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    Piston to deck height?

    M20B25 block, M52B28 crank, 130mm rods, stock M20B25 pistons.

    My math says piston should be 0.5mm below deck at TDC. This is #1 at TDC. Doesn't look below deck.

    Our cam has extra 0.050 lift. I'm worried a .040 head gasket will cause valve interference.

    Need an experienced engine builder to explain.

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

    Das Beast build thread

    #2
    Not an experienced engine builder, but I think the complicated piston shape makes deck height a relative number on an M20b25. The cam timing and duration will also affect piston to valve clearance. The only way to be sure on the piston to valve, and piston to head clearance, is to use clay/solder. I can take a picture of the stock piston height, but you still need the piston to valve measurement.
    Maybe an actual experienced engine builder can chime in on how much piston rocking can affect the clay/solder readings.

    Comment


      #3
      blocks i have measured are around 206.2mm +/-0.1mm, unknown level of molestation

      so it goes like this


      stock M20B25

      75/2 + 135 + 34.2 = 206.7mm

      so the stock stack height should sit 0.5mm above the deck and with a 1.75mm gasket the clearance would be approx ~1.25mm (this is approx because the squish band is "angled" slightly. Its actually an arc shape not an actual chamfer)

      2.8L (OE parts bin)

      84/2 + 130 + 34.2 = 206.2mm

      so it should be flush.

      yours looks flush to me (near the dowel pin on the RHS of the piston in view)

      after all that you measure it all to get the real answer. the aim is 1mm clearance nominal piston to head about 1.5mm P to V can go tighter to 1mm if you're brave

      use a 0.040 mm washer/shims under the head and check
      Last edited by digger; 01-17-2021, 09:36 PM.
      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
        Here's another reality check with your data.

        0.059 target clearance
        0.039 if you're brave

        0.069 stock head gasket
        0.049 stock piston to valve clearance
        0.010 schrick 284/272 p2v clearance w stock gasket

        -0.039 extra cam lift
        0.020 shorter rotating stack
        -----------
        -0.019 less p2v clearance

        Gasket check
        0.069 : 0.049 - 0.019 = 0.030 Too close

        Stock 0.069 gasket has 0.030 clearance with our cam and stack, so:

        0.040 : 0.030 - (0.069 - 0.040) = 0.001 Kaboom
        0.080 : 0.030 + (0.080 - 0.069) = 0.041 Brave
        0.086 : 0.030 + (0.086 - 0.069) = 0.047 Not bad
        0.098 : 0.030 + (0.098 - 0.069) = 0.059 Target

        Need to return my 0.040 gasket for a 0.086

        Thanks Digger. You saved us a set of bent valves
        Last edited by dvallis; 01-17-2021, 10:57 PM. Reason: Typos
        "And then we broke the car. Again." Mark Donohue, "The Unfair Advantage"

        Das Beast build thread

        Comment


          #5
          Originally posted by dvallis View Post
          Here's another reality check with your data.

          0.059 target clearance
          0.039 if you're brave

          0.069 stock head gasket
          0.049 stock piston to valve clearance
          0.010 schrick 284/272 p2v clearance w stock gasket

          -0.039 extra cam lift
          0.020 shorter rotating stack
          -----------
          -0.019 less p2v clearance

          Gasket check
          0.069 : 0.049 - 0.019 = 0.030 Too close

          Stock 0.069 gasket has 0.030 clearance with our cam and stack, so:

          0.040 : 0.030 - (0.069 - 0.040) = 0.001 Kaboom
          0.080 : 0.030 + (0.080 - 0.069) = 0.041 Brave
          0.086 : 0.030 + (0.086 - 0.069) = 0.047 Not bad
          0.098 : 0.030 + (0.098 - 0.069) = 0.059 Target

          Need to return my 0.040 gasket for a 0.086

          Thanks Digger. You saved us a set of bent valves
          youve only got 0.5mm clearance with stock gasket and 284/272? seems a bit tight as many have used on stock engine which should be worse. You sure the cam isnt over advanced?
          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
            piston to head and piston to valve should be dealt with as separate issues.
            Mock assemble the head with the old gasket (of known compressed thickness).
            Based on your math lay 4 short pieces of solder of the appropriate diameter near the edge of the piston equally spaced.
            Assemble and rotate the motor slowly through TDC to compress the solder. Disassemble and measure.

            Now you know what head gasket to shoot for.

