Holset Air Compressors – 012-010   Air Compressor Pin Bore Wear

Table of Contents

Initial Check

Initial Check



The unloader valve body is installed with spring tension. Use care when removing to prevent personal injury. Always wear protective eye wear.

NOTE: This procedure applies to SS and ST models only.

Hold the unloader valve body down and remove the two captive washer capscrews and the two plain washers.

Remove the unloader valve body.

Remove the o-ring seal.

Remove the rectangular ring seal.



To avoid damage to the air compressor, do not allow any debris to fall into the air compressor cylinder.

NOTE: Disassembly of the center unloader valve on Holset® two cylinder air compressors is similar to the single cylinder unloader valve.

Remove the unloader valve cap and the unloader valve spring.

Remove the intake valve seat and valve.

Remove the intake valve spring.



Do not use a screwdriver. A screwdriver can gouge the top of the piston.

Insert the small end of a 10 mm [3/8 in] drive socket extension 152 to 254 mm [6 to 10 in] long through the exhaust valve seat onto the top of the piston.


Bar the crankshaft over until the compressor piston reaches top dead center (TDC), and the extension starts to move downward approximately 3 to 6 mm [1/8 to 1/4 inch].



To prevent damage to the top of the piston, do not use a hammer.

Give a quick, hard push downward on the extension and listen for a metallic click as wear clearance is taken up.


If significant piston motion is felt or a metallic click is heard, the pin bores can be worn, and the compressor must be examined further.


NOTE: All air compressors will not exhibit pin bore wear.

If no motion or sound is heard, the compressor is in satisfactory condition and does not need to be replaced.


Remove the socket extension.


Install the intake valve spring with the tang down.

Install the intake valve.

Install the intake valve seat with the flange side up.

Install the unloader valve cap spring.

Install the unloader valve cap.

Use high temperature Accrolube Lubrication Teflon grease, or equivalent, to lubricate the outside diameter of the cap.


NOTE: The rectangular ring seal must be installed with the grooved side up.

Install the rectangular ring seal.

Install the o-ring seal.

Use clean 15W-40 engine oil to lubricate the o-ring seal.

Install the unloader valve body.

Press the unloader valve body down to be sure the tangs of the unloader valve cap are in the three slots of the intake valve seat.



Do not overtorque. Compressor damage will result.

Hold the unloader body down and install the two plain washers and captive washer capscrews.

Tighten the capscrews.

Torque Value:  14  n.m  [10 ft-lb]

Last Modified:  13-Jun-2005