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Piston and Cylinder Condition - MZ 125

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oriel dave
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Joined: 29 Jun 2006
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PostPosted: 22:33 - 11 Jun 2010    Post subject: Piston and Cylinder Condition - MZ 125 Reply with quote
Hi,

I bought this bike as a project - it's been sat for years and the piston had seized to the cylinder. I've freed it with a lot of hammering but am now unsure as to how bad the condition of the piston and barrel is.

The main damage is the ring of built up rust in the cylinder barrel and the piston looks quite marked. Other than this the surfaces are pretty smooth, no bad scoring and I think the piston moves ok in the barrel apart from in the rusty bits.

Lots of photos:

https://i763.photobucket.com/albums/xx271/oriel_dave/mz/web000.jpg

https://i763.photobucket.com/albums/xx271/oriel_dave/mz/web002.jpg

https://i763.photobucket.com/albums/xx271/oriel_dave/mz/web005.jpg

https://i763.photobucket.com/albums/xx271/oriel_dave/mz/web011.jpg

https://i763.photobucket.com/albums/xx271/oriel_dave/mz/web013.jpg

https://i763.photobucket.com/albums/xx271/oriel_dave/mz/web009.jpg

more photos

Is it possible to remove the rust in the cylinder (eg. by careful sanding) and do the piston / rings look ok?

Thanks,

Dave
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MrJames
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PostPosted: 23:34 - 11 Jun 2010    Post subject: Reply with quote
Get the cylinder rebored and fit a slightly oversized piston Smile
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evoboy
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PostPosted: 00:10 - 12 Jun 2010    Post subject: Reply with quote
Might get away with just a hone to the barrel, if there are no deep scores.

Piston and rings defo need replacing and possibly the small end bearing if its been rusty in there.
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Teflon-Mike
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Joined: 01 Jun 2010
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PostPosted: 02:14 - 12 Jun 2010    Post subject: Reply with quote
evoboy wrote:
Might get away with just a hone to the barrel, if there are no deep scores.

Piston and rings defo need replacing and possibly the small end bearing if its been rusty in there.


Honing the bore was my first thought, too, but I had a think about the piston & rings.

First off its an MZ. Second up its a two-smoke. Third up, parts in question are presumed dead anyway...

So what MORE harm could you do?

If you have a honing arm, I'd give it a go. Failing THAt, sounds horid, but east-german tolerences were't exctlt 'precise...

FINE wet & dry, wetted with thin oil. Use a large area and work it evenly round the bore.

Rings susted in the piston gfrooves will be a bugger, BUT, soak in penatrating oil.

See if you can tap the ring-pins out from inside the pistonso the rings can be turned in the groove, and then fiddle & riggle them to see if they loose.

again, fine oiled wet & dry to clean up the ally piston, & gentle attension around the rings might get them free.

If so, remove, clean, and oil-wet & dry on thr rings & the rest of the pistion.

Assemble back up with LOTS of 2-stroke oil to lube it; turn it over gently by hand, plug out to work it through...

THEN pup the plug back in, make sure the points are clean, and give it a kick... see how it goes!

You NEVER KNOW!

These engines are very tolerent of 'brutal' mechanics, and the worst that can hapen is the bores so far gone one of the rings snags and drags gouges down it....

But as its PRESUMED it needs a rebore ANYWAY... that would have been on the cards.

Best case, it works, job sorted for little more than time, patience and bit of sand-paper and old oil!

I'd give it a go!
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Paddy Blake
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Joined: 29 Jun 2006
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PostPosted: 02:38 - 12 Jun 2010    Post subject: Reply with quote
Teflon-Mike wrote:
evoboy wrote:
Might get away with just a hone to the barrel, if there are no deep scores.

Piston and rings defo need replacing and possibly the small end bearing if its been rusty in there.


Honing the bore was my first thought, too, but I had a think about the piston & rings.

First off its an MZ. Second up its a two-smoke. Third up, parts in question are presumed dead anyway...

So what MORE harm could you do?

If you have a honing arm, I'd give it a go. Failing THAt, sounds horid, but east-german tolerences were't exctlt 'precise...

FINE wet & dry, wetted with thin oil. Use a large area and work it evenly round the bore.

Rings susted in the piston gfrooves will be a bugger, BUT, soak in penatrating oil.

