See if the protective surface is intact. Most parts are factory coated with a protective layer. Such as piston pin, bearing bush with paraffin protection, piston ring. The surface of the cylinder liner is coated with anti-rust oil and wrapped with wrapping paper. The valve, piston, etc. are immersed in anti-rust oil and then packaged in plastic bags. If the seal is damaged when you purchase it. The wrapping paper is missing. Anti-rust oil or paraffin is lost. Should be returned. See if the glued parts are loose. Parts made up of two or more parts. Parts are press-fitted, glued or welded, and no looseness is allowed between them. For example, the oil pump plunger and the adjusting arm are combined by press fitting, the clutch driven and the steel sheet are riveted, the friction plate and the steel sheet are riveted or glued, and the paper filter element skeleton and the filter paper are glued. Made of. The wire ends of electrical equipment are welded. If looseness is found during purchase, it should be exchanged. See if the rotating parts are flexible When you purchase the rotating parts assembly such as the oil pump, turn the pump shaft by hand and feel flexible and free from sticking. When purchasing a rolling bearing, one hand supports the inner ring of the bearing, and the other hand rotates the outer ring, and the outer ring should be able to rotate quickly and freely. Then gradually stop. If the rotating parts do not rotate, it indicates internal corrosion or deformation, so do not buy. See if there are any missing parts in the assembly. Regular assembly parts must be complete to ensure smooth assembly and normal operation. If some small parts on some assembly parts are missing, the assembly parts will not work or even be scrapped. See if the assembly marks are clear. To ensure that the assembly relationship of the fittings meets the technical requirements, the assembly marks are engraved on the surface of some parts (such as timing gears). If there is no mark or the mark is illegible, it will bring great difficulties to the assembly, even if it is wrong. Look at the surface of the mating surface. If there is any wear mark on the surface of the mating part, or if the painted part is peeled off, the old paint is found. Most of them are disguised as scraps, and can be returned at this time. See if the surface hardness is up to standard. The surface hardness of each fitting has the specified requirements. After confirming the purchase and negotiating with the merchant, the steel saw blade can be used for trial shooting. Scratch and scratch without scratches, indicating high hardness, slightly higher hardness after scratching, and obvious marks after the stroke. Indicates that the hardness is low (note that the test surface should not be damaged when the test is performed).