What are the causes of valve rocker arm fracture?

① The valve clearance is too small.
② The pressed depth of the valve guide is not enough, and it is too long above the cylinder head plane, which causes the spring seat to hit the top of the valve guide when opening, while one end of the rocker arm is forced to press down under the push of the camshaft, and the valve rocker arm is broken due to the resistance of the guide.
③ The depth of the valve head plane sinking into the cylinder plane is not enough. When the valve is opened, the valve collides with the piston, causing the rocker arm to break.
④ The valve is stuck in the valve guide. The valve, especially the exhaust valve, is most likely to bite. This is mainly due to the poor combustion in the cylinder, and the residual flame in the cylinder is discharged during the exhaust, which causes the exhaust valve rod to be frequently burned by the discharged flame heat flow, reaching a very high temperature. In this case, the valve stem and valve guide are likely to be inflexible, resulting in seizure. It may cause bending of the valve push rod or fracture of the valve rocker arm. In severe cases, the valve head may be broken by the piston crown into the cylinder.
⑤ The camshaft is seized due to lack of oil in the bearing. At this time, the torque of the crankshaft timing gear distorts the camshaft due to the disordered valve timing during the flameout process, which causes the valve rocker arm to be jacked off.
At present, the valve rocker arm of the imported Nissan car engine often burns out, which is mainly caused by poor lubrication.
After the valve rocker arm is burnt out, the camshaft bearing bush shall be replaced. When replacing, the oil hole on the bearing bush must coincide with the valve rocker shaft oil passage on the cylinder block. Use compressed air to blow through the valve rocker shaft passage of the cylinder block and cylinder head.
Check the lubrication of the valve rocker arm frequently during engine operation, and check and adjust the valve clearance. If poor lubrication is found, remove the valve rocker arm for cleaning, and then reinstall it for use.