Hummer H1 Service Manual page 105

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2-68 Engine
MICROMETER
Figure 2-119: Measuring Piston Pin Diameter
Measure pin bushing diameter in connecting rod with small
bore gauge or inside micrometer (Figure 2-120). Record bush-
ing diameter and compare it to diameter of new or known good
piston pin. The difference between the two measurements rep-
resents the piston pin-to-connecting rod pin bushing clearance.
Required clearance is 0.0003-0.001 in. (0.0081-0.0309 mm).
Replace the connecting rod if the bushing is worn oversize.
The bushing is not serviced separately.
BORE GAUGE
(OR INSIDE
MICROMETER)
Figure 2-120: Checking Connecting Rod Pin
Bushing Diameter
Check connecting rod straightness in a rod fixture, or on a sur-
face plate or sheet of glass. The rod can be straightened if dis-
tortion is less than 0.002 in. (0.05 mm) end-to-end. Replace the
rod if severely bent, twisted, or cracked.
Measure piston skirt diameter at, or just below the pin bore
centerline. The correct gauge point is approximately 2.57 in.
(65.3 mm) down from the piston crown (Figure 2-121). Be
sure measurement is taken at a point opposite (at right angle to)
PISTON PIN
CONNECTING ROD
PIN BUSHING
the pin bore centerline as shown. Replace the pistons if wear is
such that piston-to-bore clearance would be greater than:
• 0.004 in. (0.120 mm) for cylinder 1-6 and
• 0.005 in. (0.133 mm) for cylinder 7 and 8
2.57 in. (65.3 mm)
Figure 2-121: Measuring Piston Skirt Diameter at
Gauge Point
Piston Selection and Fit
Production engines are equipped with different grade size pis-
tons. The different grades allow factory select fitting of pis-
tons. Correctly, three grades are used with the ID letters JT,
KT, and GT or J, K, G, or T. The different grades are to accom-
modate minor variances in bore size. The ID letters are on the
cylinder block pan rails adjacent to each cylinder bore, or the
piston itself. The grade sizes are described in the piston and
bore size chart at the end of this section.
Some general selection recommendations are:
• If an original piston is in good condition and meets all
specifications, it can be reused.
• If the cylinder bore is OK but the piston was damaged,
just use a new piston of the same grade size.
• If the piston grade size is not marked on the piston or
pan rail, and the original piston is not reusable, select a
piston that will provide the required piston-to-wall
clearance. It is only necessary to know the actual bore
size.
• The largest oversize piston available is 0.020 in. (0.50 mm).
• It is not necessary to rebore all the cylinders on a low
mileage engine. Only the damaged cylinder needs to be
repaired. This is permissible because factory service re-
placement and original pistons are all the same weight.
Balance is not affected. However, it is recommended
that all the pistons be replaced and the cyliners be re-
bored on high mileage engines.
®
PISTON SKIRT
MICROMETER