New piston for MZ ES250/2 Trophy

[Puffs]

No idea why this is gone but here's the
Original post by EmzedRay » 06 May 2022, 20:26
Hello. I am new to the Forum. I have recently bought a nice blue and cream ES250/2 Trophy and have removed the piston which was rattling somewhat. The piston is a Mega 7 marked 68.96. It also has a 4 and a 2.3 stamped on the crown. I think this is the original piston?

The piston sides are very worn and scored, but the bore of the barrel looks shiny and pretty much unmarked. The piston when fitted in the barrel with the rings in place can be wiggled about, but not by a huge amount. I am torn between just fitting a new standard replacement piston or getting a 1st oversize piston and having the barrel rebored and honed. What would you recommend?

Now for some more questions:-

1) What piston to fit? What make and where to get it from? I have found these people and this looks like the exact same piston that I have:-

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

Any recommendations or other options?

2) The above listing mentions an "N" marking and conrods guided at the top. I can see no N marking on my original piston. What should I fit? They have pistons with and without the N marking.

3) If I go for the 1st oversize piston and the rebore, what size piston should I fit? Should I go straight to a 70mm? Also what piston/bore clearance should I specify to the workshop? I see mention of 0.04mm and also 0.05mm for easier running in?

4) What thickness metal head gasket should I fit - 0.2mm or 0.4mm? Will the thinner one give any meaningful compression benefit? Why are there different thicknesses available?

I look forward to your feedback, comments and recommendations.

Many thanks, Ray

My reply:
by Puffs » 07 May 2022, 10:57
Hi Ray, congrats with your ES250/2 Trophy!

The standard '0' bore for the ES250/2 is 69.00mm, and that should get 0.04mm clearance, hence 68.96 would be the size of the original piston for a '0' barrel (and 68.97 for & '+1', 68.98 for a '+2'). On the marking, could it be 'MEGU'? Megu = Metallgußwerk Leipzig GmbH, one of the original piston providers.

For the ES/2, the manual recommends a 'new' piston clearance of 0.04mm, and a replacement value of 0.10mm. For more modern engines (particularly the later ETZ's) this was relaxed to 0.05mm/0.10mm. The tight 0.04mm requires very accurate manufacturing, and indeed more diligent running in, and for those reasons MZ later relaxed the new fitting clearance to 0.05mm. I do not know if this should apply to an ES/2.

There will be wear in both the piston (skirt) and borehole wall, you have to measure the cylinder in order to decide if only fitting a new piston would be sensible. Often a rebore + the 1st oversize piston is chosen, but not necessarily always. Note that barrel wear is not equal on all sides.

Pistons come in different alloys and qualities. For the tight 'new' fitting (both 0.04mm or 0.05mm) it is essential a good quality piston in K20 alloy is used, like the originals (Megu, HAL, KAL) were. For vendors see viewtopic.php?f=5&t=9257&p=47896#p47896 , but best not be tempted to buy a cheap non-K20 piston.

In a conrod guided at the top, the big-end has much sideways clearance hence is centralised at the small-end side; I believe that is the case in the original ES/2. I don't know what "N-marking" means, maybe it's related with that - ask Guesi.

If a re-bore is required, choose the smallest possible option. The maximum overbore is 2mm, so take the smallest steps possible. The manual mentions 0.25mm steps, but original pistons are rare and you may not find the required pistons for that; common modern oversizes are in 0.50mm steps.

Different sizes of head gaskets can be used to set the piston-cylinder head clearance (squish, typically 0.9 - 1.2 for the ETZ, don't know for the ES). Yes, a lower squish will give a higher compression pressure, but might result in detonation. However, these bikes were designed for quite low-grade fuel.

Good luck, and do not forget to pre-mix oil in your fuel!

(PS: I don't know where you are in the UK, but I believe there is an MZ Meet in Bristol this Sunday, at the Salutation Inn, Ham, near Berkeley, Gloucestershire, GL13 9QH, start around noon.)

[EmzedRay]

Puffs, very many thanks for your comprehensive reply. Much appreciated. Things are getting a bit clearer.

The remaining questions are whether to rebore and go first oversize and then whether I need a piston centralised at the top. Also, whether to go 0.02 or 0.04 for the head gasket, and where to get the piston from. I will do some more research, but any additional comments from the Forum are most welcome.

