What does a car valve guide mean
The function of the valve guide is to guide the movement of the valves, ensure that the valves perform reciprocating linear motion, enable the valves and valve seat rings to fit correctly, and transfer the heat of the valve stem to the cylinder head.
The working temperature of the valve guide is also relatively high, approximately 200℃. When the valve stem moves in the guide tube, it is lubricated only by the engine oil splashed out from the valve train, and thus is prone to wear. Therefore, ductile iron or powder metallurgy materials with self-sliding properties are often used to make individual parts, which are pressed into the valve guide seat holes on the cylinder head with a certain interference fit to prevent them from falling off. Some engines use retaining rings to fix and position the valve guides to prevent them from loosening.
The lower part of the valve guide extends into the intake and exhaust ducts. To prevent resistance to the airflow, the outer circle of the extending end is made into a conical shape. There is a gap of 0.05 to 0.12mm between the valve guide and the valve stem, allowing the valve stem to move freely within the guide.
The valve guide supports and guides the valves that move back and forth within it, ensuring that the valves and valve seats can fit correctly, and also serves to conduct heat.
There are two types of valve guides: integral type and embedded type. The integral valve guide directly processes the valve stem hole in the cylinder head. The embedded valve guide is pressed into a cylindrical tube through the guide hole in the cylinder head with an interference fit of 0.015 to 0.065mm.
Some valve guides have ring grooves machined on the outer cylindrical surface, which are embedded with retaining rings for positioning, limiting the depth of the lower end into the airway and preventing it from loosening.
There is generally a fit clearance of 0.05 to 0.12mm between the valve stem and the valve guide. They are lubricated by the engine oil splashed out from the valve transmission parts.
If the gap between the valve stem and the valve guide is too small, it is easy to cause the valve stem to get stuck and have poor sealing, which may lead to valve burnout and a decline in power performance, economy and emissions.
If the gap between the valve stem and the valve guide is too large, it will cause the following abnormal phenomena:
The valve seat is not stable, the sealing performance declines, and it wears out at an accelerated rate, leading to valve burnout and a decrease in power performance, economy, and emissions.
Air leakage in the intake system causes the mixture in the gasoline engine to become thinner.
Excessive engine oil seeps down through the gaps into the combustion chamber or air passage (especially during the intake process), resulting in increased engine oil consumption, blue smoke emission, and accelerated carbon deposit formation.
During the exhaust stroke, the exhaust gas enters the crankcase through the gap between the exhaust valve stem and the valve guide.
When the pressure at the exhaust port is lower than that inside the crankcase, engine oil leaks into the exhaust port through the gap between the exhaust valve stem and the valve guide.
The reason for blue smoke during startup is often that the gap of the exhaust valve guide is too large. When the machine stops, the oil seeps through the gap of the exhaust valve guide into the combustion chamber and adheres to the exhaust manifold, and is burned off during startup. This phenomenon will ease or disappear as the engine temperature rises.
The function of the valve oil seal is to prevent excessive engine oil from seeping into the combustion chamber or intake and exhaust ducts through the gap between the valve guide and the valve stem. It is a rubber sealing ring, installed at the upper end of the valve guide. If the valve oil seal is damaged, excessive engine oil will seep down through the gap between the valve and the valve guide into the combustion chamber or airway, resulting in increased engine oil consumption. This is accompanied by blue smoke in the exhaust and accelerates the formation of carbon deposits on the valves and in the combustion chamber.
The methods for replacing valve guides are divided into two types: replacing valve guides with flanges and replacing valve guides without flanges.
There is a flange valve guide
When the entire cylinder head is heated to 100-150℃ in a constant temperature box, take it out and lift the cylinder head. Use the valve guide disassembly and assembly tool to push the valve guide towards the camshaft side. Then apply lubricating oil to the new O-ring, install it on the new valve guide, and then use the valve guide disassembly and assembly tool to drive the new valve guide into the camshaft side. After the cylinder head cools down, drip some cutting oil into the inner hole of the valve guide, and use a valve guide reamer to trim the valve guide to obtain the appropriate fit clearance between the valve stem and the valve guide. Finally, thoroughly clean the cylinder head with gasoline or kerosene and it can be installed and used. Replace the valve guide without flanges
Before removing the valve guide from the cylinder, measure the height of the exposed part with a vernier caliper and make a record. When the cylinder head is heated to 100-150℃ in the constant temperature box, take it out and lift the cylinder head. Use the valve guide disassembly and assembly tool to push the valve guide towards the camshaft side. Then, use the valve guide disassembly and assembly tool to drive the new valve guide into the camshaft side. While measuring the height of the exposed part of the valve guide, drive the new valve guide in bit by bit until it meets the specified requirements. After the cylinder head has cooled down, drip some cutting oil into the inner hole of the valve guide and use the valve guide hinge to trim the valve guide to obtain the appropriate fit clearance between the valve stem and the valve guide. Finally, thoroughly clean the cylinder head with gasoline or kerosene and it can be installed and used.
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