When talking about clearances & sizes for pistons and bores, we should be talking in hundredths of mm, as the accuracy of a micrometer is at best 0.01mm. And even measuring at that accuracy is a challenge, as pre-load, elasticity & temperature effects start playing a role. So you cannot measure more accurate than 0.01mm, or ½ thou in Imperial (= 0.0127mm = 0.01mm at that accuracy), regardless if the thing has a digital readout, or not. On that, note that common callipers have a stated accuracy of 1/20mm, or 0.05mm. Now you may be able to read/estimate slightly more accurate (certainly if the thing has a digital readout, as most have nowadays), but readout is something else as accuracy.
Maybe it is different for other types, but for the 250cc ETZ, proper (K20) pistons are marked with the nominal size of the piston, in mm, to that 0.01mm accuracy. So a piston for a 69.50mm (nominal) bore, with an intended 0.05mm clearance, should be marked with '69.45', meaning it has a size of 69.45mm. MZ publish tables for this in their manuals. For the 250cc MZ's, the piston is measured perpendicular to the piston pin, some 15-20mm above the base of the piston. Note that pistons are tapered: they are narrower near the crown, and wider at the skirt. Furthermore, pistons are not circular or round, but oval, even though 2T pistons have a full skirt, as required to close off ports. The shape of the pistons is to a large extent driven by thermal expansion: it should properly fit the cylinder at the operating temperature.
If a piston is marked with '69.5' (or '70.5', '70.0' etc) that means it is intended for a 69.5 class bore, or the first re-bore, typically 69.50mm. At least for the ETZ that is the first re-bore: the ETZ has 4 re-bores, at 0.50mm increments. Note that 71.00mm is the biggest standard re-bore indicated by MZ. I suspect that the prime reason for that is that the port timing may be altered for bigger re-bore's: the ports (overflow, exhaust, intake) are only properly perpendicular to the cylinder wall close to that wall, say the first mm. Another reason might be, because of thermal and/or mechanical stability of the liner (which is pressed into the alloy casting). Nevertheless, for the ETZ there also is a 71.50mm Sondermaß, and even larger sizes, while these barrels have also been drilled up to 75.5mm, the bore for the ETZ300/301. However, this is not necessarily without problems, particularly the port timing may/will be impacted.
Anyway, a piston marked with '69.5' does probably not have a nominal size of 69.50mm, but rather 69.43mm or 69.42mm. I'm not sure if this is always true (maybe someone like Guesi can correct me here - he saw far more of these MZ pistons), but my impression is that poorer quality pistons are marked like that - with the nominal bore size, rather than the nominal piston size, while quality pistons tend to be marked with the real nominal piston size, as prescribed in the MZ manuals. Those poorer quality pistons are made of alloys with a higher thermal expansion coefficient, and/or machined to less exacting standards, and as such require more clearance. So the manufacturer then specifies the nominal bore size, rather than the piston size, and then the manufacturer decides the piston size & clearance (often 0.07-0.08mm).
For the 250cc ES & TS, and also for the ETZ250 at some places, MZ have historically specified 0.04mm clearance. But note: this is on the original piston, as made in Leipzig by HAL, KAL or Megu. Those pistons were made of a well defined alloy, called K20 (AlSi20, indicating a 20% Silicium content, causing a dull grey appearance due to dispersion of crystals), which has a low thermal expansion coefficient. (More on alloys & expansion can be read in https://en.wikipedia.org/wiki/Hypereutectic_piston.) When the engine runs, the fuel combusts above the piston crown, heating that crown, and also the sides heats up due to friction: the piston gets quite warm, and warmest near the crown. All metals expand when they heat up, and by how much depends on the thermal expansion coefficient of the alloy. However, the piston heats up more than the barrel, which is also cooled by the fins, and that's one of the reasons why you need clearance. And the taper is required due to the thermal gradient: top part of the piston is warmest, hence expands most. That the piston gets much warmer than the barrel is also the reason why you need more clearance for a piston that expands more, as lower quality pistons tend to do.
