Valve springs and valve stem seals can be replaced without removal of cylinder head; normally it requires two special tools and compressed air. If there is no compressed air the valves can be secured in place using a length of nylon rope to stop them falling into the cylinder. Start with the removal of the camshaft cover, spark plug from Cylinder head with defective component or all spark plugs where replacing all valve stem seals. Rotate the crankshaft until the piston in the affected cylinder is at the TDC of the compression stroke and if doing all the seals, start with cylinder number one and do in the firing order sequence. Take an adapter and insert it into the spark plug fitting that is frequently used in connection with your air hose, and use the compressed air in order to secure the valves. In case of lack of air pressure, place the piston as near to top dead centre as possible on the compression stroke and push into the combustion chamber a nylon rope with its end left exposed for withdrawal. Plug up the cylinder head holes with shop rags so that no contaminants can get into the engine, then, use a valve spring compressor to compress the spring. After compressing the spring, take out the cam follower and the lash adjuster, the keepers, the spring retainer, the damper assembly and the valve spring. Remove the valve stem seal and if air pressure cannot retain the valve closed, you may be looking at a hassle with the valve face or seat that would require removal of the cylinder heads to have it fixed. Put a piece of a rubber band or an adhesive tape around the valve stem with the intention of preventing it from falling off.Now, allow the pressure to fade and give the valve stem a good check to ensure it rotates in the guide as required. Reverse the valve to keep it shut by applying air pressure, apply engine oil the valve stem and fit a new umbrella type guide seal. Put the installation cap on the stem of the valve, fit the valve stem seal over the cap and screw down till the seal is in contact with the guide. Take off the cap and insert, with a special installation tool or screwdrivers, the seal to the valve guide. It fits the valve spring, damper and retainer; compress the spring then fit the keepers; lash adjuster to be fitted along with cam follower. Release from the spark plug hole the air hose and the adapter if used or pull out the used rope as well. Last, replace the parts that were unbolted, turn on the engine, and look for possible signs like dripping oil or foreign noises emanating from the region of the camshaft cover.