Read this on another site....I will provide the link............
I have an 85 k-10 305. I have oil pan and pump off how do I tell if it has a 1 or 2 piece rear main seal?
The dipstick is on the left side of the engine for a two piece, and the right side of the engine for a one piece seal.
The one piece goes on over the back of the crank shaft to replace it.
The two piece you need to remove the rear main bearing cap to replace it.
http://forum.73-87chevytrucks.com/sm...?topic=22760.0