A Honda CB125T two-cylinder motorcycle traveled more than 52,000 kilometers as shown in Figure 1. Due to an accident, the right crankcase cover of the engine was damaged. Since there was no replacement of imported parts, the right crankcase cover of the domestic CB125T had to be replaced.

When replacing, add all the lubricating oil in the crankcase and add SG =SAE-15W/40 special lubricating oil for motorcycles. Start the motorcycle, warm up the car a little, and start the test run. The engine works normally and give it to the user for use. But less than 3 days, the user came to the maintenance station to report that the engine right cylinder overheating phenomenon, the cause of the failure is unknown.

According to the maintenance experience, a single cylinder overheating fault, should be abnormal wear of the cylinder, piston and ring, or replace the quality of the parts, or the lubrication road to the cylinder is blocked. First open the cylinder head housing and start the engine. Observe the oil coming from both ends of the camshaft on the cylinder head. There is no sign that the oil is low at the end of the camshaft liner on the right cylinder.

In desperation, only the decomposition engine, remove the cylinder body, check the right cylinder and piston, there is abnormal wear phenomenon, replacement has worn piston and ring, cylinder liner using metallographic sandpaper dipped in a moderate amount of oil from the right wall make light grinding processing, and replaced the lubrication oil in the crankcase, after about 5 km run, feel in a hot car power drops more apparent, quickly touch with finger after parking cylinder cylinder part side heat sink, right than the left cylinder heat a lot.

Open the cylinder cover again and check that the right cylinder and the piston appear to pull the cylinder. Remove the left and right cylinder piston, check the big end gap of the right cylinder connecting rod is bigger than the left cylinder, it may be the right cylinder lubricating oil out of the problem?

The lubrication system of the Honda CB125T two-cylinder engine takes the form of a pressure-splash hybrid wet sump. The oil pump adopts cycloidal rotor type and selects 7:8 rotor pump with more teeth to reduce the pulsation rate of oil pressure produced by wear through increasing the number of teeth.

The oil from the engine oil pump is divided into four channels, as shown in FIG. 2. One channel is dedicated for crankcase transmission directly to the gear set of main and auxiliary shaft for lubrication of the change gears. The second way is transferred from the special lubricating oil channel to the crankcase oil pan, and then transferred to the big end of the left cylinder connecting rod and the cylinder of the crankshaft. The third route is to deliver the lubricating oil to the upper end of the cylinder head to lubricate the left and right journal of the camshaft and the camshaft lift part; The fourth way, from the right crankcase cover oil channel, through the elastic oil channel installed on the right end of the crankshaft, directly into the right cylinder connecting rod head, lubrication cooling right cylinder cylinder, piston.

With doubt, focus the flashlight on the right box cover to look after the threading opening, crankshaft right port oil channel elastic spring did not fall. Then open the right crankcase cover, its lubricating oil way detailed inspection, found the right box cover Φ oil hole 5 mm casting on the edge of a small sand hole as shown in figure 3.

Originally should flow to the right cylinder crankshaft oil, for the right guy Φ 5 mm on the edge of the oil hole casting this small sand hole, make the inside part of the lubricating oil back into the crankcase, the crankshaft to the right amount of lubricating oil.

Disassemble the crankcase, replace the crankshaft connecting rod assembly, replace the cylinder body and the right cylinder piston and ring, carefully reassemble the disassembled parts.

Since there is no ready-made right crankcase cover change, had to use A, B glue by 2:1 blending ratio, touch on cleaned right lid Φ 5 mm on the edge of the oil hole casting small sand hole, 48 hours for A, B glue adhesive solution cured completely, put on the right side of crankcase, start the engine, 10 km road test and engine cylinder overheating phenomenon disappear right, motorcycle acceleration is normal, confirm the original car right cylinder overheating fault be completely ruled out.