Recently, I met a 125 motorcycle with a single cylinder made in China, with a total driving distance of more than 40,000 kilometers. The user is specialized in long-distance transportation, the vehicle is more bitter and lazy to maintain. Once forgot to change the oil, resulting in serious wear of the cylinder piston as shown in the figure below, but in the roadside to find a nearby stand repair, replace the damaged parts, after reloading, the engine power returned to normal.

However, since then, there has always been an abnormal sound at the head of the cylinder. After returning home, I will check with a familiar maintenance worker. According to my experience, it may be the valve or small chain sound.

I came across this car by chance. I was very familiar with a mechanic in a repair shop who came to see me fixing this car. The maintenance master said, this car is very strange, the cylinder piston is newly replaced, the cylinder head part of the parts have also been carefully checked, the preliminary judgment is that the exhaust valve rocker R side serious wear, replace the valve rocker arm, the abnormal sound of the cylinder head still exists, no matter how to adjust the valve clearance, there is abnormal sound.

In fact, there are many reasons for the abnormal sound of the cylinder head. In order to find the root cause of the fault, first confirm the correct installation of the distribution timing, and under the condition of normal inlet and exhaust valve clearance, remove the valve rocker arm seat assembly and carefully check the wear condition of the valve rocker arm, camshaft lift part, camshaft journal and bushing.

Inspection results: camshaft journal and bushing wear uniform, the newly replaced valve rocker arm R30 circular arc is not a shape of wear trace, and then check the rocker arm spring is not broken, it seems that everything is normal.

So, where exactly is that cylinder head noise coming from?

With doubt, I also installed the valve rocker arm seat assembly according to the normal assembly method, adjusted the valve clearance, while turning the crankshaft with a socket wrench, while carefully observing the contact between the valve clearance screw and the top of the intake and exhaust valve rod.

Found that when check the working condition in the exhaust rocker arm adjusting screw valve clearance and valve stem apex contact is not very normal, into the valve clearance on the valve rocker arm adjusting screw front R7 ball head, basic contact with the valve at the middle of the top of the pole, and the exhaust valve clearance adjustment screw front-end R7 ball head, but off the top of the valve stem center, near the edge contact, This is obviously abnormal contact behavior.

Then disassemble and decompose the valve rocker seat for inspection, and take a spotlight flashlight to look after it. It is found that the top of the exhaust valve has been knocked into a collapsed edge by the valve clearance adjustment screw, and the top of the exhaust valve rod has been knocked into a pit by the valve clearance adjustment screw. Obviously, the top of the exhaust valve rod is not surfacing with Staili alloy powder.

Because the top surface of the valve rod is affected by the impact and sliding friction of the valve rocker head, it is easy to wear. In order to improve its wear resistance, according to the relevant technical requirements, the top of the genuine exhaust valve rod should be surfaced with the appropriate thickness of the staili alloy powder, as shown in the figure below.

Obviously, the valve that the user replaced last time in the roadside stall is a fake and shoddy fittings. Because of the low hardness of the head, the valve clearance adjustment screw can not withstand the long-term tapping and the pit, so that the valve clearance increases and causes the clicking sound.

Replace the valve clearance adjusting screw and the exhaust valve as shown in the figure above. Check as above to confirm that the R7 ball head at the front end of the valve clearance adjusting screw is in contact with the middle of the top of the valve stem. Adjust the valve clearance and reinstall the removed parts. When the engine is started, the sound in the cylinder head is basically normal, the abnormal sound of the cylinder head disappears, and the original fault confirmation is completely eliminated.