4. The size of the piston head of the regular manufacturer is slightly smaller than the size of the skirt
Regular manufacturers generally reduce the size of the piston head (the position above the first piston ring) by 0.25-0.35mm (for example: the diameter of the piston of a CB125T motorcycle is 44mm, while the piston head is 43.5mm), mainly for Prevent excessive thermal expansion of the piston head after the temperature of the combustion chamber rises, resulting in scuffing or swelling of the cylinder. Therefore, the size of the piston head above the first piston ring can be measured with a vernier caliper or a digital caliper. If there is no margin of 0.40-0.60mm, it is a substandard product.
5. The piston head produced by regular manufacturers is thicker
The piston head is generally made thicker, mainly to increase the strength and rigidity of the piston, and at the same time to reduce the thermal resistance of heat flowing from the top of the piston to the piston ring, so that the heat from the top of the piston passes through the piston ring and the cylinder wall to the heat sink or cooling. Liquid, which can effectively prevent the temperature of the top of the piston from being too high.
You may see in some engine failure pistons, there is a phenomenon of piston burning (there is an irregular burning hole on the top). After excluding the extremely low calorific value of the spark plug and the overheating factors of the engine, most of them are caused by the thin thickness of the top of the piston.
Generally, the thickness of the top of the authentic piston is between 3.0 and 4.0mm. The thickness of the top of defective or shoddy pistons is less than 2.5mm. As long as a straightedge is placed on the bottom of the piston, a vernier caliper (with a depth caliper) will measure the size from the top of the piston to the bottom of the piston, and subtract the size from the bottom of the piston to the back of the top of the piston, which is the thickness of the top of the piston.
It is more accurate to measure with digital calipers. If the measurement result is not between 3.0 and 4.0mm, and less than 2.5mm, it must not be installed. In addition, the top of some pistons is generally sandblasted. The main purpose of sandblasting is to eliminate the casting stress on the top of the piston and prevent the natural deformation of the piston during use.
