Preparing the 235/261 oil pump

On any rebuild of the Chevy 235/261 engine, especially if hydraulic lifters are to be used, it is essential to pay particular attention to several areas of the oil pump (even if it is a NEW pump!). Very few pumps actually need replacement, so a complete disassembly, cleaning and re-assembly will usually suffice, but special attention must be paid to eliminate any air leaks on the inlet side of the pump. (Small leaks on the output side of the pump are not of concern).

Air leaks on the inlet side of the oil pump will allow air to mix with the incoming oil. Experience has shown that this air collects in the lifter galley and may cause hydraulic lifters to become “SPONGY” and result in lifter noise.

Air in the oil is not good for the bearings but it is disastrous in hydraulic lifters. It will cause them to become soft, “spongy” and very noisy especially at highway speeds. (Like brakes, with air in the lines).

Three areas have been identified and are suspect even on new pumps:

1-    BOTTOM COVER: Assure flatness, freedom from nicks and burrs, and the oil pump must have a gasket or use loctite #515 or equivalent anaerobic sealer. (Do NOT use RTV!)

Assure .002” – .005” gear axial (end) clearance.

2-    PICK UP TUBE: Use loctite sealer on the threaded joint where the tube screws into the pump. Make sure the threads are in good shape and the tube has not been squashed oval in this area. An oval tube may feel tight in the pump, but it will allow air to enter the oil stream.

3-    OIL PRESSURE RELIEF PISTON: Too much clearance between the piston and its bore can cause an air leak into the pump because this area is open to air in the crankcase and could allow air into the inlet side of the oil pump. I suggest plugging the end of the bore and drilling a .060” vent across into the inlet hole of the pump. Be careful not to cover the vent hole when installing the plug.

These three areas are often overlooked by many engine rebuilders who are mostly familiar with the Chevy V8 and other modern engines which employ an oil pump which is submerged in oil. All three of the previously mentioned potential inlet side air leaks are inherently not possible on the Chevy V8.

The GMC 228, 248, 270 and 302 engines use a loose fitting pick-up tube into the pump body which, I’m sure leaks air but this engine cannot use hydraulic lifters due to lack of a feed gallery. Racing experience on this engine indicates that the bearings apparently do okay with some leakage so I wouldn’t disassemble your entire engine to revise the pump unless, of course, you’re having bearing problems or the engine is apart for other reasons.

On any rebuild of the Chevy 235/261 engine, especially if hydraulic lifters are to be used, it is essential to pay particular attention to several areas of the oil pump (even if it is a NEW pump!). Very few pumps actually need replacement, so a complete disassembly, cleaning and re-assembly will usually suffice, but special attention must be paid to eliminate any air leaks on the inlet side of the pump. (Small leaks on the output side of the pump are not of concern).

Air leaks on the inlet side of the oil pump will allow air to mix with the incoming oil. Experience has shown that this air collects in the lifter galley.

Air in the oil is not good for the bearings but it is disastrous in hydraulic lifters. It will cause them to become soft, “spongy” and very noisy especially at highway speeds. (Like brakes, with air in the lines).

Three areas have been identified and are suspect even on new pumps:

1-BOTTOM COVER: Assure flatness, freedom from nicks and burrs, and the oil pump must have a gasket or use loctite #515 or equivalent anaerobic sealer. (Do NOT use RTV!)

Assure .002” – .005” gear axial (end) clearance.

2-PICK UP TUBE: Use loctite sealer on the threaded joint where the tube screws into the pump. Make sure the threads are in good shape and the tube has not been squashed oval in this area. An oval tube may feel tight in the pump, but it will allow air to enter the oil stream.

3-OIL PRESSURE RELIEF PISTON: Too much clearance between the piston and its bore can cause an air leak into the pump because this area is open to air in the crankcase and could allow air into the inlet side of the oil pump. I suggest plugging the end of the bore and drilling a .060” vent across into the inlet hole of the pump. Be careful not to cover the vent hole when installing the plug.

These three areas are often overlooked by many engine rebuilders who are mostly familiar with the Chevy V8 and other modern engines which employ an oil pump which is submerged in oil. All three of the previously mentioned potential inlet side air leaks are inherently not possible on the Chevy V8.

The GMC 228, 248, 270 and 302 engines use a loose fitting pick-up tube into the pump body which, I’m sure leaks air but this engine cannot use hydraulic lifters due to lack of a feed gallery. Racing experience on this engine indicates that the bearings apparently do okay with some leakage so I wouldn’t disassemble your entire engine to revise the pump unless, of course, you’re having bearing problems or the engine is apart for other reasons.

Tom Langdon

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