MEASURING THE PISTON

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

MEASURING THE PISTON DURING A REBUILD
PREPARING THE PISTON FOR MEASUREMENT
THE PISTON DIAMETER
MEASURING THE PISTON CLEARANCE
IMPORTANCE OF CHECKING THE PISTON CLEARANCE
METHODS FOR DETERMINING PISTON CLEARANCE
THE PISTON RING SIDE CLEARANCE
MEASURING THE WRISTPIN

MEASURING THE PISTON DURING A REBUILD

The piston taper: pistons are not perfectly round, they do not possess straight sides. Pistons are manufactured with a slight taper, entailing that the diameter at the lowest part of the skirt is larger than the diameter of the piston head. This taper is created because aluminum pistons expand as engine temperatures rise, and the piston head, exposed to the high temperatures of combustion, will expand more than the cooler skirt does. To compensate for the greater expansion of its head, the piston is constructed with a built in taper. As the engine operates at proper temperatures, the piston shape will become round.

The cam ground piston: the piston shape is designed to compensate for both heat expansion and physical forces it will experience as the engine operates. Exploding combustion chamber gases force a piston downward in the cylinder, against the cylinder walls. The way that a piston is connected to the crankshaft causes greater force to be directed toward its front than toward its back side. The side of the piston which bears the brunt of combustion pressure is called the thrust face.

Forces during the power stroke are greater at the thrust face than elsewhere on the piston sides. Therefore a piston is often made so that its diameter perpendicular to the wristpin is larger than its diameter measured parallel to the wristpin, giving the piston a very slight oval or egg shape.

If looked at from above, the wristpin would divide the piston egg into wide and narrow halves, with the thrust face at the wider half. Pistons with this design are often called cam ground pistons. Force on the power stroke will push the piston thrust face into the cylinder wall. Force applied this way makes the cam ground piston rounder during its power stroke than it is otherwise.

PREPARING THE PISTON FOR MEASUREMENT

Once the rings have been visually inspected, you can prepare the piston for measurement. First the rings must be removed. To remove a piston ring, you must spread it open enough so that it can slide from its groove and then from the piston. It is best to spread a ring, rather than by hand, with a special pair of pliers called a ring expander. A ring expander clamps onto the two ends of a ring, and as its handles are squeezed, widens the ring circumference. Ring expanders provide much better control during ring removal and installation, and helps prevent the brittle rings from accidentally breaking.

THE PISTON DIAMETER

Diameter measurements must fall within service manual specifications for a piston to be reusable. You can measure piston diameter with an outside micrometer. Where measurements should be taken will depend on the style of piston. Specific measurement procedures and diameter specifications should be listed in an engine service manual.

Compare the measurement to its specification: because the piston has a natural taper, and because a cam ground piston is not truly round, you can get different measurements at different locations on its surface. For this reason, it is important to comply with the engine service manual instructions regarding measurement procedures. Specifications for the very same piston can change based on which engine it has been installed in.

Often you will find a service manual specification such as thrust face O.D.(outside diameter) at location D1. A pictorial illustration would then explain where on the piston surface D1 was. Another specification might be maximum wear limit of thrust face O.D. at D1; this would indicate the minimum diameter allowable for the piston at location D1.

Record the measurement: to record information, you may want to devise a crude form. By recording the piston diameter, you will not have to remeasure later when calculating the clearance between the cylinder walls and piston. If the diameter of the piston falls outside of specifications, it should be replaced. If the diameter is within specifications, and there are no signs of damage, the piston can be reinstalled.

MEASURING THE PISTON CLEARANCE

As previously noted, the piston will expand as engine temperatures rise. Since the piston normally expands more than the metal of the cylinder wall, clearance between the two must be counted for when the engine is cold. This clearance measurement is called piston to bore clearance(or simply piston clearance).

IMPORTANCE OF CHECKING THE PISTON CLEARANCE

Piston clearance is obviously critical to engine operation. If it is too small, the piston will fit too tightly in its cylinder after the engine has heated up, resulting in excessive friction. If the clearance is very small, the piston can even seize in the bore. A piston stuck this way might sometimes be freed after an engine cools, however both it and the cylinder wall will probably have sustained significant damage.

If the piston clearance is too large, the piston will rock back and forth as the engine runs. This will create engine knock, and will eventually break the piston skirt. The ability of the rings to seal the combustion chamber will also be dramatically reduced.

