A 125 span car said that the last time the car was repaired in other shops, the cylinder head exhaust port M7 thread was damaged due to improper disassembly by the repairman, which could not be repaired, and the cylinder head assembly had to be replaced, but after the replacement, the motorcycle exhaust pipe was emitting blue smoke.

The owner of the car is experienced enough to know that this is burning oil, so look for the repair shop to check and repair, but the maintenance staff according to experience, has replaced the piston, piston ring and cylinder, still no effect, the cause of the failure is unknown.

I picked up the car to try, as the owner said, by the way to look at the odometer, has run more than 50,000 kilometers. Experienced masters and owners know that under normal circumstances, blue smoke exhaust phenomenon, must be the engine oil channeling! According to my years of experience in repairing cars, from a technical point of view, there are usually several ways for engine oil channeling: first, there is leakage at the piston, ring and cylinder seal; The second is that the crankcase is filled with too much oil (more than 20% of the prescribed oil); Third, the gap between the valve and the conduit is too large or the oil seal of the conduit fails. The first two ways are easy to find, but the third way is ignored by many people, because the fault point is more hidden, so it is easy to be overlooked. It is difficult to judge the failure of the valve guide and the valve oil seal on the vehicle. The cylinder head is decomposed, as shown in the above figure, the valve oil seal is removed, and the actual clearance of the valve rod in the catheter hole is carefully checked. The concentricity between the outer circle of the oil seal installed on the intake valve guide and the catheter hole is carefully checked, as shown in the following figure.

The deviation between the two is about 0.20mm (concentricity requirement is ≤0.025mm), resulting in partial wear between the oil seal and the valve rod after the valve guide is installed, which will cause unilateral wear at the main lip of the oil seal soon after use, until serious wear, and eventually lead to the oil splashing in the crankcase being sucked into the inlet and entering the combustion chamber causing blue smoke failure.

Replace the cylinder head assembly, carefully reinstall the removed parts, start the motorcycle, the blue smoke phenomenon of the engine disappears, and confirm that the fault is completely eliminated.

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