Unless there has been some renewal, each valve and spring should be put back in the guide from which it was removed. Fit each valve and valve spring in turn, wiping down and lubricating the valve stem as it is inserted into the guide. As each valve is inserted, slip a new rubber sealing ring over the stem and onto the top of the valve guide. Slip the spring over the stem of the valve and fit the retaining cap into the top of the spring. Position the compressor so that the bottom part of it bears on the rear of the valve; depending on the type of compressor being used, it may be necessary to rig up some arrangement of wooden wedges or similar to achieve this. Whatever method is being used, be careful not to scratch the valve head, the combustion chamber, or the mating surface of the cylinder head. Compress the valve spring until the cotters can be slipped into place in the cotter grooves, and it will be helpful to grease the cotters, which sometimes have the annoying habit of falling out of the grooves at the critical moment and disappearing under the bench.