Broken valve springs and defective valve stem seals can be replaced without removing the cylinder heads, typically requiring two special tools and a compressed air source. If compressed air isn't available, a length of nylon rope can be used to prevent the valves from falling into the cylinder. Begin by removing the rocker arm cover from the affected cylinder head, and if all valve stem seals are being replaced, remove both covers. Next, take out the spark plug from the cylinder with the defective component, and if replacing all seals, remove all spark plugs. Turn the crankshaft until the piston in the affected cylinder is at top dead center on the compression stroke, starting with cylinder number one and following the firing order sequence. Thread an adapter into the spark plug hole and connect an air hose from a compressed air source. Remove the nut, pivot ball, and rocker arm for the valve with the defective part, then pull out the pushrod. Apply compressed air to the cylinder to hold the valves in place. If compressed air is unavailable, position the piston just before TDC on the compression stroke, feed a long piece of nylon rope through the spark plug hole, and rotate the crankshaft until slight resistance is felt. Stuff shop rags into the cylinder head holes to prevent parts from falling into the engine, then use a valve spring compressor to compress the spring/damper assembly and remove the keepers. Remove the spring retainer or rotator, oil shield, and valve spring assembly, then take out the valve stem O-ring seal and umbrella type guide seal, replacing the O-ring seal each time it is removed. If air pressure fails to hold the valve closed, the valve face or seat may be damaged, necessitating cylinder head removal for repairs. Wrap a rubber band or tape around the top of the valve stem to prevent it from falling into the combustion chamber, then release the air pressure. Inspect the valve stem for damage, ensuring it doesn't bind in the guide. Reapply air pressure to retain the valve in the closed position, then remove the tape or rubber band. Lubricate the valve stem, guide, and new seal with clean engine oil, and install the new exhaust valve oil seal if applicable. Install the damper spring, valve spring, and oil shield, then the valve spring retainer or rotator. Compress the valve spring assembly and carefully install the new O-ring seal, ensuring it lies flat in the groove. Position the keepers in the upper groove, applying a small dab of grease to hold them in place if necessary. Remove the pressure from the spring tool and ensure the keepers are seated, checking the seals with a vacuum pump. Disconnect the air hose and remove the adapter from the spark plug hole, or pull out the rope if used. Finally, install the rocker arm(s) and pushrod(s), spark plug(s) and wire(s), and rocker arm cover(s), then start the engine and check for oil leaks and unusual sounds.
