Note: This procedure does not apply to 16 valve engines. Broken valve springs and defective valve stem seals can be replaced without removing the cylinder head. These operations require special tools and compressed air.
1. Remove the cylinder head cover (see Section 4).
2. Remove the spark plugs from those cylinders that have defective parts. If the valve stem seals need to be replaced, all spark plugs for compressed air must be dismantled.
3. Rotate the crankshaft until until the desired piston is at TDC on the compression stroke (see Section 3). If valve stem seals are being replaced, start with the first cylinder and proceed from cylinder to cylinder in the order of operation.
4. Screw the fitting into the spark plug hole and connect it with a hose to a compressed air source.
5. Remove the rocker axle assembly.
6. Apply compressed air to the cylinder.
Attention: the piston can be driven by compressed air and the motor will suddenly start to rotate.
7. The valves will be held in place by compressed air. If there is wear in the valve/valve seat pair and there will be significant air leakage, and the compressed air pressure is not enough to support the valves, use the other method described below.
8. If a compressed air source is not available, you can use another method. Bring the piston close to TDC. Then, through the spark plug hole, insert the nylon cord into the cylinder until it fills the combustion chamber.
Make sure that the end of the cord protrudes from the hole just enough to easily remove it from the cylinder. Then rotate the motor until there is slight resistance provided by the cord.
9. Close all holes in the head with rags to prevent parts of the valve mechanism from getting there and use a puller to compress the valve springs. Remove crackers.
Note: A pair of appropriately sized wrenches can be used to compress the springs.
10. Remove the valve springs, then remove the valve stem seal from the valve guide.
Note: if, after removing the springs and the valve stem seal, there is not enough air pressure to hold the valve closed, this may indicate wear on the valve / valve seat or cylinder-piston group, which will require removal of the head and additional repair work.
11. Place a rubber ring over the valve stem or wrap the stem with duct tape to prevent the valve from dropping and relieve air pressure.
Note: If a cord was used instead of air, move the piston slightly to release the valves.
12. Inspect the valve stem. Determine by rotating the valve in the guide for wear or curvature.
13. Move the valve up and down and make sure there is no binding. If sticking is found, the valve stem may be bent or the guide damaged. In this case, the cylinder head must be removed and additional repair operations carried out.
14. Re-inject air into the cylinder (or press the valves with a piston). remove the rubber ring with the valve stem.
15. Lubricate the valve stem with engine oil and install a new valve stem seal.
Note: The intake and exhaust valve stem seals are not interchangeable: the intake valve stem seal has a white spring, the exhaust stem has a black one.
16. Install the valve springs.
Note: try to match the ends of the springs with the paint marks on the cylinder head.
17. Establish plates of springs. Compress the springs and insert the crackers from their grooves. Lightly coat the inside of the cotters with grease to help secure the cotters to the valve stem.
18. Remove the compression force from the springs and make sure that the crackers are in place.
19. Remove air pressure and unscrew the fitting from the spark plug hole (or pull the cord out of the cylinder).
20. Install rocker axle assembly (see Section 10).
21. Install spark plugs.
22. Install the cylinder head cover (see Section 4).
23. Start the engine, check for oil leaks and extraneous knocks under the cylinder head cover when the oil reaches operating temperature.