The Mystery of the EVO IX Cylinder Head Drill Holes.


Let’s say you own a Mitsubishi Evolution IX, and want nothing more than to stand there and admire the turbocharged delicacy that resides beneath your hood.  You reach down for the hood release and are instantly greeted with the firm “thud” of the hood-pop in action.  Reluctantly, you free yourself from the hug-like grip of your race-inspired Recaro seat as you hop out of your machine.  The opened-windowed door closes behind you.  You walk toward the front of the vehicle while holding intense eye contact with your freshly cleaned 14-spoke BBS 17’s.  The hood is then effortlessly lifted toward the sky and you are greeted with one of the finest examples of Mitsubishi technology to ever roll off the assembly line, the 4G63.

Within an instant, your eyes focus on something that is clearly out of place.  It is something so strange that you aren’t sure if it belongs there or not.  You have looked at nearly eleven billion EVO’s in the past, but never noticed this kind of nonsense. You are stunned, disappointed, and nearly jallywagged. “What was the purpose of these drilled out holes in my cylinder head!?” you screamed at the heavens.  Sadly you are answered with the deafening sound of silence.

Maybe one of the readers can help?

Jeremy Nutt

Hi, I'm Jeremy.

14 thoughts to “The Mystery of the EVO IX Cylinder Head Drill Holes.”

  1. I have some ideas I will just list to help fuel any thoughts others may have.

    1. They are used for centering and aligning the head in some jig?
    2. They are partially drilled for a mounting boss or bracket?
    3. They are drilled to remove some marking?
    4. They are for tuning the frequency response range for the knock sensor?
    5. They are purely a function of the production process / or manufacturing test?

  2. My best guess was similar to your # 1. I figured that maybe some kind of machining device holds it in that spot as it's being machined.

  3. We asked AMS Performance (Mitsubishi performance parts specialists) what their thoughts were and they also agreed that it probably has to do with the machining process. The holes could be where the cylinder head is held by the machine.

    Maybe we should contact Mitsubishi themselves and solve the mystery once and for all?


  4. The strange thing is that it looks more like an afterthought than something that would occur early in the manufacturing process. Its also offset at a point which doesn't seem like its near the center of gravity which doesn't seem to me like a sensible point to use as an anchor during machining. Contacting Mitsubishi would certainly lead to some answers. Does anyone know if there are similar holes elsewhere on the casting?

  5. I would have to guess that it is a precaution taken to prevent cracking. The drilling could be a relief or a means to stopping the crack that forms from spreading. I’m sure we have all noticed our fair share of casting imperfections and counter measures.

  6. So I put this question to Ed Clancy of Mitsubishi R&D America who in turn passed it on to his counterparts in Japan. The answer is the holes are for automatic detection on the assembly line. The holes pass by a sensor and tell the system what motor family the head belongs to.

    your welcome 🙂

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