A pair of movable fittings, the valve, and the valve guide. The valve’s job is to manage the opening and closing of the intake and exhaust passageways in order to accomplish the ventilation task; the valve guide controls the valve’s movement, prevents the valve head from skewing, and ensures that the valve and the valve seat work together closely. The matching gap widens due to frequent movement and friction between the valve and the valve guide, causing the valve stem to wobble in the guide, generating partial valve headwear and causing the valve to close loosely. At the same time, an excessive fit-gap will not only cause poor heat dissipation of the valve but also accelerate the ablation of the valve; another hazard is that the oil can enter the cylinder from the valve guide along the valve stem, resulting in increased oil consumption, exhaust smoke, accumulation The increase in carbon accelerates the wear of diesel engine cylinder liners and pistons.
1. Increased Valve and Valve Guide Wear Clearance
Phenomenon: Oil leaks into the cylinder, resulting in blue smoke from the burning oil, severe carbon buildup in the cylinder, eccentric valve wear, and air leakage due to poor sealing.
Reasons: (1) The valve reciprocates at a rapid rate in the valve guide, causing natural wear from persistent friction. (2) Poor lubrication, dirty air and oil, and a lot of contaminants will cause the valve and valve guide to wearing out faster.
2. Valve Stem Top Bend
Phenomenon: The Yanmar 4tnv88 valve clearance varies, the valve timing is off, or the valve stem becomes stuck in the guide.
Reasons: (1) The valve and the piston crown collide, bending the valve stem crown because the valve clearance is too small, the decompression clearance is too little, the valve phase is incorrect, and so on. (2) The valve stem is unevenly stressed and bent because the valve seat ring is not concentric with the guide tube, the guide tube is eccentrically worn, the head of the rocker’s arm is eccentrically worn, and the valve movement is blocked owing to excessive carbon deposition in the guide tube.
3. Valve Guide Crack
Reasons: (1) Incorrect installation position; the conduit is too high above the plane of the cylinder head, and the spring seat collides with it and breaks when the rocker’s arm opens the valve. (2) Improper installation method; use special tools to press in, not forcibly knock-in; especially for air-cooled diesel engines, the cylinder head should be heated and then pressed into the conduit; otherwise, the conduit hole is damaged and the interference of conduit fittings is small, causing the conduit to move up and down, cracking or breaking. Inspection method: The components crack inspection method may detect obvious cracks and inspect minor cracks.
4. Repair and Replacement
If the valve stem and valve guide are too worn after inspection, they are usually trashed. Brush plating and a coating of wear-resistant and high-temperature resistant materials can also be used to repair the valve stem.
The previous valve guide must be knocked out of the cylinder head with a special gun when disassembling it. Pay attention to the direction in which the cylinder head’s bottom plane is facing up and punching down. Straighten the bottom plane of the cylinder head downward after punching out the old valve guide, and check whether the interference between the outer diameter of the new valve guide and the cylinder head’s guide hole is within the acceptable range.
Check whether the fit clearance between the valve guide rod and the valve guide fits the requirements after installing the valve guide. It can be measured with a dial indication if possible. The empirical method outlined before can be used to check whether something is unconditional. The inner hole is somewhat reduced once the guide tube is driven into the cylinder head. If the space between the decreased guide tube and the valve stem is less than the minimum assembly clearance or the valve cannot be fully inserted, the inner wall of the guide tube must be reamed using a reamer until the clearance fulfills the assembly requirements.
Today’s introduction to the inspection and maintenance of valves and valve guides ends here. If you have any ideas, please leave us a message, and welcome to subscribe to Landtopmall.