Model #921166400 Craftsman air compressor

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Question and Answers

Q:

Compressor #921-166400 will not build pressure

A:

Thank you for your question.

Your air compressor probably has something broken inside the cylinder. Disconnect the power to the air compressor and remove the plastic cover around the motor and compressor.

Before you take the compressor apart, listen and feel for air leaks. Check to see if air is coming back out of the intake filter. If air is coming back out of the intake filter, the tank check valve is the problem. If you feel air coming out of the intake when it is running I suspect the valve plate is broken.

I recommend checking the tubing connections to the pump. If the connections are fine, you have to remove the head. The head is secured with four bolts. Directly under the head is the pressure plate. The pressure plate has reeds on it that direct the air into the discharge line and keep the air from going back into the cylinder of the compressor. Check the reeds for warping or breaking. If the reeds are damaged, you must replace the pressure plate. Make note of how the pressure and seal sit on top of the cylinder.

If the pressure plate looks fine, I recommend setting it aside and inspect the piston and the cylinder. What usually happens is the piston ring wears and allows air to blow past the piston. When this happen the compressor will run longer and may never come up to full pressure. To repair this problem I recommend replacing the piston ring and the cylinder. Most compressors have a kit available that contains the piston, ring, cylinder, and the seals.

Please feel free to reply with more details if you need further assistance on the repair of the compressor.

Here is a link to order any parts you need. Sears Parts Direct .

If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website: Sears Home Services .

Thank you for using Manage my life.

Mark T.

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Mark T -
Sears Technician
March 19, 2012
Q:

pressure won't build higher than 35psi. I checked the reed valves they are good. excessive pressure comming out the oil fill cap.

A:

The piston rings could be bad and cause the problem you have. The cylinder walls could be scored. Inspect the reed valves for warping and the pressure plate for scoring or warping.

Your air compressor probably has something broken inside the cylinder. Disconnect the power to the air compressor and remove the plastic cover around the motor and compressor.

Before you take the compressor apart, listen and feel for air leaks. Check to see if air is coming back out of the intake filter. If air is coming back out of the intake filter, something is wrong inside the cylinder.

You can see a parts diagram at [www.searspartsdirect.com]. The parts diagram will help you locate the parts that build air pressure. These are the parts you need to inspect.

Turn the motor shaft by hand and be sure the piston goes up and down.

If the piston is ok remove the 4 screws holding the head on top of the cylinder. Remove the head.

Directly under the head is the pressure plate. The pressure plate has reeds on it that direct the air into the discharge line and keep the air from going back through the intake side of the compressor. Check the reeds for warping or breaking. If the reeds are damaged, you must replace the pressure plate. Make note of how the pressure plate sets and the o-rings that seal around it.

Remove the pressure plate and inspect the cylinder walls for scratches or wear, if the cylinder is damaged you need to replace the cylinder, piston, and rings. Make note of the piston and the rings on it. If a ring is worn or broken the air goes back through the intake filter and no pressure is built. If you have to replace the rings, replace the piston and sleeve also.

To replace the rings and piston, you have to remove the nuts that hold the piston to the eccentric.

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Jimmy K -
Sears Technician
July 22, 2010

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