In most engines, the proper piston clearance is roughly 0.002 inches for every inch of piston diameter. For most small engines, the proper clearance will fall between 0.003 and 0.005 inches. Check service manual specifications.

METHODS FOR DETERMINING PISTON CLEARANCE

The subtraction method: to determine clearance, you will need to measure the diameters of both piston and cylinder bore. Then the outside diameter of the piston can be subtracted from the inside diameter of the bore, the result of which is the piston clearance. Compare your calculated clearance to the engine service manual specifications. If the piston clearance falls outside of specifications, both the piston and cylinder must be resized.

Suppose that when you measure the cylinder bore, you find its diameter to be 2.3750 inches. First you would need to compare this number to the service manual specifications for acceptable cylinder bore dimensions. If the bore falls within specified parameters, you can move on to the next measurement.

Next measure the piston. If its measurements are also within the specified service manual range, piston clearance can be calculated. To calculate piston clearance, you would subtract the piston diameter from the bore diameter. Once piston clearance has been calculated, you must check it too against service manual specifications for piston to bore clearance.

Never assume that just because the piston diameter and the bore diameter are within specifications, that the piston clearance is also within specifications. This will not always prove true. If the piston diameter and bore diameter both fall within specifications, then piston clearance probably will too. If the piston diameter is at the low end of its acceptable range, and the bore diameter is at the high end of its own acceptable range, the difference between them can occasionally exceed specifications.

The feeler gauge method: the best way to determine piston clearance is by subtracting the piston diameter from the bore diameter. Some technicians prefer using two feeler gauges instead(though the results can be inaccurate for somebody inexperienced in the technique). First a piston without rings is slipped into the bore. Then a blade from each feeler gauge is inserted into the bore along either side of the piston. Different size blades are inserted until those which are found which produce a slight drag when pulled free. The measurement combined from the blades is your piston to bore clearance.

To make this feeler gauge measurement easier, most who practice it use longer than normal feeler gauges. Some technicians also prefer to insert the piston upside down from the top of the bore, a procedure which allows you to grip the piston sturdily by its wristpin holes to prevent it sliding to the bottom of the bore. It is usually more difficult to acquire accurate results using the feeler gauge method as it compares to the subtraction method, the reason most service manuals do not recommend it.

THE PISTON RING SIDE CLEARANCE

Once the piston ring grooves are cleaned, and the piston wiped off, side clearance for the rings can be checked. This dimension is the clearance between a piston ring and the upper edge(usually called the inside edge) of the groove in which it rests. Piston ring side clearance performs an important function. During the power stroke, as combustion pressure pushes the piston down in the bore, some of the combustion gases, expanding as they burn, are forced into the space between the piston and cylinder walls. These gases should be prevented from passing the piston and reaching the crankcase by the rings. Pressure from combustion will force a ring hard against the cylinder wall, helping to seal the combustion chamber during each power stroke.

Piston ring side clearance will allow a ring to tip slightly in its groove under normal operating conditions. As the piston goes down in the cylinder during its intake stroke, the oil control ring tips and scrapes excess oil from the cylinder walls. During the compression and exhaust strokes, as the piston rises, the oil control ring tips in its groove and glides over the film of oil on the cylinder walls with modest friction. And during the power stroke, combustion pressures against the ring or rings help seal the combustion chamber and enhance engine power.

If ring side clearance is too large, the ring will tip excessively, reducing its ability to seal. Excess movement of a piston ring may also cause it to break. If clearance is too small, the ring may bind in its groove when the piston heats up and expands.

To measure the piston ring side clearance, the ring does not need to be installed on the piston. It can simply be held in its groove, resting against the lower edge, and then a feeler gauge inserted between the ring and the upper edge of the groove.

Most piston side clearances will be between 0.004 and 0.007 inches. Refer to an engine service manual for precise specifications. Because each ring groove may be worn differently, you should always check for proper clearance in them all.

MEASURING THE WRISTPIN

The wristpin is used to link a piston to its connecting rod. The connecting rod bearing for the wristpin allows the rod end to rotate effortlessly around the wristpin as the piston moves. Manufacturers usually make wristpins out of sturdy, high quality steel. Therefore you will rarely find wear on them. To guarantee that a wristpin is not worn, you should measure its diameter with an outside micrometer, and compare those measurements with service manual specifications.