crank case and distance peiece???

Not sure why you posted this in automotive. Car engines don't have a distance piece. In a car, the cylinder bolts directly to the crank case and the piston is connected directly to the crankshaft via the connecting rod. the crankcase is the block and bearings that support the crankshaft.

In a reciprocating pump or compressor, the cross head guide is bolted to the crankcase. In this case we have a slider crank mechanism where the crank turns and the connecting rod connects the crank to the crosshead and then the cross head reciprocates. Attached to the cross head is a piston rod and at the end of the piston rod is the piston. A distance piece bolts to the crosshead guide, and the cylinder bolts to the distance piece. In a recip compressor the distance piece is normally vented to flare as the piston rod packing is in this area and any packing leaks go into the distance piece. The distance piece also serves to separate the cylinder from the crankcase preventing hazardous gas from the cylinder from entering the crankcase.

Hope that helps.