market dynamics
Motorcycle engine impact abnormal sound replacement assembly is invalid, the original is the omission of peripheral accessories related inspection
Release date:2021.09.02

A CL125-3 Chunlan Tiger cross-riding motorcycle traveled more than 50,000 kilometers, due to the use of too long, a long-distance riding process, abnormal impact sound in the crankcase, disassembly inspection found that the crankshaft left and right cylinder crank shaft connection slip, two cylinder crank shaft is not in the 180 degree range, but had to replace the crankshaft components.

After replacing the crankshaft, after a short run-in, the main performance of the vehicle such as start and acceleration is basically normal, but there is abnormal sound in the right crankcase cover. The maintenance personnel opened the right cover, checked the meshing part of the clutch big gear and the high-speed gear, confirmed there was abnormal friction trace, and replaced the clutch big gear and high-speed gear respectively, but the fault sound remained unchanged.

After receiving the feedback from the maintenance station and arriving at the scene, after brief communication and basic inspection of the faulty vehicle, it was confirmed that there was abnormal sound of the right cover gear of the vehicle.


Cl125-3 Chunlanhu motorcycle is a copy of Honda CB125T. In the crankshaft structure, as shown in the above two pictures, the right end face of the crankshaft is equipped with high-speed gear, and the part connected with the transmission is the clutch active hub big gear mounted on the right end face of the main shaft, as shown below, and the connection mode is the inner hole spline connection.

Due to the crankshaft itself processing or because of other reasons, the spline shaft on the right end face of the crankshaft produces radial runout, so that the high speed gear installed on it produces abnormal runout, which produces the movement abnormal sound when meshing with the clutch gear.

From the above analysis, it is not difficult to see that the general maintenance personnel often only know to replace the high-speed gear or the clutch active hub gear or the high-speed gear with it, while ignoring the inspection of radial runout of the spline shaft on the right end face of the crankshaft (the bottom right picture below).

Open the right cover under the cold condition of the engine and remove the lubricating oil in the crankcase. After being lubricant basic flow, with a clean cloth to the crankshaft to the right gear tooth surface wipe, magnetic table mountain and the dial indicator trying to suck on the exhaust pipe, take 2-2.5 mm in diameter of needle placed in high speed gear tooth surface, at the same time with black marker pen gear at high speed (244 fmi engine high-speed gear tooth) of 18 for a starting point on the top row.

Managed to make the dial indicator measuring head pressure needle tip is about 0.20 mm, take out the needle, use of edge size is 22 mm socket wrench slowly rotate the crankshaft to the right fastening nut about 40 °, also put the needle on the gear tooth surface measurement again (note that at this point without touching the magnetic table and dial indicator, lest affect the accuracy of the measurement).

In this way, one tooth is measured at an interval until all measurements are completed. The radial runout value of the gear is obtained by subtracting the maximum value from the minimum value and then dividing by two (the radial runout value of the high-speed gear is within 0.05mm, which is normal).

After several times of repeated measurement, the radial runout value of high-speed gear is above 0.30mm, it is deduced that the radial runout value of spline tooth on the right end face of crankshaft assembly is out of tolerance. Only by replacing the crankshaft assembly can abnormal sound be eliminated.

Through communication with the user consultation, to minimize the damage to the user, to replace a group of other fault engine (scrap) by the failures of the other components of the crankshaft, carefully after the disassembly parts, start the motorcycle and engine sound back to normal, slightly after the warm-up, the road, engine running voice soft, confirmation of gear noise in the original car right cover completely ruled out.