Do you know where I can get a manual for my ES2? Is there a free download anywhere?

Thanks again, Ray

PS: I live in East Sussex, but have a flat in Bristol. Not there this Sunday though. How often are the Bristol meets?

[Kruh]

Here you go, I've attached the manual below.

As far as the headgasket goes. It varies a bit from cylinder to cylinder and the base gasket thickness. Best practice is to buy both (they're cheap) and then when assembled, check the squish clearance using the solder method. When you dissassemble the top end, measure the headgasket thickness, most likely, thats the size you will need.

[Puffs]

On your question to re-bore or not, you have to measure the barrel. If you do not have the tool for measuring such a bore ID accurately, you have to go to a workshop that has.

Head gaskets come in tens of mm, not hundreds. Do as Kruh said, for reference the solder method:

Bild 72. Ausmessen des Spaltmaßes = squish

See the strip of solder (I use rosin cored) is placed at the left or right side, not front or rear, which should be even easier with your decentralised plug hole.

Some people actually take the head off & wedge a piece of solder in the top of the barrel, to prevent loading only 1 side:

Then fit the head & rotate the crankshaft (at the flywheel, or kick it) to take the piston through TDC. Subsequently you can measure the squish size by the thickness of the squashed solder tip.

For the ETZ I use the top method, which is from the ETZ manual.

If you fit the same gasket size as before, tighten the head & measure, with a bit of luck you're done.

Info on that meet: https://www.mzridersclub.com/club-secti ... loucester/

[EmzedRay]

Many thanks for the additional input from various parties. All much appreciated.

I guess the remaining question I have is whether the conrod is guided from the top or the bottom on ES250/2. Here: https://www.guesi-motorradteile.de/inde ... 9/c/_/_/?_ they talk of the piston being marked with an N to indicate if the piston is top or bottom guided.

There is no N I can see on my existing piston which seems to suggest it is a "guided from the top" piston.

Does anyone know the answer to this one?

Cheers, Ray

[EmzedRay]

Just looked at Page 21 of the Manual and is says "the connecting rod is guided by the small end bush" in bold!

I guess that is the answer then and is in line with there being no N on my piston?

Any thoughts?

[Blurredman]

Sorry to get here so late, esspecially as I have (relatively recently) come under ownership of an ES250 myself and have gained a fair amount of information about the bikes since.

I am glad you have the colour manual, provided by Kruh, initially I had the b+w (plus the original book) but the colour scanned copy is great quality and helps somewhat for the wiring diagramme (the colours aren't related to the wire colour though, but their job (lighting, charging, ignition etc), so bear that in mind!


My FTP also includes the ETS250 manual (as well as many other MZ stuff), which serves additionally as an addendum to the later imported ES250/2 bikes, with their different jetting and slight improvements that are obviously not included in the original '69 manual. Essentially, the later ES250's are the same rated (apparently) 21hp as the TS, the ETZ etc as opposed to the 17.5hp of the original '69 release models. Which also brings me onto the wiring diagramme in the manual briefly, because it seems like some colours of wires were changed a little after the manual came out, but it's fairly intuitive (any help required let me know).

Anyway, you're in the UK, and you're therefore likely to have a 1970/1971/1972 bike (likely registered later) which is what my experience tells me based on bikes I have seen both in person and on the Internet on what was typically imported to the UK at the time. And I say this (please confirm?) because I suspect you will also require the ETS250 manual as mentioned. Link below. (EDIT: I have attached the document to this post incase my server in unreachable).

ftp://blurredmanswebsite.ddns.net/Vehic ... Manual.pdf

I didn't notice any marking on the top of my piston either, when I re-assembled it, but I honestly wasn't looking, though it is definitely centred from the top. I noticed mine was standard '0' first size though.


The MZ club meet (bristol section) is every 2nd sunday of the month. I was indeed there yesterday and there was a nice turn out. If you happen to be in the area on the particular dates, then it is a nice day so do visit. I actually live in S. Wales, and it's a 110 mile round journey so for me it's a nice enough ride too.

[EmzedRay]

I have worked out that I need a 1st oversize (69.50mm) piston, con-rod top guide version (to suit an ES250). Searching around I have found one manufactured/branded "Almot".

