Semi-hermetic compressors are frequently used on light commercial air conditioning and refrigeration equipment. With the cast iron body and bolted head, oil pan, oil pump, and end cover that opens the electric motor section for service, certain field repairs can be made. Only two types of repairs should be attempted in the field. Replacement of valve plate assembly and/or the oil pump, if the unit has separate pump. No other type of repair should be attempted in the field.
The semi-hermetic compressor has many of the same component parts as an automobile. To rebuild a compressor thoroughly, special tools and test instruments are needed. If a compressor is not pumping properly, the head bolts and head should be removed carefully from the block. Looking into the cylinders, you can turn the crankshaft and see if all of the pistons move up and down. If there is a broken piston or connecting rod, put the head back on and replace the whole compressor. Some say that a compressor with the above condition would make a lot of noise. In some cases, this is so; however, I’ve come across many compressors that had broken rods or pistons and didn’t make any noise.
If the pistons are alright, examine the valve seats and the valve reeds. Also, examine the head gaskets. There is one gasket between the block and the valve plate and another gasket between the valve.