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Am I insane? M20B30 budget (sub $200) piston option?

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    Am I insane? M20B30 budget (sub $200) piston option?

    Need some input from those who have more sanity than I do. Looking for pistons for my m20b30 build, with an 85.8mm stroke and 86mm bore. Here are measurements between stock m20b25 and the setup I'm thinking about using.

    m20b25:

    Crank throw = 75

    Rods = 135

    Piston KH = 34.2

    Total stack height from center of crank to top of piston = 206.7

    m20b30 concept

    Crank throw = 85.6

    Rods = 130

    Piston KH = 33.53

    Total stack height from center of crank to top of piston = 206.33 (0.37mm lower than OEM, well within block deck height tolerance. Basically identical)

    So with this new setup, I would end up with a flat top piston with same diameter wrist pin, and same size compression rings. CR would be between 9:1 and 10:1 depending on dish volume which is currently unknown, but small. The piston is cast aluminum and costs $170 for two sets of four.


    https://www.summitracing.com/oh/part...jR7rWt17lRClX0

    Any thoughts?
    "Tires are our fuse" -Matt Juengel
    1986 325e Sedan | M20B27 parts available for sale |

    #2
    One thing to remember when building a stroker is that the piston skirt length also matters for the m20. With the increased stroke, sometimes the clearance of the piston to crank counterweight becomes a problem. I'm no expert on this so take that with a grain a salt. But also, don't be cheap when building an engine.

    Instagram: Reichart12

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      #3
      you'll need to clearance the skirts and add valve reliefs but should physically work with some mods
      personally id rather adapt stock ones even though they are only 84mm. quality not quantity
      89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

      new build thread http://www.r3vlimited.com/board/showthread.php?t=317505

      Comment


        #4
        Originally posted by Reichart12 View Post
        One thing to remember when building a stroker is that the piston skirt length also matters for the m20. With the increased stroke, sometimes the clearance of the piston to crank counterweight becomes a problem. I'm no expert on this so take that with a grain a salt. But also, don't be cheap when building an engine.
        Already aware of this issue, and it would indeed be an issue. I'm a machinist, and have all the equipment to properly modify these pistons in my home shop. Including adding valve reliefs.
        "Tires are our fuse" -Matt Juengel
        1986 325e Sedan | M20B27 parts available for sale |

        Comment


          #5
          You could look for some good used OE Pistons from another model to
          89 E30 325is Lachs Silber - currently M20B31, M20B33 in the works, stroked to the hilt...

          new build thread http://www.r3vlimited.com/board/showthread.php?t=317505

          Comment


            #6
            I've heard bits and pieces of people trying to use an M54 rotating Assembly, but II'm not sure how well that combination would work. Seems to be the "cheapest" way to get to 3L from what I've read, but others on here have way more experience than I.

            This thread may be of interest: https://www.r3vlimited.com/board/for...ld-in-progress

            Comment


              #7
              Originally posted by sert57 View Post
              I've heard bits and pieces of people trying to use an M54 rotating Assembly, but II'm not sure how well that combination would work. Seems to be the "cheapest" way to get to 3L from what I've read, but others on here have way more experience than I.

              This thread may be of interest: https://www.r3vlimited.com/board/for...ld-in-progress
              I use the same rotating assembly as described in zoomer's thread. M54b30 crank and rods, shaved m54b25 pistons. Block needs a bit of clearancing as mentioned in the first post. Be careful not to go too deep into the cylinder wall. Generally worked out for me. Combustion is not ideal, but works. Zoomer's engine died because of assembly error. Mine rattles right now because god knows why, most certainly some stupid user error as well. But I did get a few thousand kms out of it. There is another guy in Romania who used the same setup , that I know beats regularly on it and still hasn't blown up.
              '85 Alpine Weiß 2-door with m20b30 ground up build

              Comment


                #8
                I am taking shaved M54 pistons that came out of a running/driving M20b30 and will be selling on here as soon as the machine shop disassembles the engine that you could theoretically just drop in if you're on a budget. Engine only has ~1K on them since being re-reinged and shaved to fit int the M20.
                1989 Zinnoberrot M3 Build Thread
                1990 Brilliantrot 325iS Build Thread

                Comment


                • Andre3127
                  Andre3127 commented
                  Editing a comment
                  PM me if you want to sell them. But, my build will have an 86mm bore and m54b30's bore is 84mm. It would not work with my crank unless I got the bigger crank.

                #9
                Your stack height looks close. Mine was 206.2. Make sure you clay the pistons during assembly and measure valve clearance. M20 is an interference engine and will bend valves if you screw up. Trust me, I know. :-)

                I used a M20B25 block, M52B28 crank, 130mm ETA rods, M20B25 pistons with shaved skirts, 140 mil mls head gasket and balanced the rotating assembly to +\- 1g per piston.

                Das Beast
                https://www.r3vlimited.com/board/for...19#post6548819
                Last edited by dvallis; 09-21-2019, 10:37 AM.
                "And then we broke the car. Again." Mark Donohue, "The Unfair Advantage"

                Das Beast build thread

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