PISTON RINGS AT WORK

Press, Volume CIV, Issue 30738, 30 April 1965, Page 9

PISTON RINGS AT WORK

ONE of the hardest-worked parts of the engine is the piston ring. It is also a vital component, for without piston rings an engine would be of very little use. Just what does a piston ring have to do?

The piston is really just a plug which moves up and down the cylinder. The rings provide a seal between the cylinder wall and the piston, and prevent gas and pressure from leaking one way, and oil from leaking the other way, into the combustion chambers.

While a piston ring looks simple, it is in fact a precision component. The ring is held by grooves in the piston, and each piston has two types of ring: compression rings and oil control rings. The former seal the gas in the combustion chamber, and the latter scrape oil off the cylinder walls and push it down into the sump. Each ring has a joint, which

permits it to be expanded enough to slip over the piston, and also to move a little in use. When the rings are in the engine, these joints are nearly closed, and the springiness of the ring keeps it pressed against the cylinder wall. The compression rings are made of cast iron, and consequently they must be handled with care when being installed or they may break. They are often coated with a special material when new, to assist them in run-ning-in against the cylinder wall. The compression rings can move a little in their grooves. When the piston travels downwards under the pressure of the burning fuel, gas fills the groove and pushes the ring against the cylinder wall, giving a good seal. The oil control rings, when the piston moves downwards, scrape oil and carbon off the cylinder walls. They are usually slotted, so the oil can pass through them and escape back to the sump. Carbon and dirt goes with it, and is filtered out by the oil filter.

There are special rings for worn engines; they are designed to handle large amounts of oil, and prevent

it from reaching the combustion chambers.

When the piston moves upwards, the oil control' rings are designed to move slightly in their grooves, and slide easily over the walls of the cylinder. This prevents them from pushing oil up towards the chamber. HIGH SPEED

Engines may use three times as much oil at high speed as at low speed. This is because of the rings, but is not necessarily a sign that new rings are needed. What happens is that the oil gets hotter, and therefore thinner. It is harder for the rings to scrape thinner oil from the cylinder walls. Added to this, the piston is travelling faster, and the ring has less time to try to scrape off the oil. Also more oil is thrown on to the cylinder

walls at high speed. As rings have to withstand movement in their grooves, high temperatures, and considerable friction, it is not surprising that they wear out. If the cylinder walls are in good condition, special rings can be installed instead of reboring the engine. If the bore is slightly worn, the rings must often change their shape to conform to it as they move up and down. Consequently replacement rings are often very flexible, and are backed by a spring which makes sure they exert enough pressure on the cylinder wall. FAULTS Faulty rings can cause noise: a clicking or rattling. This can be caused by ring tension that is too low, a broken ring, or rings that are very worn. A broken ring must be replaced immediately, because it can badly damage the cylinder wall. Rings are sometimes broken when they are installed carelessly. To test for a faulty piston ring, put about an ounce of engine oil in each cylinder through the spark plug hole. Crank the engine to work the oil to the rings, then start the motor. If the noise has been reduced, it is very likely that the piston rings are at fault. Treating each cylinder in turn may reveal if there is a broken or damaged ring in one particular cylinder. A broken ring has a different sound from piston slap, which usually sounds like a muffled bell, and is usually worst when the engine is cold.