See if you can tap the ring-pins out from inside the pistonso the rings can be turned in the groove, and then fiddle & riggle them to see if they loose.

again, fine oiled wet & dry to clean up the ally piston, & gentle attension around the rings might get them free.

If so, remove, clean, and oil-wet & dry on thr rings & the rest of the pistion.

Assemble back up with LOTS of 2-stroke oil to lube it; turn it over gently by hand, plug out to work it through...

THEN pup the plug back in, make sure the points are clean, and give it a kick... see how it goes!

You NEVER KNOW!

These engines are very tolerent of 'brutal' mechanics, and the worst that can hapen is the bores so far gone one of the rings snags and drags gouges down it....

But as its PRESUMED it needs a rebore ANYWAY... that would have been on the cards.

Best case, it works, job sorted for little more than time, patience and bit of sand-paper and old oil!

I'd give it a go!


Long story short.
Hone the cylinder,heat the piston to get the rings out then sand the piston with wd40 and p120 sand paper.
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27cows
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Joined: 01 Nov 2009
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PostPosted: 05:43 - 12 Jun 2010    Post subject: Reply with quote
I've had bores like that. Don't even bother trying to rescue it. Get a rebore and save yourself endless hassle. Besides which, that piston has seen way better days Shocked
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Pete.
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PostPosted: 06:33 - 12 Jun 2010    Post subject: Reply with quote
Hard to tell from the pictures if the barrel is rust-pitted. If it is, you need to bore & hone or replce it.

I don't see why spares should be any morer difficult to find than usual. Take a look around for oversize pistons and agood used piston 7 barrel and buy whichever comes up first.
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temeluchus
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PostPosted: 07:33 - 12 Jun 2010    Post subject: Reply with quote
https://www.startright.co.uk/index.php?main_page=product_info&cPath=145_165_259&products_id=11083

first oversize for an ETZ 125.

Also has 2nd oversize.

If its a TS125 parts are harder to come across, but can be found.
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stinkwheel
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PostPosted: 08:00 - 12 Jun 2010    Post subject: Reply with quote
Teflon-Mike wrote:


So what MORE harm could you do?


Well...

From past experience on my Jawa (and a mates experience on his MZ), it could nip up again just enough for the small end needles to make a break for freedom from a wrecked piston but not quite enough to stop it running instantly. In the intervening fractions of a second, the needle bearings could fly up through the transfer port and embed themselves in the edge and crown of the piston.

It could be carrying just enough energy to hammer half a dozen needles into the cylinder head and put several, deep scores in the bore before the piston lets go totally.

If you were really unlucky, the stress this caused would lead to one of the main bearings collapsing causing shortly after the next rebuild leading to the crank and conrod eating themselves and knocking lumps out of the crankcase.

That piston looks like it was actually seized in the bore before the rusting happened.

While you're ordering a new piston. Get them to throw in a couple of crank seals too, they are cheap and you may need them.

Consider splitting the engine. All that hammering will have caused powdered rust and detritus to drop down into the crankcase. You will need a clutch puller and an impact driver.
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oriel dave
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Joined: 29 Jun 2006
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PostPosted: 09:41 - 12 Jun 2010    Post subject: Reply with quote
Thanks for all the replies.

Teflon-Mike: Those were my first thoughts too but now I think it's not worth it for the price of a new piston.

27cows wrote:
I've had bores like that. Don't even bother trying to rescue it. Get a rebore and save yourself endless hassle.


I feel compelled to follow this advice but reluctant to spend that much money. I'm under the impression that a rebore is around £100 and would probably require sending it away?

Temeluchus: It's an ETZ so those are perfect Thumbs Up

Will a bike garage do honing or is it more specialist than that?

Also if I get the barrel honed and it's ok should I get first or second oversize rings? I don't know what it's got on it.

stinkwheel wrote:
Consider splitting the engine.

Was planning on it. There's probably half a pint of WD40 in there as well. Where do you think is the best place to get seals and gaskets from?

Dave
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Walloper
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PostPosted: 10:25 - 12 Jun 2010    Post subject: Reply with quote
It looks facked.
But if parts are going to be a problem then it may be worth the effort to clean off all the shite and measure up the bore, piston diameter and ring groove clearances.
It may have been seized only because water got into it.
If you are not sure of history on the beast then it may be worth fannying around first.
If time is an issue and you can source a new o/sized piston and rings then hone/re-bore to new size and ride on.

