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3.0L, 84mm bore, 89.6mm crank?

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    3.0L, 84mm bore, 89.6mm crank?

    I'm rapidly gathering parts together for my 3.0L M20 build, first step was obtaining a complete M54B30. I'm saving the crankshaft and rods, hoping to use the stock M20B25 pistons as I'd rather not change the bore. I recall reading about this setup quite a while ago, but haven't seen any real details in searching for the last few weeks.

    I know MM uses the B30 crank and rods along with their 86mm pistons for their 3.2L builds... would the same crank/rod setup with 84mm pistons give me working 3.0L? Or are the stock B25 pistons too tall with those rods and that crank?

    Thanks for any knowledge, I'm planning on testing and confirming anyways for future builders but any tips going in are greatly appreciated.
    Last edited by Raxe; 03-08-2012, 10:23 PM.

    >> 1988 3.1 ITB E30 /// 2002 E46 M3 6MT / 2001 E38 740i Sport / 1991 S38B36 E30 (sold)

    #2
    You will need custom pistons of some sort I'm pretty sure.
    -Alex

    Comment


      #3
      For what it's worth, I tried it out yesterday... B25 pistons obviously didn't work at all as the skirts are too large. M54 pistons on the other hand turned fine with the M54 rods and M54 crank, however at tdc they're about 1mm too high.



      Oh well, pretty interesting that it's so close anyways.

      >> 1988 3.1 ITB E30 /// 2002 E46 M3 6MT / 2001 E38 740i Sport / 1991 S38B36 E30 (sold)

      Comment


        #4
        subscribed! looking for info on this as well...
        sigpic
        1986 "C2 2.7 Alpina" Sedan
        1987 325ic Black Vert
        1986 327i Red Track Car RIP 10/10/10
        1989 325is Henna S50 track car SOLD


        Transaction feedback here please!!
        http://www.r3vlimited.com/board/showthread.php?t=170548

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          #5
          You could have the pistons machined down probably ($$$$) but the biggest concern IMO is valve/piston clearance, no? Those pistons are made for 4 valves per cylinder, and doesn't have the valve reliefs like M20 pistons
          -Alex

          Comment


            #6
            you need the piston dome to suit the 885 chamber. IMO get some pistons from JE they will replicate the correct dome
            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


              #7
              you can machine the M54 pistons to work - the crowns are really thick. you'd obviously cut M20 valve reliefs in it.
              Build thread

              Bimmerlabs

              Comment


                #8
                Chris,
                could the m20b25 piston skirts be machined to suit easier than machining the crowns of the m54 pistons?
                Or do the m20b25 pistons still sit too proud of the block?
                I haven't had any of these combinations together in a block. I can guess that the stock m20b25 pistons might require a lot of removal to clear the weights...but would they work at all?

                Maybe I'll just get the m54 pistons and put them on the mill.
                Port matching and >272 cam help with gains?
                RD headers?
                Last edited by supernaughtIX; 03-28-2012, 04:48 PM.
                88 325 IX DiamantSchwarz - TOURING - waiting...
                89 325 IX AlpinWei├č - Gone
                88 325 IX DiamantSchwarz - Parted
                89 325 IX RoyalBlau - Parted
                88 325 IX RoyalBlau - Gone
                88 325 IX DiamantSchwarz - Parted

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                  #9
                  I think it would be better to machine the M54 pistons regardless. also, I'm certain the M20 pistons won't work, the pin height will be way off with an 89.6mm crank. the rod length is the same, so the pistons would end up sticking way out of the block.
                  Build thread

                  Bimmerlabs

                  Comment


                    #10
                    Took a comparison pic of the M20 and M54 pistons for reference.


                    >> 1988 3.1 ITB E30 /// 2002 E46 M3 6MT / 2001 E38 740i Sport / 1991 S38B36 E30 (sold)

                    Comment


                      #11
                      yeah, there's no way the M20 piston is going to work.

                      see this thread:
                      http://www.e30tech.com/forum/showthread.php?t=75938
                      Build thread

                      Bimmerlabs

                      Comment


                        #12
                        Originally posted by digger View Post
                        you need the piston dome to suit the 885 chamber. IMO get some pistons from JE they will replicate the correct dome
                        Yes! I honestly don't get why people think that it is acceptable to not match the two. Even and complete combustion is not only better for the longevity of the motor, but it is essentially more efficient.
                        sigpic

                        A man chooses, a slave obeys... Would you kindly?

                        Comment


                          #13
                          Provided that machining 1mm off the top of the pistons didn't take off so much material it compromises the integrity of the piston the process itself would actually be very simple on a lathe.

                          The valve recesses shouldn't be too much of a challenge either.

                          Originally posted by pantelones View Post
                          Yes! I honestly don't get why people think that it is acceptable to not match the two. Even and complete combustion is not only better for the longevity of the motor, but it is essentially more efficient.
                          If that's the case then why aren't eta pistons or any other M5x/S5x piston domed but yet the cylinder head is?

                          The dome is there to create a target compression ratio not even out the combustion. You can create a similar effect by aggressively decking the cylinder head.

                          EDIT: Also, you might have to machine out some material from the lower side wall of your block for the bottoms of those rods to clear similar to how the IE 3.1l stroker kit does.
                          Last edited by RobertK; 03-29-2012, 08:13 AM.

                          Comment


                            #14
                            Originally posted by RobertK View Post
                            Provided that machining 1mm off the top of the pistons didn't take off so much material it compromises the integrity of the piston the process itself would actually be very simple on a lathe.

                            The valve recesses shouldn't be too much of a challenge either.



                            If that's the case then why aren't eta pistons or any other M5x/S5x piston domed but yet the cylinder head is?

                            The dome is there to create a target compression ratio not even out the combustion. You can create a similar effect by aggressively decking the cylinder head.

                            EDIT: Also, you might have to machine out some material from the lower side wall of your block for the bottoms of those rods to clear similar to how the IE 3.1l stroker kit does.
                            M5x use a pentroof combustion chamber, and a centered spark plug. M20 885 is hemispherical with an offset spark plug. the shape of the M20 piston is related to that.

                            there's tons of material to machine off the M54 pistons. just go read the link I posted.

                            I think you only need to clearance the block for the 84mm M20B28 crank, which is actually larger in diameter than the S52/M54 crank?
                            Build thread

                            Bimmerlabs

                            Comment


                              #15
                              Originally posted by nando View Post
                              M5x use a pentroof combustion chamber, and a centered spark plug. M20 885 is hemispherical with an offset spark plug. the shape of the M20 piston is related to that.

                              there's tons of material to machine off the M54 pistons. just go read the link I posted.

                              I think you only need to clearance the block for the 84mm M20B28 crank, which is actually larger in diameter than the S52/M54 crank?
                              Completely forget about the spark plug orientation but still, the eta pistons and aftermarket pistons that come with the IE kits are not domed so what's up with that?

                              http://www.iemotorsport.com/bmw/E30-engine/m20-32.html

                              According to IE the 89.6mm stroke of the S52/M54 crank is what has to be accomodated. I think you are thinking of the counter weights on the M52 crank that must be shaved to clear stock "i" piston skirts.

                              Comment

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