MZ/MuZ Riders

I’m in the proces of rebuilding my 1984 MZ ETZ 250. About 9 years ago I bought two bikes from a private seller, one in pretty good shape, which I managed to restore, the other for spare parts. I was never quite pleased with the way the way the restored one ran, so I took it apart again a few years back. I couldn’t find the time to take up the project again until now.
I’m trying to piece the best pieces of both engines together but it seems the two engines are different and this raises a few questions. One engine (engine n°: 1164203) has the external oil connection and one (engine n°: 1082331) has the internal oil connection.

Question 1: Can I use the same oil pump on both engines or are they different?

Both engines have a cylinder head but seem to be different in size. The cylinder head which matches engine 1082331 does not have a piston. The piston that matches the cylinder head of engine 1164203 is too big for the other cylinder head. Measuring the bore of engine 1082331 gives me a reading of approximately 68,6mm. Measuring the piston which fits the head of 1164203 gives me a reading of exactly 70mm.

Question 2: Is that the reason why the pistons have different numbers on this site?

https://mz-b.net/index.php?route=journal2/quickview&pid=2100000104901

https://mz-b.net/index.php?route=journal2/quickview&pid=2000000007465

https://mz-b.net/engine-and-parts/etz-250-piston-70-00-etz-250-2100000104925

https://mz-b.net/engine-and-parts/etz-250-piston-70-00-kit-etz-250-2000000007489


I can probably use the cylinder head from engine 1164203 on the case of 1082331 but…

Question 3: How do I know for sure the right size piston for both heads since my reading of 68,6 mm does not seem correct as the smallest diameter seems to be 69.

Attached are a lot of pictures of both engine cases and cylinder heads. I’m guessing the engine with the internal pump is the latter of the two and matches my 1984 MZ, so I would prefer to rebuild that one. (The one with the external pump also has a dent in the clutch-case.)

Can anyone shed some light?

Thank you for the welcome and the answers Puffs. I live north of Antwerp, you are situated in the glorious Ardennes so it seems. Every year in oktober we have a long weekend in Bütgenbach en when Porsche was still racing in the WEC-championship we would spend 4 days at Spa-Francorchamps, lets be honest, the most beautiful race-track in the world. I’m not a religious man but if I would have to seek the almighty’s throne, Eau Rouge would be my place to start.

I’ve managed to come up with some answers by reading the workshop manual. It made clear what the different sizes are all about, strangely though the vendor seems to sell pistons up to 75 mm. But at least now I know what to do. I will probably use the cylinder with the matching piston I have already.

Could have checked the WSM but I was to lazy to read it and hoping someone would know the answer on this forum I’ve printed the WSM when I started the project many years ago but most pics are a bit dark so it’s not always clear. Because I didn’t work that much on this project I’m feeling a bit rusty, I’ll manage to get going though.
I did buy the Haynes manual but there was no explanation of the different piston-types, but it will definitely be useful in the future.

Might start a restoration-thread one of these days. Cheers!

For a start, as you have detected. One of those engine's has an internal oil galley.
It is an older type. Years and numbers are unknown but I personally think that these internal controlled oil galleys are the bikes from 81-83 ish. Coinciding with the sale of pre-mix models (with the disc brakes AND a tacho!). At least in the UK and most of Western European countries the non-('luxus' models as they were known) were soon removed for sale. Perhaps due to lack of demand? They carried on the non-luxus models in Eastern Europe for long after '83 ish however.

During this time I think they decided to cut the costs, by creating this 'external' line because it was a lot cheaper than the casting process of the internal galley on the clutch cover, but also the machining into the crankcase too. Also... The external pipe gives one positive however.. You can clearly see whether the oil is getting from the oil pump to the crankcase... A quick visual 'yes the oil is pumping'.. When the internal galley was replaced with external tube? I don't rightly know..

I wouldn't say both those pumps are interchangable however- the outside tube type obviously has different way of getting the oil back up to the outside of the case, whereas the earlier one would seem to fit directly on the inlet for the oil in that compartment. I don't have an internal galley oil pump so I don't have experience there. You can see the inlet in the picture however.


Just for the purposes of other people's interest (puffs), please would you make a picture of the inner side of the internal oil clutch cover?

Interestingly- I have one myself that I bought as a replacement for a mesed up cover on one of my bikes... And I never took a picture so I could never show those who were interested and found nothing on the internet. This would be an opportune time to get a reference picture! Pleeease



Oil pumps: As far as i'm aware there was only one model of pump used from 81 until mz's demise. However I am unsure. However, I can see no reason why they would be interchangable. I myself also have two on my shelf- I have them removed also. I wouldn't worry about the nay-sayers and the heresay that is spouted around concerning the oil pump removal and the running of the engine post removal regarding "running lean" etc. Even the factory book does not have a differentiation of settings or jet sizes between oil pump and pre-mix models. There is only a 2% less petrol being put through the system in place of oil. I myself have gone 15,000 miles on my 251 and 10,000 miles on my 250.


Pistons: Official piston oversizes only go up three. 68.6 I do believe is the size of the piston. 69 would be size of the bore.
However like Puffs has said- those pistons are cheap. Both he and I have had experience with those cheap pistons produced by an Indian company 'RAM'. Personally I haven't had issues with them, but then my wear seems to be considerably less- maybe because my cylinder rebore shop actually re-bored to the size I told them too..
Both my Mz's have those cheapo RAM pistons in them, and each machine has done 4,000 miles since installation.. One of the bikes is still silent as a Honda 90, but the other is not silent, but I am not regarding the piston to be the issue there.

Thank you both very very much for the info on the MZ!!! @ Puffs, thank you for the Eau Rouge picture. Racing a Panigale V4 around Spa must have been amazing!!!
@ Blurredman, thank you for putting al those docs online. It will keep my books clean but my keyboard greasy (which is cleanable )

This is what it looks like for the internal pump on my ETZ.



I found this in a Dutch publication, the engine with the internal pump indeed seems to be the older one.



Looking at the drawings it looks like there's two different pumps but when searching online indeed one can only find one type of Mikuni pump.