Piston choice for ts250/1

[DAVIDGUEST123]

Hi all.
I'm now at the point where I need to rebore a very tidy barrel purchased from ebay. The old one I had rebored to 3thou(iknow too big) had fins broken, so it's a bit of start from stratch. I have the barrel, I now know what size to do rebore it, all I need is a piston. 70.00mm for a second oversize will be the size, but I'm unsure where to buy from and which one (Almot K20) It all a bit confusing. This time I want it to be right. Thanks Dave

[Guesi]

There are original GDR pistons available.

https://www.guesi-motorradteile.de/inde ... 9/c/_/_/?_

[Puffs]

Hi Dave,
shop around, but Guesi is right, Almot is not the best. I'd also avoid even cheaper Indian or Chinese produce.

Remember what happened to me (see elsewhere) and discuss your history with the workshop that will do the rebore. Make sure that he realises that you are very serious about the clearance you're asking, and that you will not accept something like 3 thou. The standard is 0.04mm for the TS250 on an original Megu:

Piston clearance TS250.jpg (36.56 KiB) Viewed 7 times

0.04mm = 1.575 thou.

[DAVIDGUEST123]

Hi again,
Wel lI've pulled the barrel off and it seems to be very different from the ebay one. I suspect I have an old ts250 rather than the 250/1. Question is will it fit. Although it has been rebored once the diameter is very different from the orginal, as shown in the pictures. It's a real shame as the 'new' one is in far better condition. As ever any infomation wil lbe greatly recived.
Many thanks Dave

[Guesi]

Which one is the new one ?

Can you post pictures of the intake side ?

And how is the diameter of both ?

[Puffs]

Yes, there are some differences in the casting between the two; please tell which is the one you bought (and is it a 05-42.062 ??). The one on the right seems to be identical to the pic above, taken from "MZ_TS250-1977-Official_Service_Manual.pdf", taken from the section on the 4-speed engine.

On your question if it fits: the differences I see should not necessarily be a problem. I do not know if the barrels from the 4-speed & the 5-speed engine are different, and if so: how? Anyway, measure & compare all details (including porting, distances stud holes, height of the barrel), and maybe try to fit with an old piston, just to see how it all fits (fit, not start).

Further to Guesi's Q's: if the one on the right is your old one, why not just rebore that one?

[Guesi]

They both can be rebored as long as it is not already on its last oversize.

[Blurredman]

I do have a spare piston.. Used for only 50 miles. It was for an ETZ250 however..

I think it's 71 mm actually..

[Puffs]

ETZ & TS pistons are different (?).

Can you PM me details on that piston?

[Kruh]

Would 0.05 mm clearence be to little for an Almot piston?
It would probably require a more careful/longer break in process.

[Puffs]

A clearance of 0.05mm for an Almot piston in a TS: I do not know.

For the TS, MZ specified 0.04mm on original pistons, and during the ETZ period that changed to 0.05mm, again on original pistons. The reason might be that the ETZ's barrel has more cooling surface, so that the barrel operates at a slightly lower temperature, hence expands less than the TS barrel. The piston will obviously be at quite the same temperature, hence expanded the same.
If that is so, the ETZ needs a little more cold clearance. It might also be that they ran into problems with the changing clientèle, which over time became less diligent in running-in, and that the tighter 0.04mm gave more & more problems, so that they changed their recommendation.

Almot K20 pistons are claimed to be cast of the same alloy, hence have the same thermal expansion coefficient & friction parameters, but they may not be machined to the same level of accuracy. To cover thermal expansion of the various parts of a piston, when it is at an operating temperature of maybe 300°C, they are machined both elliptical (so not round, when seen from above) and conical (more narrow at the crest). Anyway, to cope with production accuracy, a clearance of up to 0.06mm seems prudent in an ETZ; so 0.01 more than the original. That would suggest 0.05mm in an TS.
There is also hearsay that their K20 alloy is not actually K20, but rather more like K12, but it would surprise me if that were true, as that would be forgery (and Si is not very expensive). I know Almot make pistons for other engines in K12, but those are marked as K12.

Regardless, running-in should be done very carefully, and it's probably wise to have a look after, say, 500 miles. Not sure if the manual says something about that. Personally I like to mix richer during running-in, but I know the manual does not suggest that.

[Blurredman]

Regardless, running-in should be done very carefully, and it's probably wise to have a look after, say, 500 miles. Not sure if the manual says something about that. Personally I like to mix richer during running-in, but I know the manual does not suggest that.

Yes it does. Though it doesn't explicitly mean oil. But more fuel = lower temp.

[Puffs]

Yes that's an option too. Indeed, more fuel will cool more, and probably particularly the piston (which is hottest of the lot). Also, the mixture with more fuel/less oxygen will burn at a slightly lower temperature. Yet how big the effect of raising the needle 1 notch is, may remain difficult to underestimate.

But with 'richer' I meant 'more lubrication'. There is a debate about that, and some people actually advocate adding graphite for extra lubrication. Again true: if you premix (as opposed to using an oil pump) & make the air/fuel mixture contain more fuel, there will also be more oil, but the effect of that is substantially smaller than just mixing richer (say, going from 1:50 to 1:33). Anyway, there are 2 main camps, one saying: use same mixture during breaking-in, and the other says: use a richer mixture during running-in.

Btw, looking at manuals for also the ES, I saw that in those days MZ still recommended 1:33... Probably with DDR mineral oil.

[dave47]

I am about to run in a TS125 with an Almot K20 piston, and have had the cylinder bored +0.03mm as per the MZ manual but now I'm starting to worry that this may be too little! Anyway its done now, and I will just need to be careful.

My real concern is what sort of 2T should I use? My instinct was to use the best quality, but I have read somewhere that modern fully synthetic oils are so efficient that the engine never runs in. So perhaps old fashioned mineral oil is best. Does anyone have an opinion?

[Puffs]

No, in my experience piston & barrel also wear if you use a top quality fully synthetic. Maybe same, maybe less quick, I wouldn't know. IMO the main benefit of a fully synth is temperature stability.

I'd just use a semi synth, same as normal, bit richer (but you've read that). After a careful few 100 miles take off carb & manifold to have a look at the piston through the intake. If you see length-wise friction marks on the skirt, take out piston & treat those lightly w/ sandpaper, as described in some manuals.