No matter the head was brought to the automotive repair shop for valve service, make sure it is clean during the process of reassembly. If the head was serviced the valves as well as the related components will already be installed. Lubricate each of the valves and put new seals on each valve stem but it is important not to twist or cock the seals for they have to seat well. Place the spring seat or shim(s) over the valve guide, and install the valve spring and retainer to be in place. There is also need to use valve spring compressor to compress the springs and fit keepers in the upper groove before you slow down the compressor so that the keepers can seat better. If needed, a small amount of grease may be applied to the keeper so that it will stay in the proper position. Perform the same for the other valves and as much as possible the components should be put back in their respective stations without swapping them. Inspect the installed valve spring height with a ruler which is graduated to get 1/32 of an inch or a dial caliber. If the head was sent for servicing, then the height installed should be correct, nonetheless, this should not be taken for granted. The distance is between the top of the spring seat or shim(s) to the bottom of the retainer and should be checked for the amount of valve spring installed height for each valve. In case it goes past this figure, the shims can be placed under the springs to adjust for the height. It is advisable to avoid taking shimming to extent of the springs installed height being below the specified ones.