Rapid piston and cylinder wear may be due to too much or too little clearance, to a bent connecting rod, to oil not being delivered to the cylinder wall,to a piston pin fitted too tight,to too much vertical clearance of piston ring, to a broken piston ring,to a collapsed piston skirt, or to a cylinder-head gasket protruding over the cylinder bore and inter fering with piston travel. Piston Rings. The two reasons for replacing piston rings are excessive oil consumption and loss of compression. Either one of these difficulties can be the result of
several other engine troubles. To be certain that piston rings are responsible, the following test should be made: Run the engine until it reaches normal operating tem perature; stop
it; take all the spark plugs out and open the throttle wide insert a compression tester in the spark plug holes, one at a time, while turning the engine over with the starter. The reading
on the compression tester should be within 20 pounds per square inch ( lb psi ) of the specifications for that engine. If the reading is more than 20 lb psi below specifications, it
shows that either the rings, cylinder-head gasket or valves are the cause of a corn pression leak. ( A low reading between two adjacent cylinders generally indicates a faulty
cylinder-head gasket.) To prove that faulty rings are the cause, a little heavy oil should be poured dyer each piston and the compression test made again. If the test shows up to
specifications, the rings are to blame,because the oil acted as a seal for the' rings. If the addition of oil through the spark-plug holes does not increase the pressure, the valves are
leaking .Since installation of new pistons has been discussed previously, only the installation of the rings will be considered here. Piston-ring manufacturers differ somewhat as to
just how much taper or out-of-round a cylinder may have to have their rings function properly. Depending on the design of oil and compression rings, recommendations are that oil pumping and comperes sion can be taken care of by new rings in cylinders ranging in taper from 0.005 to 0.020 in. and out-of-round by 0.003 in. However, some ring manu
facturers state that new pistons should be installed after the cylinders are re-bored only when cylinder taper exceeds 0.005 in.
affects the steel expander. Some replacement oil-control rings are of the multiple-piece, steel-segment type which consists of several ring sections in one groove, some of which are made of steel. Before pistons are removed for ring replacement, the ridge formed on the top of the cylinder must be removed to prevent damage to the piston lands as the piston is pushed out of the cylinder, and when it is replaced. This ridge marks the portion of the cylinder that is not worn by ring travel. It is re moved by a specially designed ridge cutter which, when properly used, does not cut into the ring-travel area.
Piston rings may be obtained in sets for the particular engine being worked on They come ready to install to fit cylinders ranging in taper from 0.000 to 0.020 in. for the standard size pistons. Other sets may be obtained in sizes UM to 0.020 in. and 0.020 to 0.030 in. to fit oversize cylinders.
Before rings are put on the piston, they should be checked for correct end-gap clearance by placing them in the cylinder in which they are to be used, and measur ing the gap with a feeler gage. This usu ally is done in the part of the cylinder having the smallest diameter.Correct end-gap clearance is important to prevent the ends from butting when
the rings are heated in operation. The rings are also fitted in the piston grooves to in sure freedom to expand, and to prevent too much up-and-down motion of the ring.
This side clearance is also measured with a feeler gage. See Fig. 5-9. When the rings are placed on the cleaned piston grooves and oil holes free of carbon and gum, the Fig. 5-8. Measuring ring end-gap piston is installed in the cylinder with the clearance,aid of a ring compressor which compresses the rings sufficiently to allow the piston to be pushed into the cylinder without breaking the rings.
several other engine troubles. To be certain that piston rings are responsible, the following test should be made: Run the engine until it reaches normal operating tem perature; stop it; take all the spark plugs out and open the throttle wide insert a compression tester in the spark plug holes, one at a time, while turning the engine over with the starter. The reading
on the compression tester should be within 20 pounds per square inch ( lb psi ) of the specifications for that engine. If the reading is more than 20 lb psi below specifications, it
shows that either the rings, cylinder-head gasket or valves are the cause of a corn pression leak. ( A low reading between two adjacent cylinders generally indicates a faulty
cylinder-head gasket.) To prove that faulty rings are the cause, a little heavy oil should be poured dyer each piston and the compression test made again. If the test shows up to
specifications, the rings are to blame,because the oil acted as a seal for the' rings. If the addition of oil through the spark-plug holes does not increase the pressure, the valves are
leaking .Since installation of new pistons has been discussed previously, only the installation of the rings will be considered here. Piston-ring manufacturers differ somewhat as to
just how much taper or out-of-round a cylinder may have to have their rings function properly. Depending on the design of oil and compression rings, recommendations are that oil pumping and comperes sion can be taken care of by new rings in cylinders ranging in taper from 0.005 to 0.020 in. and out-of-round by 0.003 in. However, some ring manu
facturers state that new pistons should be installed after the cylinders are re-bored only when cylinder taper exceeds 0.005 in.
Replacement Rings. Replacement rings differ In design from those used in new and re-bored cylinders in that they are more flexible and exert greater pressure against the cylinder walls. This permits them to follow tapered and out-of-round cylinder walls more completely than conventional "snap" rings which have only sufficient built-in tension to follow very accurate cylinder walls. Replacement rings generally use steel springs called expanders behind the cast-iron rings to provide the increased flexibility and pressure. One exception, however, is the top compression ring where the high temperature
affects the steel expander. Some replacement oil-control rings are of the multiple-piece, steel-segment type which consists of several ring sections in one groove, some of which are made of steel. Before pistons are removed for ring replacement, the ridge formed on the top of the cylinder must be removed to prevent damage to the piston lands as the piston is pushed out of the cylinder, and when it is replaced. This ridge marks the portion of the cylinder that is not worn by ring travel. It is re moved by a specially designed ridge cutter which, when properly used, does not cut into the ring-travel area.
Piston rings may be obtained in sets for the particular engine being worked on They come ready to install to fit cylinders ranging in taper from 0.000 to 0.020 in. for the standard size pistons. Other sets may be obtained in sizes UM to 0.020 in. and 0.020 to 0.030 in. to fit oversize cylinders.
Before rings are put on the piston, they should be checked for correct end-gap clearance by placing them in the cylinder in which they are to be used, and measur ing the gap with a feeler gage. This usu ally is done in the part of the cylinder having the smallest diameter.Correct end-gap clearance is important to prevent the ends from butting when
the rings are heated in operation. The rings are also fitted in the piston grooves to in sure freedom to expand, and to prevent too much up-and-down motion of the ring.
This side clearance is also measured with a feeler gage. See Fig. 5-9. When the rings are placed on the cleaned piston grooves and oil holes free of carbon and gum, the Fig. 5-8. Measuring ring end-gap piston is installed in the cylinder with the clearance,aid of a ring compressor which compresses the rings sufficiently to allow the piston to be pushed into the cylinder without breaking the rings.
