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Thread: Rod side clearance: opinions needed

  1. #1
    I'm putting together my stupidly expensive 540 BBC (Merlin tall deck, Diamond blower pistons, Eagle rods, Scat crank) and discover my rod side clearance is 0.028 to 0.029. Way more than the recommended clearance in the range of 0.018 to 0.020. The bearing clearances are fine, 0.0025 all rods. So, how much will excess side clearance screw things up? Too much oil spewing out between the rods, lowered oil pressure, blah blah but is it worth getting another crank? Also, the deck height was set at 10.185, and now at assembly the pistons are still 0.017 in the hole! I'm guessing that the stroke is not actually 4.25 or the rods are short. Lesson learned: CHECK EVERYTHING AND ASSUME NOTHING! AFTERMARKET PARTS ARE OFTEN WRONG!

  2. #2
    spectras only
    DJ ,if everything else is within specs,I only expect a drop in oilpressure ,mainly at idle and full operating temperature,diff opinions are velcome http://free.***

  3. #3
    Detroit Jim,not to get ya in a HUFF,but with all the time I've spent with the local racers (ears wide open)...Scat crank is a no no.Especially on a Blower motor.Quality control issues? Thats were your clearance may have gone.

  4. #4
    Run it, .028-.029" side clearance is fine. I run .035" on my rods, no problem and oil pressure is fine.

  5. #5
    It will work. When you build or freshen up many engines, you encounter many situations that you're not pleased with, but you have to live with it. Too much rods side clearance will throw more oil in the cylinders but .028-.029 is not .050. Some times we get nut with some numbers and when we try the thing, we are surprised at how good the results are.
    Rod to rod clearance is not the worst where you see oil pressure loss, I mean not when you talk about .029 instead of .025. Oil pump, distributor shaft to block clearance, lifter type, crank end play are also factors that can affect oil pressure.
    In your case, may be your rods are worn out, tired or simply not right. Or may be the crank...Don't throw away parts for that small clearance difference.
    With no offense, may be your measurements are not exacts, or may be your tools...

  6. #6
    Thanks for all opinions. That's what this board is about. This clearance is probably not a big deal, but I'm used to delivering 100% perfect tooling, gages and fixtures to the Big Three and I question everything not dead nuts. Same applies to my engines. All parts are brand new, clearance was checked with a feeler gage, the rods are perfect so maybe aftermarket stroker cranks are made with more side clearance built in. If so, fine with me. So, what does rod side clearance do? Obviously for expansion but my math shows about 0.004 growth due to heat. Need some clearance between the rods since they rub against each other on the side faces. So it seems that the gap between the rods mostly gives an exit for the oil to flow across the bearing journal and cool it. Then the oil is thrown onto the cylinder walls for lube and cooling. More rpm, supercharged, more load, need more oil. Should be just right.
    DeetroitJim O.O.A.E. (Overly Obsessive Automotive Engineer)

  7. #7
    My take on this mater is a little common sence. While I do agree that there are tolerances nesessary + or -, and that one should not be too anal, the extremes are their for good reason!! If you are building a fresh engine with tolorances of a old worn out one you can expect balls out performance for a very short period of time (two seasons of, THIS HAUL'S ASS! ) however due to the financial investment of the engine you are building don't you think you owe it to yourself to at least hold proper spec's? When you do loose a rod bearing (<~~facts of loose performance) the first thing that you are going to say is, damn why didn't I fix that rod clearence! I have to agree with "phillyray" chances are the guy that machined your crank thought it was for a VW didn't speak english, and your crank will be FARFRUMWERKEN in no time.
    [This message has been edited by bajaruner (edited August 31, 2001).]

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