Freshly built M20 stroker: oil problem

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  • _lestat_
    Member
    • Oct 2012
    • 44

    #1

    Freshly built M20 stroker: oil problem

    Hi

    Just rebuilt an M20 engine and converted into a M20B29 stroker using the M52B28 crank with Mahle motorsport pistons, forged internals, new flowed and ported old stock 885 head with all modifications you can think of and so on.

    This week we started it up for the first time on Motul 10W40 mineral break in oil (car has not driven yet) but we immediately saw a lack of oil pressure. Cold we have 72psi at 2000 RPM. Warm on 2000 RPM we have 26psi, 3000 RPM 35psi, which is not enough.
    - Crankshaft bearings (ACL) tolerance 0,0027inch. Due to the application chosen for this but still within BMW specs.
    - Conrad bearings (BMW) tolerance 0,0015inch, which is well within specs.
    - Intermediate shaft bearings were also replaced as these where shot. Placed new ones with tolerance 0,0017inch or 0,045mm. My mechanic rebuilt engines for a living (S50, S54, M30), however never replaced intermediate shaft bearings on an M20 so this is the only thing we didn't ourselves.
    - oil pump is new.

    We already replaced the pressure relief valve by a new one, replaced the new oil pump by a spare one, checked the two pumps internally but everything is ok. Headwise everything is also ok, however we still have this lack of oil pressure. We tested also with different oil pressure meters, which all show the same result.
    As for our understanding we don't think that some oil wholes should be plugged in this stroker setup. I also don't think that this is a problem from using break in oil, as viscosity is 10w40.
    The only thing we can think of is the only thing we didn't do ourselves: the intermediate shaft bearings. The person who did this used honing stones, however I must admit it was fairly rough done, as you can see in the pictures. I still remember that I felt the contours with my fingernail.
    Could this result in the problem we have?
    I have a spare of new bearings, so we are thinking of placing them ourselves this time. I already sent a pm to #ForcedFirebird so I hope he reads it as he manufactured some tools to do the job. I know he uses 0,001inch (0,026mm) tolerance so this is different from the 0,0017inch we applied. What do the racers here to increase oil pressure?

    Anyone else who's thinking at other causes or already had this problem with the shaft bearings?

    Thanks for you help.

    Gregory
    Attached Files
  • ForcedFirebird
    R3V OG
    • Feb 2007
    • 8300

    #2
    I can answer publicly so that others with issues can reference. :)

    Your pressure numbers are that of a very tired engine! We consider them ready for rebuild when we have less than 10psi per 1000rpm.

    That tolerance is large on the intermediate bearings. BMW calls for .02mm, or .0008". The smaller a journal is, the tighter the oil clearance needs to be.

    Your rods are fine. I typically see .001-.0015" (.0254-.038mm) and a max of .002" (.05mm) on the mains, yours seems a little loose. The block machinist should have shaved the underside of the main caps and align bored the mains, or mix-matched the colored BMW bearings.

    A "rule of thumb" in engine building is .001" oil clearance for every 1" of journal diameter. So for example the rod journal is 1.77", so you will want to target .0017" max, mains are 2.36", so you want a .0023" max.

    The intermediate shaft is 1.43", but BMW specifies .02-.03mm (.0008-.001"), probably because of the slot in the bearing that feeds the distributor dummy gear spits some oil. My bearings have a narrower slot to help keep more oil going to the crank.

    We have been able to modify the oil system to get a lot more pressure. The oil relief has a piston that is staked inside it with a spring. Cut the stakes off, put 2mm thick worth of washers behind the springs, and stake the piston back in. Alternatively, you can block that hole entirely as the pump itself also has a relief - so I remove the c-clip in the pump relief, add an m20 head bolt washer and reassemble. This will raise the relief to ~85psi from ~65psi. Doing this on a fresh engine, we will see 90psi cold start, 25psi at hot idle and 80psi at 7000rpm.



    Last edited by ForcedFirebird; 10-08-2022, 11:08 AM.
    john@m20guru.com
    Links:
    Transaction feedback: Here, here and here. Thanks :D

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    • _lestat_
      Member
      • Oct 2012
      • 44

      #3
      Thanks for the answer.

      We will replace the intermediate bearings with clearance of .026mm.

      The main bearings still fall within spec (.03mm-.07mm equals .0027") however I must admit .05mm or .002" would have been better and sits in midrange.
      We played initially with colored BMW bearings, however tollerances were .04mm. After multiple opinions of builders we decided this was to tight as the car will be used for hillclimb / trackdays. After break-in we will use 20W50 oil.
      I thought Bimmerheads uses even bigger tolerances then .0027" for their turbo builds.

      Regarding the oil pressure, If I understand correctly you do 2 things?
      - In the oil relief the stakes are cut off and replaced by 2mm thick washers behind the springs. (hole should not be blocked entirely)
      - you remove the c-clip in the pump relief and add an m20 head bolt washer

      Pressure will increase, but flow is not increased, is this correct?
      Does anyone have experience with an M21 oil pump from a 524td? part number 11411275093. Some people say this pump is a bolt on fix which gives more flow then an M20 pump?

      Comment

      • ForcedFirebird
        R3V OG
        • Feb 2007
        • 8300

        #4
        Some of my friends/customers have blocked the oil reliefs off completely. They see about 100psi as a dead head. My shop caters to local racers, primarily endurance.

        We use LiquiMoly 10w60, same spec as the s54 if you look for a substitute.

        Not sure what other shops do, can only share my experiences.

        I do not have experience with the m21 pump, but would be curious to take one apart and see the differences. It's hard to get authentic m20 pumps these days. I've been rebuilding them.
        john@m20guru.com
        Links:
        Transaction feedback: Here, here and here. Thanks :D

        Comment

        • _lestat_
          Member
          • Oct 2012
          • 44

          #5
          So for endurance racers you also apply main bearing tolerances of 0,04mm-0,05mm (0,0015"-0,002") together with the use of 10w60? This is not to tight for engines who are seeing alot of 5000 to 7000rpm?

          Comment

          • ForcedFirebird
            R3V OG
            • Feb 2007
            • 8300

            #6
            When running races that long in the tropical heat here in Florida, it's not uncommon for us to see 300f peak temps on the oil pan.

            The s54 has the same oil clearance as m20, but the crank journals are larger, so theoretically s54 should use thinner oils. The s54 specs 10w60, so we started using it with good results. Prior to that we used Mobil1 15w50, but it got really thin and we would lose pressure once it reached much over 275f. Conventional oil turns to water after 260f.

            I can only give you my experience. YMMV

            john@m20guru.com
            Links:
            Transaction feedback: Here, here and here. Thanks :D

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