The piston consists of the lowest part of the combustion chamber. In order for the pis-ton to move, there needs to be clearance between the piston and the cylinder wall. The construction is designed to maintain the proper clearance when the piston expands due to high temperatures during combustion. As the piston boss part is thicker, it is easily affected by heat expansion. Therefore it is made to be slightly oval than the direction of right angle for the piston pin (B) so that the diameter in the direction of the piston pin (A) expands to make a circle during heat expansion. The piston head is exposed to high temperature during combustion, and it is not directly cooled by coolant and air. For that reason, the piston head reaches greater temperatures than the piston skirt. Taking into account the heat expansion during combustion, the piston head is slightly tapered when compared to the diameter of the piston skirt.
HINT: For the piston diameter, measure the area specified in the Repair Manual. Note that the location for measurement is not the maximum diameter. For that reason, keep in mind that the standard oil clearance of the piston in the Repair Manual is not the actual gap between the cylinder and the piston.
2. Thrust force
When the pressure during compression or combustion acts on the piston, a part of that force acts on the piston skirt and causes it to push against the cylinder wall. This is called thrust force. Thrust force is divided into two typesmajor thrust force and minor thrust force. The former occurs during the combustion stroke and the later occurs during the compression stroke.
3. Piston slap (Side knocking)
Piston slap is the noise that occurs when the piston hits against the cylinder wall. This is also called side knocking. Piston slap occurs when the direction of the thrust force changes from the compression to combustion strokes. Piston slap is affected by the amount of piston clearance. When the piston clearance is large, there is a greater piston slap. In some engines, the centerline of the piston and the centerline of the piston pin are slightly offset in order to reduce piston slap.
4. Operation of offset pistons
In engines with offset pistons, the direction of thrust of the piston changes from the minor thrust direction to the major thrust direction near the end of the compression stroke. In this way, the piston slap is reduced because the piston’s thrust direction changes before the piston receives combustion pressure.
5. Piston size
When the piston and cylinder wear beyond the allowable limit, it is necessary to replace the cylinder block or piston, or to rebore the cylinder block or cylinder liner and use oversize pistons. The piston and piston pin are usually provided as a piston assembly.
The standard size, which indicates the piston diameter, is inscribed.
The piston must be installed facing the correct direction.
The direction is inscribed on the piston head.
The side with the mark represents the front side.
The exact location of the inscription differs according to the engine model.
(1) Standard size piston
When the engine is assembled, each standard size piston is set for each cylinder bore size to achieve piston clearance precision. As the number gets larger, the piston diameter increases incrementally by 0.01 mm (0.0004 in.) units. Recently, engines have only one cylinder bore size and one piston size.
(2) Oversize piston
The size of an oversize piston is determined by the extent of wear on the cylinder. Normally, only the 0.5oversize piston is available as supply parts. The 0.5value in the oversize piston means that it is 0.5mm (0.02in.) larger than the standard size piston. Besides the oversize piston written above, there are engines that require supply parts for 0.75 and 1.0sizes. For some engines, oversize pistons are not available as supply parts.
Striations on the Piston Skirt
The piston skirt has been striation finished to improve the lubrication properties. The striations on the piston skirt are not a poor finishing. In some engines, the striated piston skirt has been coated with a composite resin to reduce friction.