Yes to crack the engine open and do a thorough overhaul.
There could be skeletons in there you don't know about 'YET'. Crying or Very sad
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temeluchus
World Chat Champion



Joined: 01 Oct 2008
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PostPosted: 10:44 - 12 Jun 2010    Post subject: Reply with quote
oriel dave wrote:

I feel compelled to follow this advice but reluctant to spend that much money. I'm under the impression that a rebore is around £100 and would probably require sending it away?



More than £40 quid for the one cylinder is daylight robbery.

Try these guys for spares also
https://www.mzspares.com/

These guys are in germany, but have everything.
https://www.mz-b.com/etz150/eetz150et.htm
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27cows
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PostPosted: 11:20 - 12 Jun 2010    Post subject: Reply with quote
If you can live with the motor possibly destroying itself and it wouldn't be any huge loss, get some wet and dry in the bore and clean up the piston too. See how it goes. Could well be OK - MZs are pretty tough old buggers.

Might be worth having a scout round - could find s/hand spares cheaply.
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oriel dave
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Joined: 29 Jun 2006
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PostPosted: 21:06 - 12 Jun 2010    Post subject: Reply with quote
Cool thanks for the help.

I think I'll phone around and get some quotes for reboring / honing when it's not the weekend. I'm hoping I can get away with just a hone.

Pistons are available so I'll replace that. If it just gets honed do I need an oversize piston / rings?

Walloper wrote:
There could be skeletons in there you don't know about 'YET'.


Hopefully not but we'll see.. it was last taxed in '97 and presumably sat since then.

Thumbs Up
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stinkwheel
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PostPosted: 00:16 - 14 Jun 2010    Post subject: Reply with quote
The hell with just honing it, you'll never know if the bore is worn past tolerance 'till you get it back. Get it rebored and the next size up piston. Then it's right.

Hand-held honing tools aren't really the right way to hone a bore anyway. To do it properly, you do it with a stone in a boring bar. If it's set up to do that, you may as well have it rebored then honed. It'll be like a new motor then.

Incidentally. If you're splitting it. MZs take standard size wheel bearings as mains, which are cheap and easily obtained.

To re-iterate and add some tips born out of experience. Don't start to strip it unless you have a) An impact driver and b) A clutch puller. The clutch fits on the end of the crank and they are really fucking tight. I've seen people bend 3-leg pullers on them.

When it comes to splitting the cases. Take the bolts out and put the whole thing in the oven until it's "spitting hot". One tap and the crankcase halves will fall apart. Try it cold and you'll land up hammering at it and breaking something when you eventually resort to jamming your biggest screwdriver in the top of the crankcase join and prying it apart.

When you rejoin it, get someone to slowly turn the crank as you tighten the bolts or you'll find it nips up tight and won't turn. If they start to feel resistance, give it a light whack on either end of the crank with a deadblow hammer and carry on as before.

Check the gear lever return spring. If it looks at all ropy, replace it while it's apart. They commonly break, you used to see lots of MZs with a bungee on the gear lever.
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Odie
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PostPosted: 07:05 - 14 Jun 2010    Post subject: Reply with quote
If you dont bother listening to stinkwheel you deserve everything you get.
If you ask a question and dont get the answer you want, for what ever reason dont come back crying because its broke again, do it once and do it right.
odie
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MarJay
But it's British!



Joined: 15 Sep 2003
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PostPosted: 08:28 - 14 Jun 2010    Post subject: Reply with quote
First off your rings are shot, the piston is far far past its best and the barrel is beyond rescuing without a rebore.

If you want to ride the bike, get a rebore, an oversized piston, new rings, small end, circlips etc.

If you don't want to do that, scrap the bike otherwise you'll end up scrapping it anyway further down the line so why not cut out the middle man.
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oriel dave
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PostPosted: 09:00 - 18 Jun 2010    Post subject: Reply with quote
Sorry for delayed response. Getting oversize piston and taking it in to get rebored.

Thanks for the advice, esp. stinkwheel! Thumbs Up
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MarJay
But it's British!



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PostPosted: 09:09 - 18 Jun 2010    Post subject: Reply with quote
Well done, you know it makes sense! You should have a nice reliable bike once the work is complete. I love MZ's! Smile
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