Page 296 - Reciprocating Compressors Operation Maintenance

Overhaul and Repair a/Reciprocating Compressors 281

cylinder diameter. Thus, a 10" cylinder bore requires piston rings to have
a gap of .020" when new. An increase of .040" would triple the ring gap
to 0.060", This increase in ring gap would be due to an increase in bore
size of .013" (.04'73.1416" = .013 in.) This three-fold increase of ring
gap also governs when worn rings should be replaced even though the
cylinder bore has not worn. Table 4-7 shows the maximum increase in
bore, size before oversize piston rings are required.

TABLE 4-7
MAXIMUM INCREASE IN BORE SIZE BEFORE OVERSIZE
PISTON RINGS ARE REQUIRED

Maximum Bore Diameter
Increase Allowed
Before Oversize
Nominal Bore Piston Rings
Diameter, In. 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

Oversize rings are required to overcome an increase in ring gap due to
an increase in bore dimension. Also, oversize rings are needed to over-
come the inability of the piston ring lands to properly support the rings
(see Figure 4-33). As the ring moves out to accommodate a larger bore
size, less of the ring side is in contact and supported by the groove wall.
The ring is thus susceptible to breakage.
Note: From a practical point of view, it is recommended that piston
rings and rider rings be replaced whenever the piston is removed, regard-
less of cylinder bore size or ring wear. These are inexpensive components