose. 2 . Before using any type of boring bar, the top of the cylinder block should be filed to remove any dirt or burrs. This is very important.
If not checked, the boring bar may be tilted which would result in the re-bored cylinder wall not being at right angles to the crankshaft. 3 . The instructions furnished by the manufacturer of the equipment used should be carefully followed. 4. When reboring cylinders, all crankshaft bearing caps must be in place and tightened to the proper torque to avoid distortion of the bores in the final assembly. Always make sure the crankshaft is out of the way of the boring cutter when boring each cylinder. Crankshaft bearings and other internal parts must be covered or taped to protect them during the boring or honing operation. 5 .
When taking the final cut with a boring bar, leave 0.025 mm (0.001 in.) on the diameter for finish honing to give the required position to the cylinder clearance specifications. (The honing or boring operation must be done carefully so the specified clearance between pistons, rings, and cylinder bores is maintained). Honing 1 . When honing the cylinders, follow the hone manufacturer's recommendations for use, cleaning, and lubrication during honing. Use only clean, sharp stones of the proper grade for the amount of material to be removed. Dull, dirty stones cut unevenly and generate excessive heat.
When using coarse or medium grade stones, use care to leave sufficient metal so that all stone marks may be removed with the fine stones used for finishing to provide proper clearance. 2. Occasionally, during the honing operation, the cylinder bore should be thoroughly cleaned and the piston selected for the individual cylinder checked for correct fit. 3 . When honing to eliminate taper in the cylinder, full strokes of the hone in the cylinder should be made in addition to checking measurement at the top, middle and bottom of the bore repeatedly. NOTICE:
Handle the pistons with care and do not attempt to force them through the cylinder until the cylinder has been honed to the correct size as the piston can be distorted through careless handling. 4 . When finish honing a cylinder bore to fit a piston, the hone should be moved up and down at a sufficient speed to obtain very fine uniform surface finish marks in a cross-hatch pattern at the specified angle of 45° to 65°. 5 . The finish marks should be clean but not sharp, free from imbedded particles and torn or folded metal. 6 .
By measuring the piston to be installed at the sizing point specified in the proper Section, and adding the average of the clearance specification, the finish hone cylinder measurement can be determined. It is important that the block and the piston be measured at room temperature. 7 . It is of the greatest importance that refinished cylinder bores are trued up to have the less than specified out-of-round taper. Each bore must be final honed to remove all stone or cutter marks and provide a smooth surface. 8 . Refer to “Specifications” in the proper Section piston to bore clearance tolerances. 9 .
After final honing and before the piston is checked for fit, clean the bores with hot water and detergent. Scrub with a stiff bristle brush and rinse thoroughly with hot water. It is essential that a good cleaning operation be performed. If any of abrasive material is allowed to remain in the cylinder bores, it will wear the new rings and cylinder bores in addition to the bearings lubricated by contaminated oil. After washing, the dry bore should then be brushed clean with a power-driven fiber brush. 10 . Permanently mark the piston for the cylinder which it has been fitted. G643 (E) Service Manual Chapter 3. Engine Mechanical System 75