Two Stroke Big End Bearing Failiure - Ideas?

Discussion in 'UK Motorcycles' started by sweller, Oct 20, 2003.

  1. sweller

    sweller Guest

    I'm currently rebuilding an MZ TS 250/1 engine, with unknown history,
    with a seized _big_ end.

    This is fairly unusual for an MZ. My understanding of two strokes and
    big end seizures is this is usualy due to lubrication failure.

    I've posted two pictures

    The top picture shows brown discolouration, I've not really seen that on
    an MZ before. Normally it's a reasonably clean grey.

    Top picture also has ringed a small 'rubber' oil way separator, between
    the cases which wasn't as normal and appeared to be split/deformed. It
    normally blanks that passage between the cases.

    The bottom shows the result of me dropping the piston (FUCKING ARSE!!)
    but also displays more blow by burning than I'd expected.

    2 stroke gurus, why did it seize?
    sweller, Oct 20, 2003
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  2. Because it's a two-stroke? ;-)

    Odd, I admit.
    The Older Gentleman, Oct 20, 2003
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  3. sweller

    sweller Guest

    It's unusual for fairly under stressed two stroke engines to take out
    their big ends, without appreciably damaging their mains. The crankcase
    doesn't look right, certainly I've not seen an MZ like it.

    Those who know the symptoms may be able to add something? I haven't a
    clue why the case is brown. Do you?

    My experience of MZ engine/gearbox failures are pistons (ignition), mains
    (pre Supa 5 ES/TS do that), gearbox (learn to adjust chains), random
    parts (learn how to rebuild them) and selectors (they all do that, sir).
    sweller, Oct 20, 2003
  4. sweller

    Muck Guest


    Maybe he was running it on old engine oil or worse?? That blow by does
    seem to be a little bit extreme though.

    Bandit 600n(With added ducktape); CG125(MSOHPR)
    "I've got a CG125, and I'm not affraid to use it."
    DS#1 ; DOMO#1 ; SH#2 ; ICQ: 166144431
    Remove _TEETH_ to e-mail
    Muck, Oct 20, 2003
  5. sweller

    mups Guest

    The amount of blow by on the piston and what looks like sludge type stuff
    on the bottom ring plus the colour of the bearing and cases looks to me
    like somebody's filled the oil tank[1] with old engine oil or something
    similar.What's the crank like?

    [1] MZ's do have oil tanks don't they :)
    mups, Oct 20, 2003
  6. sweller

    sweller Guest

    No, petroil
    sweller, Oct 20, 2003
  7. sweller

    sweller Guest

    Should have added it's the B/End that's seized. The mains look good but
    the engine has been split recently (the last 10K or so).

    Oddly the small end is a plain bush, not sure of the history but MZ's
    only seemed to undergo major change every five years (this does not
    suprise me)

    AFAIK 1978 is not one of those years. Later, or should I say
    contemporary ('78-'79), engines had needle small ends which is what I'd
    have expected.
    sweller, Oct 20, 2003
  8. sweller

    mups Guest

    It's prolly been running on a 25:1 petrol/chipfat premix. The last owner
    didn't own a chippy did they?

    Seriously that knackered seal would it have let oil from the gearbox into
    bottom end?
    mups, Oct 20, 2003
  9. sweller

    sweller Guest

    No, the seal I referred to isn't a crankcase seal, IYSWIM. It
    deliberately blanks an oil passage. Why I don't know.
    sweller, Oct 20, 2003
  10. sweller

    Pip Guest

    i know fuckall about MZs.However, I've seen that brown discolouration
    before and attribute it to an engine that has been seriously abused by
    neglecting oil change intervals and running the level too low, leading
    to serious overheating. The brown is burnt-on oil, I reckon.

    Does it smell bad?
    Pip, Oct 20, 2003
  11. sweller

    sweller Guest

    It is a two stroke, so engine oil changes aren't exactly high on the
    sweller, Oct 21, 2003
  12. sweller

    Muck Guest

    Maybe it's been running on really shitty 2 stroke oil, or something?

    Bandit 600n(With added ducktape); CG125(MSOHPR)
    "I've got a CG125, and I'm not affraid to use it."
    DS#1 ; DOMO#1 ; SH#2 ; ICQ: 166144431
    Remove _TEETH_ to e-mail
    Muck, Oct 21, 2003
  13. sweller

    Filth Guest

    Viagra, worked for TOG, allegedly...
    Filth, Oct 21, 2003
  14. Yebbut, two-strokes don't *use* oil unless the crankcase is leaking. And
    even if it were, the big end is lubed by the two-stroke mix, not the
    gearbox oil.

    On the early four-speed MZ250s, the oil seals were inboard of the mains
    so the mains were lubed by the crankcase oil and not the two-stroke
    mixture, but even here the big end would be lubed by the two-stroke.

    The discolouration smacks of heat, though, yes. Something failed and the
    heat was caused by friction???
    The Older Gentleman, Oct 21, 2003
  15. sweller

    JH Guest

    I presume it's a varnish type deposit rather "rusty water" kind of deposit?

    Is there any of the premixed fuel left in the tank? Does it smell odd? Using
    completely the wrong oil, i.e. vegetable oil, could cause it i guess (and
    there's been a lot in the press in recent months about using cheap
    supermarket oils in diesels, perhaps previous owner thought it was a good

    If the premix ratio was widely out (far too much oil) would that lean it off
    and run hot (no, i guess not, the top end would show more probs..)

    On balance, if there's more blow by than expected, it does sound like lube
    related (no sh*t!) and i'd plump for wrong premix oil. Can't help but think
    the rubber grommet has a part to play though...

    JH, Oct 21, 2003
  16. sweller

    Sean at work Guest

    I'd hazard at using cheap oil causing bore wear/bearing wear then excessive
    blowby and thrashing causing overheating. Could overheating in the
    crankcases cause that seal to fail?

    Perhaps the PO used lead replacement petrol :eek:)

    Only time I've seen that discolouration, it has been due to cheap oil and
    excessive heat.
    Sean at work, Oct 21, 2003
  17. sweller

    Pip Luscher Guest

    In the photo it *looks* as though the case was scored, possibly by
    the crankwheel. Seems unlikely, though.

    It also sort of looks as though the geearbox was welded up at the top
    but that might just be a casting artefact. Or my eyesight.
    Pip Luscher, Oct 22, 2003
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