How to replace a part in the engine?

I have a..........
Craftsman Eager-1
6.5 Horse Power Walk Behind Lawnmower
Power Gear Drive
22 inch cut

Engine Model Number: 143.996510
Engine Family: WTPXS.1951 BA
Displacement: 195
This mower is in good shape and was given to me, but I need to replace a part and I need to know how to do it, and where the part is located in the engine. The part is in a small plastic bag that says on it:
ORIGINAL SERVICE PART
TECUMSEH
# 27897 (number on the bag)
The part is an O-ring of some kind. It's made of aluminum and about as big around as a silver dollar. There is a black rubber washer attached to it's inner circle, and that washer is encircled by a small steel spring. The previous owner said there was a problem with the dip stick tube coming out during use, and that the part is supposed to fix that problem.
I hope you can help me.

Skip Mathews
December 2005