EVO: Engine Mechanicals - Sub-04S

Top End Oiling

Rocker Box Oiling

1991-1999 engines

  • Crankcase pressure (air and oil mist) is routed up the pushrod tubes (bypassing the falling oil from the rocker boxes) into the rocker box.
  • Collected air pressure and oil mist in each rocker box is routed up into a sealed cavity in the lower portion of the box.
  • This mixture passes up from underneath a rubber one-way valve (umbrella valve) sitting over the cavity inlet.
  • The oil is designed to separate from the air by;
    • Hitting the underside of the umbrella valve and falling back into the sealed cavity in the middle rocker box.
    • From there it should drain back into the lower rocker box through a tiny hole behind the umbrella valve.
    • This tiny hole is in a recessed area behind the umbrella valve.
    • There is a certain amount of backpressure in the sealed cavity away from the air exit hole.
    • This backpressure should help push the oil into the tiny hole and back into the lower rocker box area to be carried back down to the lower end.
    • The upstroke of the pistons causes a vacuum to form in the system.
      This vacuum is also pulled through the oil passage from the heads / rocker box.
      (in theory, it should help pull oil through the drains in the heads to the bottom end.
  • Air pressure is designed to continue up past the umbrella valve and exit a hole in each head on the intake valve side.
  • Pressure escapes the head through the hollow bolts (one in each head) that hold the air cleaner mount.

2000 and Up engines

Rocker Arm / Valve Oiling


  • Oil flows up the holes in the pushrods from the lifters and enters the rocker arms through a hole in the pushrod end of the arm.
  • Oil is supplied to each valve stem and springs from the drilled holes in the underside of each rocker arm above them.
    • The holes are only on the valve side of each rocker arm.
  • Oil is also forced through the clearance between the rocker arm shafts and bushings for lubrication there.
Valve oiler holes in Rockers 1) Rocker arm oiling path 2)
Valve Squirter Hole Locations 3)

1) , 2)
photo by Hippysmack
photos by Hippysmack
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