Two Stroke Big End Bearing Failiure - Ideas?

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
http://www.sweller.co.uk/sob/xeu-eng.htm

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?

 

Because it's a two-stroke? ;-)

Odd, I admit.

 

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).

 

<snip>

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

--
Muck
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

 

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

 

No, petroil

 

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.

 

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?

 

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

 

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?

 

It is a two stroke, so engine oil changes aren't exactly high on the
agenda.

 

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

--
Muck
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

 

Viagra, worked for TOG, allegedly...

 

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???

 

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
idea.

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

 

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 )

Only time I've seen that discolouration, it has been due to cheap oil and
excessive heat.

 

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.