If the valve seats are badly worn or cracked, new valve seat rings can be inserted, and the cylinder head must be drilled to the appropriate dimensions. The valve seat rings must be run in concentrically with the valve guide bushings.
Valves
Check the valve stems for excessive wear, pitting or cracking. Place a steel ruler along the well-cleaned valve stems and check if the stems are straight. Deformations are not allowed, as otherwise the stems may jam in the guides. Check the valve plates for scale or cracking. The distance between the upper surface of the valve and its upper edge (the width of the valve edges) must be at least 0.8 mm.
Valve guide bushings
Fig. 74. Checking the clearance between the valve stem and the valve guide bushing
The valve guides are built into the cylinder head. If the clearance between the valve stem and the guide is too large, the valve guides must be turned. First check the clearance. To do this, insert the valve as shown in Fig. 74 into the cylinder head and move it up and down. If the clearance seems too large, countersink the guides to use valves with larger stems. It is generally recommended that the clearance should not exceed 0.50 mm for intake valves and 0.60 mm for exhaust valves.
Valves come in two large overhaul sizes. Always countersink the valve guides on the valve plate side. The size of the reamer depends on the degree of wear. Use the smallest reamer first and then try the smallest overhaul valve to determine if the clearance is within the specified values.
The valve seats must be finished with a milling cutter after the guides have been reamed to maintain straightness between the guide and the seat.
Valve springs
It is better to replace the valve springs with new ones if the engine has significant mileage. The springs can be checked by putting the old and new springs end to end on a long screw and screwing the nut onto the screw (with appropriate washers). Tighten the nut so that the springs are under pressure and measure the length of the springs. If the used spring is 10% shorter than the new one, then replace the entire set. Install the valve springs so that the closed end points downwards.
Cylinder head
Check the cylinder head for cracks or other defects. Check the mating surface of the cylinder head for deformation by placing a ruler lengthwise, crosswise and diagonally on a well-cleaned surface and measuring the gap with a feeler gauge. If the gap is more than 0.15 mm, grind the surface of the cylinder head. In this case, it is necessary to ensure that too much metal is not removed during grinding. The maximum removal for grinding is specified in the appendix.
Measure the camshaft bearing bores. The bearing diameters should be measured in conjunction with the camshaft journal diameters to determine bearing clearance.