GMC T Series 1994 Repair Manual page 530

Figure 33— Checking the Cylinder Bore
• If the engine is at high hours or miles, and cylinder
bore wear is past specifications in all or most cylin­
ders, then consider reworking all bores to the ser­
vice oversize.
• If the engine is at relatively low hours or miles and
the bore distress is limited to one or two bores,
then the distressed bores are the only ones that
need to be reworked.
• Fine vertical scratches will not by them selves
cause excessive oil consumption. Therefore honing
to remove them is unnecessary.
• Note if the cylinder block has the letters J or K
stamped on the pan rails. These letters refer to the
cylinder above each letter, and identify what grade
size piston goes into that bore during the produc­
tion process to ensure precise piston to bore clear­
ance.
• If a significant area of the bore is polished through
the hone marks, but otherwise serviceable, break
the polished surface lightly with a finish hone and
install new rings. Refer to " Finish Honing."
• The pistons should be inspected for cracks, scuff­
ing, heavy or deep scratching, ring groove damage,
or other signs of excessive wear.
• Since different grade size pistons may be used in
the same engine, it is important to mark pistons in
reference to their bores.
• Use the smallest possible oversize service pistons
when reconditioning.
• All service oversize pistons are of the same weight
as the production pistons, therefore engine balance
is not affected by using service oversized pistons.
• If honing is not required, prepare the cylinder bores
for reuse by cleaning with a hot water and deter­
gent wash. Apply clean engine oil to the bore after
cleaning.
• If boring and honing is required Refer to " Boring"
or " Honing."
The following is general information on the subject of
boring and honing. A detailed description of types of
equipment, speeds, feeds, grid sizes of honing stones,
cycle times, etc. is beyond the scope of this manual.
BORING
• The boring operation is done with a single point
tool that leaves tool marks similar to those found
on a phonograph record. The speed and feed of
the boring tool should be controlled to minimize the
depth of these tool marks.
• In general, the boring operation imparts the founda­
tion for the proper bore diameter, taper and round­
ness and the rough honing operation removes the
boring tool marks. Refer to " Rough Honing."
• Completely disassemble and clean the block prior
to boring.
• Refer to " Service Piston Specifications" for selec­
tion of service replacement pistons and corre­
sponding bore diameters.
• The pistons to be installed should be on hand and
marked as to bore locations. Install reused pistons
in their original bore locations. If the bore has not
been re-honed, then the original rings with low
hours or miles may be reinstalled on the original
pistons in the original locations. If the bores are
re-honed all new ring sets should be used.
• Before using any type of boring bar, the top of the
cylinder block should be " stoned" using a flat stone
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
a right angle to the crankshaft.
• Carefully follow the instructions furnished by the
manufacturer of the equipment being used.
• When reboring cylinders, all crankshaft bearing
caps must be in place and tightened to the proper
torque specifications to avoid distortion of bores in
the final assembly.
• When taking the final cut with a boring bar, leave
approximately 0.13 mm (0.0050 inches) on the
diameter for rough and finishing honing.
ROUGH HONING
• Honing stones are most often spring loaded and
therefore tend to introduce local out of specification
diameter, taper and roundness conditions quickly.
Therefore it is important to minimize the amount of
time required for the rough honing operation.
•T h e hone travel should allow portions of the hone
to leave the top and the bottom of the bore. This
will ensure that a barrel shape bore will not be
formed.
• When honing the cylinders, follow the hone manu­
facturer's recommendations for use, cleaning and
lubrication during honing.
• Use only clean, sharp and straight stones of the
proper grade for the surface texture to be imparted.
Dull, dirty and worn stones cut unevenly and gen­
erate heat
• When taking the final pass with the rough hone,
leave approximately 0.005 mm (0.0002 inches) on
the diameter for finish honing.
FINISH HONING
• The purpose of the finish hone is to remove just
the " peaks" left by the rough hone.
• The removal of these peaks leaves flats or pla­
teaus on the surface of the bore. This " plateau"