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  • main cap bore specs

    Hi,


    Does anyone have crankcase main cap bore diameter spec/tolerances? without bearing
    -I'm looking for bore diameter +/- tolerance and also out of round specs

    Mains were recently align honed and I want to double check my numbers.

    TIA,

    Joseph
    Joseph

  • #2
    I was hoping someone would weigh in...but I guess not.

    I don't know the main cap specs...but I know with S14 builds people generally do not align bore. The tolerances are normally tight enough as the blocks come from BMW that it is not necessary. Much of the "engine blueprinting" steps engine builders follow were established in the days when people where building hot rod motors from iron block American V-8s....which had very loose and inconsistent tolerances.

    My question....if a block has been align bored...and any measurable amount of material removed...how does one obtain adequate bearing crush? Typically building an S14...no one measures main cap bore diameter...they just measure bearing crush. It seems like if the caps have been align honed, you would need thicker bearings....which AFAIK are not available for the S14...unless you are talking a smaller inside diameter for a re-ground crank.

    Or maybe your shop just told you they align honed it to make you feel good....and didn't do it?

    Maybe Ron or one of the other engine builders will weigh in.
    Last edited by Ironhead; 03-26-2018, 04:52 PM.

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    • #3
      I am not 100% sure in this but think the main caps are removed and the flat surface of the cap face is machined to first make the bearing hole oval shape when bolted back on to the block then the honing tool is centred and honed until bearing ID diameter is round and the correct ID, which would end up at same main cap bore diameter as what it started out at +- a few tenths of a thousand inch.

      One of the reasons as Brian mentioned the machine tolerances from factory are still very good even by todays standard that combined with the two different thickness bearings RED and BLUE the crankshaft oil clearance can be adjusted to with in spec for a rebuild.

      Red bearings in the Block and if required red or blue in the main caps.

      I think you would have to get a dimensional drawing of the block to get a definite answer to your question.
      Dave.

      E30 M3 1987
      Mini Clubman GT
      BMW E36 323 Msport
      Toyota Corona
      KTM 200EXC
      Honda CB50 (1979)

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      • #4
        You are probably correct Dave....as I know that is more or less how machine shops repair con rods that are out of round.

        I do know that with BMW, Porsche, and most Japanese engines there is rarely anything to be gained by align honing.....unless the block had severely overheated or otherwise been damaged....in which case replacing it (if possible) is generally the best course of action.

        Comment


        • #5
          Thanks for the reply Ironhead and dave.

          My engine suffered oil pressure loss and loss of all main and rod bearings. The main caps #2 and 3 were slightly loose - normally they are a snug fit but I could wobble them back and forth. The machine shop repaired the caps by welding small amount of material to the edge and then ground the caps to make them square and straight, then align honed the main line bore straight again. They are very experienced in cast iron blocks - but more american stuff. I measured the main bores and they are around 0.0003-0.0005'' larger in the vertical dimension than before, and around 0.0008 wider at the parting line. Bearing crush is around 0.003 to 0.004 which looking at the sig email list seems ok. Main bearing clearance measures at 0.0025.

          From what I can gather reading some technical articles from Mahle, the bore diameter variance seems to be ok, but its nice to have a spec sheet to compare to.
          Joseph

          Comment


          • #6
            Hi, The bearing crush sounds good but I am not so sure on the 0.0025Ē clearance for a road car, possibly a re-measure with the shells bolted in the block at the correct torque. I take it you are using out side micrometer and telescopic gauges not venier callipers.

            Blue bearing shells can be brought from BMW to reduce the clearance and installed in the bearing caps.


            E30 M3 1987
            Mini Clubman GT
            BMW E36 323 Msport
            Toyota Corona
            KTM 200EXC
            Honda CB50 (1979)

            Comment


            • #7
              Hi Dave. Itís for a race engine. I ordered red bearings specifically to open up the clearance. Iíll be running 10w50 oil. Clearances are measured using a micrometer on the crank journal and using that measurement to zero the dial bore gauge. What are your thoughts on 0.0025 with that oil weight for a race engine. Iíll be using a dtm oil pump.

              Joseph

              Comment


              • #8
                I must admit I don't have experience building race engines and I have heard the clearances are larger at mains and big ends to reduce friction, I have looked in the group A manual they list 0.04 to 0.08mm which is 0.0016" to 0.0031Ē so based on that and the good work your engine reconditioner has done 0.0025 should be ideal.

                E30 M3 1987
                Mini Clubman GT
                BMW E36 323 Msport
                Toyota Corona
                KTM 200EXC
                Honda CB50 (1979)

                Comment

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