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    #1

    Engine builders. Need your assistance.

    I need some questions answered by people who have HANDS ON knowledge from building motors. The motor in question is a 2.5 m20. OEM shortblock with 150-175k.
    I decided to measure my engine bore on all cylinders since my head is off at the moment. Cylinder #2 and #5 have me concerned. Both were off by roughly the same amount(within .001" of each other). The top deck bore came to 3.315" which is out of spec by roughly .008". Half an inch lower, cylinder bore is still close to within spec(3.307). I have three questions...

    1. How bad off is this? My compression was strong prior to pulling the head but, there was also ALOT of carbon buildup that I removed.

    2. Is there a chance I will be burning off oil if left in this condition for around 2-3 years? (I will be rebuilding a short block over the next 24months)

    3. Is this normal wear for 150-175,000 mile m20 or, is it possible I am getting piston slap on full stroke?

    -Brandon

    This picture is just to show the gauge I used. Actual measurement shown is not the engine bore.

    Originally posted by Ryan...
    It now emits a beautiful blue-ish yellow/green smoke from the exhaust?? No idea what would cause that color, but I assume its good.
    Tags: None
    #2
    Youre using the wrong gauge. Also, from what I've been told, you measure roughly an inch down into the bore for the top bore measurement.

    Comment

      #3
      You need to use a bore gauge like this with a micrometer:


      Or a bore gauge with dial like this:


      Calipers aren't really accurate enough and can't measure far enough down into the cylinder. You can use calipers with the first style bore gauge I posted, but it is more difficult to get an accurate reading.

      Diameter alone also won't tell you much. What you need to measure is bore taper (measure top and bottom of the cylinder - compare numbers) and bore concentricity (is the cylinder round or slightly elliptical?). These specs are much more important then the actual size of the cylinder.

      Does the Bentley have a run through of all of this?

      Comment

        #4
        Nice calipers none the less.
        Ma che cazzo state dicendo? :|

        Comment

          #5
          You are measuring the top near the head(with the wrong tool, as stated). There is a slight rounding of the bore where it exits. You are just fine, just make sure there is no bore damage and get it honed & surfaced. Also replace the piston rings when you hone.

          Rings: 11-25-1-713-192-M42 (you need 6)

          Rod bearings: 11-24-1-284-849-6CYL-M41

          Order everything from Steve @ blunttech, he saved me hundreds on the complete stroker rebuild.
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          Comment

            #6
            First off, kudos for measuring. Secondly, as others stated you are doing it incorrectly but your bores are probably fine. The rings only wipe so far up the bore, so you'll need to factor in ring-land height to your figures.

            Comment

              #7
              yea, I ended callin a buddy I know still in shop who brought a proper bore gauge. everything checked out okay. none of them had ovaled and oil rings shouldn't be exceeding that tapered lip at the top. Since the compression checked out okay, I'm not too concerned about the top ring exceeding the taper. I'll probably check the compression after my next oil change just to confirm I'm still in the clear. from the looks of it, I should be just fine and then some until my spare short block is done being assembled. thanks for all the advice fellas.

              -Brandon

              Originally posted by Ryan...
              It now emits a beautiful blue-ish yellow/green smoke from the exhaust?? No idea what would cause that color, but I assume its good.

              Comment

                #8
                By the way, from what I've read, piston slap/knock is due to usually one of four things... Timing too advanced. Timing too retarded. Oil has begun to demineralize. Oil pressure too low. I'm pretty sure three of these are fact. Anyone care to comment on the "demineralizing?" And also, is there any other common problems that can be diagnosed due to engine knock?

                Originally posted by Ryan...
                It now emits a beautiful blue-ish yellow/green smoke from the exhaust?? No idea what would cause that color, but I assume its good.

                Comment

                  #9
                  Shit gas?

                  I'd guess oil 'de-mineralizing' could be from a leaky injector contaminating it. Or possibly coolant/water contamination as well.

                  Comment

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