# Craftsman Air Compressor



## RKDOC (Jun 20, 2006)

A customer just brought me a Craftsman air compressor model 921.153101 Serial 540215. I have never worked on the compressor part of a compressor before. I have a IPL on the unit. The compressor will only build 30lbs of pressure. The customer installed a new piston and rings. (compressor looks brand new, but is a couple years old) When I start the compressor I get air coming out a hole at the top of the oil fill plug.

My thinking on the working of the compressor is that as the piston goes down, it draws air through the filter through one reed valve. Then as the piston goes up one reed valve closes and the air is pushed past the other reed valve to the storage tank. If this is right then there should not be air pressure in the crankcase part of the compressor?

That said then the problem more than likely is the cylinder is worn to were the new piston and rings are not sealing against compression? Sorry for the lengthy post but any help, Advise or suggestions would be greatly appreciated. If I pull the head off is there somewhere I can get the specs for the cylinder bore?

Thanks RKDOC


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## 30yearTech (Sep 14, 2006)

Just like an engine, when the piston moves downward in the cylinder, there will be some pressure developed below the piston in the crankcase. When the piston moves upward there will be a vacuum. If the crankcase is properly sealed then this is just a small back and forth pressure situation in the crankcase. If the pressure in the crankcase is developed on the compression stroke of the piston, then you are correct and it's likely an issue with the cylinder and or piston and rings. (maybe the ring end gaps are lined up?)


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## RKDOC (Jun 20, 2006)

Thanks 30 year. The pressure coming out the vent hole is quite a bit and continuous. I will have to pull the compressor apart and see what I have. Thanks again.

P.S. any way of getting specs for the cylinder bore?


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## nnyparts.com (Dec 28, 2009)

I always cringe when a customer brings in a machine either in parts or letting me know he or she has already replaced this and that. In your case the piston and rings that were put in could have been done improperly and I would start from scratch beginning with ....did the customer do the overhaul correctly. Was a gasket damaged in the process. You should be able to get the spec's and a parts breakdown at the Sears Parts Direct site http://www.searspartsdirect.com


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