Engine Knock Under Load

Discussion in 'Airheads' started by Boxer Metal, Apr 13, 2013.

  1. supershaft

    supershaft because I can

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    Not very common at all. I suspect those oversize bearings sold ten years ago at Ozzie's where ordered on mistake. I personally have changed a lot of airhead rod bearings and I have never seen one take oversize bearings. I have never heard of it from any mechanic I have ever worked with?

    There is still no need to plastigauge rod bearings. Measure the journals. You don't even need a mic. .010 or .020 or .030" is plenty easy enough to determine with a vernier caliper. Wrong size bearing? They will be too loose or stuck solid after you tighten the rod bolts. Who would do that?
    #41
  2. Boxer Metal

    Boxer Metal Mad Scientist

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    I agree 100%:D I'll pull the sidecar off tonight and tomorrow start pulling the top end.
    The one thing I should mention-when I first started this motor after it had been sitting for almost 10 year once it warmed up it smoked like it had a stuck ring so I added a small amount of Marvel Mystery oil to the oil and to the gas. After a few miles the smoking went away so I changed the oil, but left the fiter(BMW 20/50). The next day it started to knock.
    #42
  3. Happy Wanderer

    Happy Wanderer Day Dreaming.. .

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    My G/S developed a stuck ring after sitting on the side stand in my tight workspace for a month or more while I worked on a new seat. Smoked like a broken chainsaw. Just terrible and got even worse after warmup.

    I dumped the oil and put in 1/4 liter of ATF and 1.75 of regular 20/50 MC oil. Within a minute it stopped smoking. Ran it like that for a couple hundred kms, dumped the oil again and put in fresh stuff. No problems since! :D

    Mike at Ozzies told me about Marvel mystery oil and how it was good for oilhead fuel injectors. Never thought to add some to the motor oil. Thanks for the tip Chris.
    #43
  4. Hookalatch

    Hookalatch Born Under Bad Sign

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    This is obviously not Chris' problem and I don't want to belabor an off-topic subject but I was curious about how commonly cranks were underground from the factory and posed the question on the Airheads list. Tom Cutter and Snobum both indicated that it is not common but that it certainly occurred. Just a heads up for anyone else replacing the con rod bearings. The oversize bearing shells were not stamped with an oversize number like the automotive ones I am more familiar with. You had to look up the part number (or measure) to determine they were not standard size.

    Chuck
    #44
  5. supershaft

    supershaft because I can

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    Anything is possible and Tom might actually know. It's just that I take all info from BMWAG, B+S, and NA with a grain of salt. It has payed off numerous times!

    Chances are this isn't Chris's problem but it is very much on topic IMO.

    My point is that if there ever was a crank that came out of the factory underground to spec, it would be impossible to assemble it with a different size bearing and not notice it. The rod would either rattle on the journal or lock solid depending on if the bearing was too big or too small. It would be impossible to put an oversize bearings on a standard crank. The engine wouldn't turn over. Save your plastigauge for when it matters.
    #45
  6. Boxer Metal

    Boxer Metal Mad Scientist

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    Here is a picture of both rods.

    [​IMG]
    #46
  7. ME 109

    ME 109 Long timer

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    Nothing out of the ordinary to my eye bmweuro.
    #47
  8. Boxer Metal

    Boxer Metal Mad Scientist

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    The left wrist pin slid out a little to easy and the right took a little persuading. The left rod small end is pretty beat up.
    #48
  9. Plaka

    Plaka Brevis illi vita est

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    What do the crank journals measure?

    What does some plastigauge say about the big end clearance?

    What does the small end measure? (you replace those bearings sometimes too.)

    What do the pins measure?

    What about the piston OD, cylinder and taper?

    Bore ID, cylinder and taper?
    #49
  10. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    #50
  11. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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    It looks like the problem was not rod bearings. Also, 30 psi at 1100 rpm and 70 psi at 3000 rpm is good oil pressure. Are you sure the problem is not pre-detonation knocking? How do the heads and pistons look - is there a lot of carbon? Any indication that the valves are contacting the piston (or carbon on the piston)? Could this be a piston slap problem? Now is the time to address any and all of these (somewhat unlikely) causes. Could this 'knock' be timing chain slap? Good luck!
    #51
  12. Boxer Metal

    Boxer Metal Mad Scientist

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    I raised the needle position up one notch and now it runs a little rich and took it for a good ride today. The noise is back. So I have oil pressure, timing is correct, fuel mixture good.
    I don't think it's the timing chain because you would here it at idle. I'm starting to wonder if there is something rattling under the tan or next to the starter.
    #52
  13. Plaka

    Plaka Brevis illi vita est

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    Did you measure up everything before you put it all back together?
    #53
  14. Boxer Metal

    Boxer Metal Mad Scientist

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    The motor measured out like it had very low miles.
    #54
  15. Plaka

    Plaka Brevis illi vita est

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    Run a tank of high test. That will put detonation to bed.

    Then work on making it knock at idle. If you can find a big deserted parking lot, try it without a helmet and bounce it around. You may well find something to your 'something rattling' theory.
    #55
  16. Boxer Metal

    Boxer Metal Mad Scientist

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    I only run 91 octane and I have been riding it around the shop without a helmet for weeks trying to pin point it.
    #56
  17. Plaka

    Plaka Brevis illi vita est

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    Check plugs w/ magnifying glass for signs of detonation (typically the insulation is white with tiny beads of molten metal).


    Unless you just want a shop bike, try a parking lot, a bumpy one.

    Check the usual suspects: missing center stand bumpers, Ovalised shock bolts, loose subframe bolts, loose battery box bolts, any bolt on the front end, coil mounts, etc.
    #57
  18. chasbmw

    chasbmw Long timer

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    I don't know how old the bike is but, the original timing chain tensioner fitted to the single timing chains, used to make a bloody awful noise when worn, BMW later rejigged the design and the problem went away.
    #58
  19. Boxer Metal

    Boxer Metal Mad Scientist

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    Update.

    I checked the jetting and 170 mains, 45 idle. I raised the needle one notch. Still runs fine and I can start it without the chokes on. I pulled the plugs and they look a little on the lean side. The motor is a 1976 R90/6 with 40mm Bings(odd combo).
    Still trying to find the noise.
    #59
  20. chasbmw

    chasbmw Long timer

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