G643 (E) Service Manual Chapter 3. Engine Mechanical System 75
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.
To Next Page
Loading...
