Why There is a Gap in the Piston Ring?

The most important part of the piston is the Piston ring. Piston rings are fitted into the grooves of the piston to maintain a good seal between the piston and the cylinder wall. Piston rings control the flow of oil to the skirt and ring themselves in adequate quantity while preventing an excessive amount from reaching the combustion chamber with consequence waste and carbonization. 

Piston ring gap

Piston ring gap
Piston ring gap

Piston rings have gaps so that they may be installed into the piston grooves and removed when worn out by expanding them. The gap ensures radial pressure against the cylinder wall thus having an effective seal to prevent leakage of heavy combustion pressure. This gap must be checked because if it is too great due to

cylinder bore wear, the radial pressure will be reduced. To check this gap clean the carbon from the ends of the ring and then check it with feeler gauges. This gap may be in the region of 0.178 — 0.50 mm governed by the diameter of the bore but if it exceeds 1 mm per 100 mm of bore diameter, new rings must be fitted.

The gap between the ring and the groove in the piston should also be checked by feeler gauges. This gap is usually 0.038 —0.102 mm for compression rings and a little less for the oil control rings. Wear in the piston ring grooves causes the rings to rise and fall during the movement of the piston, causing a pumping action and resulting in heavy oil consumption. Excessive gas blow by, and loss of compression will also take place if this gap is too much.

During service, the piston ring may have lost some of its elastic properties due to which radial pressure will be reduced on the cylinder wall. This property can be checked by pressing together a worn and a new ring, as in Fig, and observing whether the gap of the worn ring closes more than the new ring.

Removing and fitting of the rings

The piston rings should be carefully removed from the piston either with a special removal and installation tool or with three brass strips. The tool expands the ring so that it may be easily removed from the piston. In the latter case, three strips are spaced around the circumference of the piston and the rings are fitted over them. The strips can be withdrawn and the ring is dropped into the groove. A ring compressor is used to compress the rings for inserting them into the cylinder while assembling the piston and cylinder. The gaps of the rings should not be in line, but they should be staggered so as to prevent the compression from having a direct path to leak by the piston skirt.

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