Chevrolet 70 1969 Series, Shop Manual

Page: 660 / 913

POWER BRAKE 5-51
Figure 10— Measuring Piston Diameter
check ring gap with ring in cylinder bore. Rings are
of cast-iron m aterial.
1. C learance between rings and sides of ring grooves
in 7 1/4 cubic-foot com pressor should be from
0.0015 inches to 0.0030 inches as hown in Figure
12. Ring gap should be from 0.002 inches to 0.010
inches.
2. C learance between rings and sides of ring grooves
in 12 cubic-foot com pressor should be from 0.0035
inches to 0.0055 inches for wide rings and from
0.002 inches to 0.004 inches for narrow rings as
shown in Figure 13. Ring gap should be from 0.002
inches to 0.010 inches.
CON N ECTIN G RODS AND BEARINGS
Check fit of connecting rod bearing inserts on cran k ­
shaft journals. C learance between bearings and crankshaft
journals should not exceed m ore than 0.0021 inches.
Checking Clearance by Plastigage Method
(Refer to Figure 14)
To check fit of bearings on crankshaft, install bearing
inserts into connecting rod. Clean the crankshaft journals
Figure 12— Piston Ring Arrangem ent a n d Clearances (7 1/4 cu. ft.)
Figure 11— Measuring Piston Ring Gap
and the inserts so that all oil is removed. Plastigage is sol­
uble in oil, therefore oil will affect the accuracy of the
reading. Plastigage is placed on the lower half of the
bearings (fig. 14).
IMPORTANT: To check fit clearance properly,
using Plastigage, it is essential to not move the
connecting rod or crankshaft. Install connecting
rod caps and tighten connecting bolt nuts to
torque. This should flatten Plastigage to not less
than 0.0003 inches or not more than 0.0021
inches. Check measurement on Plastigage scale
(fig. 14). Replace bearing inserts if clearance is
excessive, or if bearings are cracked or flaked.
Connecting rod caps are not interchangeable.
Position caps so that locking slots are both locat­
ed adjacent to same cap screw.
Figure 13— Piston Ring Arrangement and Clearances (12 cu. ft.)
CHEVROLET SERIES 70-80 HEAVY DUTY TRUCK SHO P M A N UA L