Page 276 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair of Reciprocating Compressors 261

Oversize Piston Rings

How much honing or reboring can be done before oversize rings are
needed? This is determined by considering piston ring gap. When honing
or reboring a cylinder, oversize rings become advisable when the ring
gap becomes three times normal due to the bore increase.
The ring gap will be increased 3.1416 times the amount of material
removed from the cylinder bore.
Example:
Cast iron ring gap .003 inches per inch of diameter. Hence, a 10-inch
cylinder would have a normal piston ring gap of 0.030 inches.
Since 3 x 30 = .090" or .090 - .030 = .060", an increase of .060"
would make the gap three times normal.
Therefore, .060 divided by 3.1416 = .019" would represent the maxi-
mum allowable increase in cylinder bore size, without resorting to over-
sized rings.
Table 4-2 can be used as a guide for the use of oversize rings, although
the example above is more conservative.

TABLE 4-2
GUIDE FOR OVERSIZE RINGS

Max. Bore Diameter Increase
Nominal Bore Allowed Before Oversize
Diameter, In. Piston Rings are Required, In.
3.99 .015
4.000 - 5.999 .015
6.000 - 7.999 .020
8.000 - 9.999 .030
10.000-14.999 .040
15.000-17.999 .050
18.000-22.499 .065
22.500 - 29.999 .085
30.000 - 34.999 .100
35.000 - 42.000 .120