In the picture at right, notice the original valve stem seal is just a small rubber O-ring. The O-ring should be installed inside of the spring retainer cap and against the bottom of the split cotters. This makes a seal between the bore of the spring cap and the valve stem to prevent oil on top of the spring cap from running down the valve stem into the guide.

For original square groove valve stems, the O-ring is placed in the bottom of the groove with the split cotters placed above the O-ring. For later production MGB with smaller round groove stems, the O-ring is placed on the OD of the valve stem just below the split cotters.

When you first see this little O-ring you may think it is never going to fit over the valve stem. Dimensions below are in inches.
O-ring Th = 0.115"
O-ring OD = 0.385"
O-ring ID = 0.155"
Stem Dia. = 0.342"

Fear not, as it only requires a little finesse. I place the O-ring across one edge of the valve stem tip. put two thumbs on the sides, press down and shove it across to stretch the ring and push it down on the other side of the stem. It goes right on with the first try (once you get the hang of it).

The original deflector is a thin metal shell fitting inside the valve spring and close around the top of the valve guide. The deflector serves to shield the top end of the guide from oil splash from the springs. There is originally no seal around the top of the valve guide. This arrangement served well for decades, but does allow a small amount of oil into the valve guides. When the guides are in good condition the oil infiltration is minimal and serves to lubricate the original iron valve guides. (As a matter of curiosity, use of the oil deflector was discontinued in December 1962 after production of 4284 MGB with 1800 engines).