Boring; Honing; Piston And Connecting Rod Assemblies - Chevrolet 1991 Light Duty Truck Repair Manual

BORING

1. Before the honing or reboring operation is started,
measure all new pistons with the micrometer con­
tacting at points exactly 90 degrees from the pis­
ton 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 free.
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
rebored cylinder wall not being at right angles to
the crankshaft.
3. The instructions furnished by the manufacturer of
the equipment being used should be carefully fol­
lowed.
4. When reboring cylinders, all crankshaft bearing
caps must be in place and tightened to the proper
torque to avoid distortion of bores in the final
assembly. Always be 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
0.03 mm (0.001-inch) 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 that the speci­
fied clearance between pistons, rings, and cylinder
bores is maintained.)
m Important
• Refer to the proper section for additional informa­
tion.

HONING

1. When the cylinders are to be honed, follow the
hone manufacturer's recommendations for the use
of the hone and 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 clear­
ance.
2. Occasionally during the honing operation, the cyl­
inder bore should be thoroughly cleaned and the
piston selected for the individual cylinder check 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.
GENERAL ENGINE MECHANICAL 6A 5
NOTICE: Handle the pistons with care and do
not attempt to force them through the cylin­
der 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 suffi­
cient speed to obtain very fine uniform surface fin­
ish marks in a cross-hatch pattern of the specified
angle.
• 2.5L: 45 degrees
• 2.8L: 20 to 32 degrees
• All others: 45 to 65 degrees
5. The finish marks should be clean but not sharp,
free from imbedded particles and tom or folded
metal.
6. By measuring the piston to be installed at the siz­
ing 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 both the block and
piston be measured at normal room temperature.
7. It is of the greatest importance that refinished cyl­
inder bores are trued up to have the less than the
specified out-of-round or 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 for
piston to bore clearance tolerances.
9. After final honing and before the piston is checked
for fit, clean the bores with hot water and deter­
gent. Scrub with a stiff bristle brush and rinse thor­
oughly with hot water. It is essential that a good
cleaning operation be performed. If any of the
abrasive material is allowed to remain in the cylin­
der bores, it will wear the new rings and cylinder
bores in addition to the bearings lubricated by the
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 to
which it has been fitted.
11. Apply clean engine oil to each bore to prevent
rusting.
PISTON AND CONNECTING ROD
ASSEMBLIES
DISASSEMBLY (ALL EXCEPT 6.2L ENGINES)
|*y»| Disassemble (Figures 4, 5 and 6)
Tools Required:
J 24086-B Piston Pin Remover and Installer
1. Piston rings. In most cases the rings should be
discarded and replaced with new ones at assem­
bly.
2. Connecting rod bearing inserts. If the inserts are to
be reused, place them in a rack so they may be
reinstalled in their original connecting rod and cap.
3. Piston pin (figure 6).
• Place the piston/connecting rod on support fix­
ture J 24086-20. Make sure the connecting rod
is fully supported. Use J 24086-900 and
J 24086-280 for 2.8L pistons.