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    M20 Rebuild Help

    Have an 87 325is and a year ago lost compression on a couple of cylinders. After a tear down saw that the bores were grooved due to ring failure and a few months ago purchased the motor out of a 86 325 eta that had good compression.

    Tore down the 86 block and cleaned everything up. All looks very good from the cylinder bores to the bearing areas on the block and crank. I tried looking up the numbers stamped into the bearings for replacements however I cannot find any that match. Could someone please help me find a good source for all bearings as well as how I can identify the ones I need?

    The head from my 87 325is is in good condition and I will be using it, however I read that the i cylinders on models that have metal bumpers (my version) require that the i cylinder skirts be machined otherwise they will not work with the e crank. Is this true as I only read this on one of the many posts?

    As I understand the i cylinders were designed to work with the shape of the i head… however from what I read, I can use the e cylinders with the i head which will end up with lower compression. Considering that this will again be a turbo application… would using the e cylinders be a bad decision?

    And finally… could someone assist in a good source for the cylinder rings as well as how I can identify the ones I need?

    I apologize in advance for the NEWB questions. Although I did all of the work to get my E30 going Turbo with MS this is my first engine rebuild. I know that this effort could cost a ton of money depending upon the work done… but I don’t have a big budget and really want to get new bearings and rings on her, and get up and going again. Stock i engine with 100K already on the clock lasted 5 years as a daily driver post turbo… if I get another 5 out of a “freshened” engine I will be a happy camper.

    Thanks in advance.

    #2
    There are numerous threads that outline your questions but for a quick answer, if you are going to turbo it you would want to use the eta block, crank, rods, and pistons. The eta pistons give you lower compression than the i pistons which gives you more headroom for boost.

    Search and read the forum here, many people have done this and documented it pretty well.
    My son has the 1987 325e, 2 door, 5speed
    I daily the 1989 325i, 4 door, 5speed

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      #3
      I would suggest using the "i" pistons, the m20b25 is only 8.8:1 stock, so no need to lower compression any more, plus the matching pistons/head shape always makes more power.

      For the bearings, there are only two types. Early engines had a single tang on the main bearing cap, later models have two tangs. I have been using aftermarket Glyco and/or KS bearings for years without issue.

      Rings you need will depend on which pistons you go with. The b25 uses thicker rings than the b27's (even the seta used the thinner rings).
      john@m20guru.com
      Links:
      Transaction feedback: Here, here and here. Thanks :D

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        #4
        Thanks... Just ordered the Glyco with double tang and GOETZE rings for the E pistons. I know I am in for less compression... but I would need to tear down the I block and pull and clean the I pistons provided they didn't get messed up when my rings went. I know the E pistons are pristine and cleaned and ready to go. Also I'm still not sure on whether the I skirts need to be machined for clearance since I have the metal bumper 87 235IS. I saw that in an online article... but not sure whether that is true or not.

        None the less for 260$ I got those parts and hopefully that and a gasket kit gets me up an going in my E30 for 5 more years. Thanks John... I think you emailed me before on this projects but I'm starting it next weekend after my parts come in. I'm the guy who lives in Indian Rocks Beach.

        Thanks again...

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          #5
          These should help. You can also see a lot of detail on engine rebuild from the Das Beast blog. Start on this page



          "And then we broke the car. Again." Mark Donohue, "The Unfair Advantage"

          1987 E30 3L Turbo Stroker Das Beast
          2002 E39 M5

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