Are these considered to be any good? Any other suggestions?

Many thanks!

Ray

[EmzedRay]

Blurredman, your comments about the ES250/2 and the later sportier ETS are interesting.

My ES250/2 was registered in the UK in Feb 1974, but I do not know when it was manufactured. It is the Trophy style with the fancy all in one headlamp cowl that marries up with the tank.

The Engine number is 4748331 and the Frame 1429262. Would those give a clue to what motor it has in it? The badge on the rear mudguard definitely says ES250/2 and that looks original.

Would be nice if it was the later 21hp motor!

[Puffs]

Some more comments:
- your conrod is guided (centred) at the top side (small end side) if your big-end (conrod) is original. Regardless of when it was built exactly, the bike is older than some of the members here, and it will have a history. It may well have received another conrod set, and now be bottom guided? You have to look & see. I do not know what the sideways clearances are for the ES (in either case), for the bottom-guided ETZ it is typically 0.5-0.6mm.

- Almot is K20 & fair IMO, but for a truly classic bike like yours I would prefer a NOS original (Megu?)... if you can find it! Does it really need a rebore? Are you sure Almot is compatible with top guided?

Pics please...!

[Blurredman]

Hi,

Mine was registered in May 1974! They were officially manufactured between Feb '67 until March '73 apparently. So, if it's original the bike, you've got the later engine.. In terms of piston guidance, I had lots (like quarter inch) bottom conrod sideways movement.. If you have that, then I suppose you have top guidance.

There are a few tell tale things to look for, If your coil is original, it should have the manufacture date on the back of that, if your generator is original, you should also have a date on that (if it hasn't rubbed of) and also the rear of the headlamp bowl. But additionally, is if you remove the rear light lens (and surround) - (probably the easiest and quickest to remove that). The bulb holder unit will have a date stamp on it too. This will give you the manufacture of that part. Of course, all these items may not be original to the bike and been replaced in the last 50 years, but you could look at them all and gleam any correlation from them.... It might also say on the frame stamp information plate. But, if it's like mine it is unreadable. I personally haven't come accross a database/document that lists engine/frame numbers for manufacture dates, not to say one doesn't exist of course.

Interestingly though despite your earlier registration, I just had a look at what my V5 says my frame and engine number are, and mine appears like it might be earlier than yours- my rear lamp bulb holder is dated last quarter (IV) 1972. Engine: 4743106, Frame: 1424612. So yours could be a '73? The above information about finding the age will hopefully tell us! It just goes to show, registration date is not to be believed.

[Puffs]

Well indeed, I wouldn't know what the UK registration date should mean. In other countries it normally indicates the date the vehicle was first registered for road use, which is obviously later as when it was produced, but even that seems unreliable with the DVLA - for mysterious reasons my YZ lost several years when I registered it in the UK.

At any rate: it's not necessarily close to the bike's age.

[EmzedRay]

Returning to this thread, here is an update. I have managed to source an original 1st oversize piston from these good people:-

https://www.guesi-motorradteile.de/inde ... 250_2/_/?_

It is a 69.50 top guided piston and the barrel and piston have just gone away for the rebore - 0.04mm clearance as per the manual.

Exciting stuff!

Now for a question: When I reassemble the top end I intend to check/set the ignition timing to 3mm BTDC, which will be easier to measure while the head is off (especially with the angled plug hole out of the way). I understand this must be set with the advance flyweights set fully open. What is the way to fix them open? Is there a particular technique for that?

Any other tips for getting the timing spot on? Is a timing disc easier/better?

Many thanks for any tips or advice,

Ray

[Blurredman]

I honestly don't know if you'd have the starting flyweights. Are you able to take pictures of the bike (for fun) and the generator to see if you have them? Mine doesn't and I seem to have an earlier vehicle.

Good job on the piston.. But be sure to run it in gingerly!

[Puffs]

I don't know how to fix those weights open (if you have them), maybe wedge a suitable piece of wood?

Yes, you are probably right in saying that on a mm scale, setting it with the head off is easier. But with points, adjustment is a recurring operation. I'd make a timing disk.

Guesi has several 'old stock' pistons which seem very suitable for your bike. As BM says: gingerly, at 0.04mm clearance.