Oil Pump Blueprinting

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Here are the gears for the Gerotor oil pump on an SR20VE.  To have a more steady oil flow, the Nissan oil pump has a lot more than just four teeth on the impeller like the animated examples shown on the last page.

In most engines the Gerotor oil pump is driven off the nose of the crank, but on some engines like the Honda K series it’s driven off the crank via a set of spur gears driving a shaft to the oil pump in the pan area.  Pumps driven off of the nose of the crank in particular are subjected to severe backlash stress due to torsional harmonics.  This is especially common in engines spun at much higher rpm than stock.  Critical harmonic periods where the gear can especially be punished exist somewhere above 8000 rpm for most 4 cylinder engines and somewhere above 7500 rpm for an inline 6.  This stress can cause the gears to fail and even split causing an immediate loss of oil pressure and destruction of the engine.  This is common in some engines like the Nissan RB26.  It can also happen in Honda B series engines and the Nissan SR20. Engines that have been stroked are more prone to these sorts of issues.

The assembled oil pump with the cover in place over the housing.  The oil pressure relief valve is housed  in the cylinder with the plug on the end on the right part of the housing cover.  The philips head screws and 10mm bolts must be removed to disassemble the pump.

Another somewhat common problem is a stuck oil pressure relief valve.  Oil pumps have a pressure relief valve that opens at high rpm to prevent excessive head pressure in the oil system.  Sometimes debris like shreds of Teflon tape, gasket fragments or flakes of bearing material common in engines can jam the valve open or shut.  If jammed open the engine will lose a lot of oil pressure causing a failure.  If jammed shut, the oil pressure can spike, breaking oil pump gears or blowing off oil line fittings, rupturing oil lines or blowing off oil filters.  All of these will cause sudden and immediate failure of the engine.

The plug is removed to access the oil pump relief valve and spring.  The valve must be absolutely clean, burr free and move very freely in the bore.  Sticking can cause damaging overpressure or loss of pressure.

What can be done?  Well many engine building projects begin with the rebuilding of a failed engine.  So the first step is to thoroughly clean all parts of the old engine to be reused.  Bearing bits and other failed engine shrapnel lurk in oil galleys and coolers.  A failed engine must have all of the galley plugs removed from the crank, head and block and be physically brushed out and ideally ultrasonically cleaned afterwards.  Just flushing with solvent won’t do it.  Believe me.  Skip this step and I will say I told you so. Coolers and lines should be replaced but in a pinch can be ultrasonically cleaned.  For sure you cannot just flush these out.  I have seen scores of engines ruined this way and have said I told you so more times than you can believe.

A 2-3mm thick shim can be placed between the spring and the plug here.  This will slightly increase oil pressure.  We would probably only do this on an engine built with loose bearing clearances.  Don’t overshim, you may blow out the front main seal.