Fitting Rings To Pistons

Fitting Rings To Pistons

Fitting rings by hand

Fitting rings with pliers


Fitting rings to pistons…… the most common mistake made when fitting rings to pistons is to fit the compression rings upside down. The standard rule I learnt as an apprentice was “inner chamfer upward, outer chamfer down, unless otherwise marked.” On most occasions this rule will be correct. Almost all rings that the chamfer rule can’t be applied to will have a mark that goes upwards. Always read the manufacturers instructions if available.

As an assembler it is wise to learn what type of ring designs are available and what materials they are made out of. This information can assist you to choose the most appropriate ring for the engine you are assembling. As an example, a top compression ring that has an outer chamfer or bevel and is marked to have the bevel upwards is very obviously used in re-rings and would not be the best choice for a reconditioned engine. (called a “ridge dodger ring”). Like wise a chromium plated top compression ring would be a poor choice for a re-ring as it may never bed in.

Fitting the oil ring: There are two basic types of oil rings, cast iron and segmented. The cast iron oil ring comes in various designs. Some cast iron oil rings rely on a small coil spring slotted into the rear of the ring to hold tension against the cylinder wall. Segmented oil rings come in two basic designs. The three piece, two rails and an expander being the most common. The other is called a unitised oil ring. This style has two rails separated by a spacer and uses a sinewave expander. Fitting the three piece segmented oil ring is as simple as placing the expander in the groove, ensuring the ends do not overlap (butt together only). Now wind in the upper rail leaving the gap 90 degrees from the expander join. Next, wind in the lower rail leaving the ends 180 degrees from the upper rail ends. Recheck that the expander has not overlapped. Take your thumb and third fingers on opposite sides of the assembled oil ring and move the ring around in the groove as a unit. The correct feel is free, but not loose or sticky.

Fitting the compression rings: Compression rings can not be wound into the grooves. Even if they don’t break this will damage them. Compression rings should be opened at the ends just enough to allow them to be placed over the piston into the grooves. The gaps should be placed opposite, but in service they may move and even line up. This does not cause any concern.(contrary to the popular myth) The ends can be opened with your thumbs, while supporting the rings with your fingers. Lower the compression ring into its groove. It is easier to use a ring fitting tool. Always recheck the correct location and that the rings are the right way up after fitting. A 30 second inspection will ensure no mistakes are left unrectified.

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