That value of 0.04mm clearance was later revised to 0.05mm, with the introduction of the ETZ251 (though also for the ETZ250 0.05mm was mentioned at some places), and I have the impression that it retroactively also applies to the ETZ250 in full. The reason is either that the original pistons are no longer available, and/or that running in with just 0.04mm clearance is actually quite a challenge, as well as time consuming. With too little clearance you may get a higher area of the piston, which acts as a pressure point and experiences more friction, which causes that location to heat up more and expand more, --> even more heat, which may cause seizing.
To summarise this: clearance depends on the piston used, and for the best pistons one should aim at 0.05mm. For poorer pistons, usually made of a shinier (lower Si-content) alloy, one commonly aims at 0.07 - 0.08mm, said 3 thou. That 0.07mm clearance is actually similar to what is used for forged pistons made of 4032 alloy (AlSi12,5MgCuNi), as used in racing, but those forged pistons are much stronger (and more expensive).
And now many shops drill to 3 thou, just because it's easy & less risky for them. If an engine seizes shortly after it has been rebored, surely the owner will go back to the workshop and complain. So for the shop, it seems more attractive to give it more clearance, for then the risk on seizures or other immediate problems is much smaller. Of course there's a down side to that coin too: with a bigger clearance, the set is much closer to the replacement clearance. MZ specify 0.09mm clearance as the maximum, between piston and liner, at some other places 0.10mm. In my mind, you can probably go to 0.12mm without too much risk, and with a rich lubrication. Richer lubrication helps here, as a thicker oil film damps a slapping piston. It reduces the noise of piston slap, as well as the risk on a crack in the piston skirt. With 'richer' I mean typically 1:32, as compared to the 1:50 prescribed by MZ. But still, going above the 0.09mm means the engine will be noisy, with piston slap audible when idling or on the overrun, and there will be a risk.
Over time, both liner and piston wear, obviously. How much depends on how the bike is used, and on lubrication, but in general both the intake side and the exhaust side of both piston & liner will wear, as well as some wear all around in the liner, due to the rings. Particularly the intake side will wear, as that is where the conrod pushes the piston up against during the power stroke, so when pressures above the piston are highest. But also the exhaust side, which is under load during the compression stroke, will show wear. In all, typical wear is in the order of maybe 0.01mm/10000km (but as said, with big variations). Now if you start off closer to the replacement tolerance, it just means that you, the owner, can ride far fewer miles before you need another re-bore.
So where you, the customer and owner of the bike, really get into problems, is when the shop bores 0.08mm clearance on a proper K20 piston, designed to be 0.05mm below the nominal new bore of the barrel. For in that case, the barrel has just been drilled too big, and a properly sized piston can no longer be found. While for the original bore of the Zylinder, 69.00 (+2/-1)mm, there are several dedicated pistons (=Kolben), each 0.05mm below the bore, yet for the each oversize there is only 1, sized 0.05mm smaller than the overbore it is intended for:
- Tabelle Zylinder + Kolben.jpg (30.04 KiB) Viewed 53 times
So the barrel should really be drilled to the proper size, of 69.50, 70.00, 70.50 or 71.00mm, while the piston takes care of the required clearance: proper pistons are 0.05mm under, poorer pistons more (typically 0.07-0.08mm, but those expand more while warming up). With too much clearance, the piston will slap, also when warmed up and in maybe 10k miles you need another re-bore.
This is precisely the error my workshop made the last time I had my barrel re-bored (by Mr. Kevin Kaut from LuxMoto, on the Aachener Straße in Sankt Vith), even though I asked explicitly for 0.05mm clearance on my 70.95mm K20 piston, with an absolute maximum of 0.06mm. Yet he bored it to 71.03mm, giving 0.08mm clearance, and a loud engine due to piston slap.