            I would shoot for .035-.040 piston to head for optimum squish. I would not go tighter than .030 assuming a good tight bottom end and conservative revs or strong rods.


            Check piston to valve in the same manner. with a single piece of solder. Piston to valve is corrected / adjusted via cam timing or cutting the valve pocket deeper / larger dia.
            Pay particular attention to the area the intake valve leads into the pocket.

            You can get both initial measures at the same time.

            Shoe

            Comment


              #7
              Schrick 284/272 specs here. Says 0.25mm valve clearance, so that's why I'm using 0.010 as a baseline p2v clearance.
              "And then we broke the car. Again." Mark Donohue, "The Unfair Advantage"

              Das Beast build thread

              Comment


                #8
                0.25 mm is the ‘valve adjustment’ clearance, between the valve tip and rocker arm. Similar to the stock valve adjustment clearances.

                Comment


                  #9
                  Well that changes everything. :-) Will redo my numbers with this data. Just need the stock specs to calculate a lift delta.

                  Intake Valve Lift: 11.4mm
                  Exhaust Valve Lift: 11.0mm
                  Intake Valve Lift @ TDC: 2.2mm
                  Exhaust Valve Lift @ TDC: 1.7mm
                  Valve Timing (IO-IC-EO-EC): 32-72 66-26
                  "And then we broke the car. Again." Mark Donohue, "The Unfair Advantage"

                  Das Beast build thread

                  Comment


                    #10
                    Here is the actual piston to head clearance for an M20 with M52B28 crank, 130mm ETA rods, 885 head and stock 0.069 head gasket: 0.078



                    With a stock cam we'd be able to shave ~0.030 off the head gasket and get closer to 0.040 piston to head clearance for an 8.8:1 compression ratio, 170 psi.

                    However, we're running a Schick 286/272 turbo cam with increased valve lift of 0.050/0.047 (I/E). Since the stock piston to valve clearance is 0.049 we'd have interference with a 0.069 head gasket.

                    To compensate, we increased the head gasket by 0.051 to 0.120 (answers Digger's earlier WHY?), bringing us back to stock piston to valve clearance. But piston to head clearance increases to 0.129, lowering compression to ~8.3:1. This was evidenced by our 150 psi compression.

                    Moral of the story? No free lunch. For higher compression we'll need custom pistons with +0.050 extra valve relief and 0.040 less piston to head clearance.

                    Thanks for all the input.
                    Last edited by dvallis; 01-18-2021, 02:27 PM.
                    "And then we broke the car. Again." Mark Donohue, "The Unfair Advantage"

                    Das Beast build thread

                    Comment


                      #11
                      the stock P to V is not 0.049, not sure where you got that.

                      you need to do the following with a known/verified gasket thickness or a shim of known thickness
                      - check the piston to valve clearance; and
                      - check the piston to head clearance.

                      then you can see what will work best, no guessing, no assumptions.
                      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


                        #12
                        Originally posted by digger View Post
                        the stock P to V is not 0.049, not sure where you got that.
                        Got it searching R3V. What's the correct number?
                        "And then we broke the car. Again." Mark Donohue, "The Unfair Advantage"

                        Das Beast build thread

                        Comment


                          #13
                          Originally posted by dvallis View Post

                          Got it searching R3V. What's the correct number?
                          IDK all i know people have used the 284/272 with stock m20b25 without P to V "issues",

                          you don't need to know either given you have all the bits to measure
                          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


                            #14
                            This guy measured clearance of 0.024 intake / 0.079 exhaust with a 284/272 and stock head gasket
                            Another post with 0.020 intake clearance

                            So we've probably got about 0.020 / 0.080 piston to valve, but like you said, "Measure, damn it !" Will check next session an post it here.
                            "And then we broke the car. Again." Mark Donohue, "The Unfair Advantage"

                            Das Beast build thread

                            Comment


                              #15
                              Originally posted by dvallis View Post
                              This guy measured clearance of 0.024 intake / 0.079 exhaust with a 284/272 and stock head gasket
                              Another post with 0.020 intake clearance

                              So we've probably got about 0.020 / 0.080 piston to valve, but like you said, "Measure, damn it !" Will check next session an post it here.
                              it is certainly possible that there is bugger all clearance and people get lucky. Note that the dynos i've seen of 2.8L and 3.1L both liked about 4* retard for best power pretty much across the board to ,so that would increase intake clearance and reduce on the exhaust so take note of cam timing.
                              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

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