Piston/Barrel Clearance sizes

[Blurredman]

Hi Guys,


This morning I took two barrels (a 250 and a 300) with over-size pistons for re-bores.

I also gave the sheet of 0.035 (for 300cc piston) and 0.04mm (for 250cc piston) clearances accordingly with each piston and the guy that served me said he personally thought they were a little bit too tight for an air cooled bike, and normally they bore them a bit larger than that..


Just wondering what people's opinions are ?? Haynes and official service manual both state 0.04 (0.0015) for the 250cc... ?

I am not using original MZ pistons. The replacement ones seem to have a glossy silver shine.. Does that mean they expand faster and potentially more too??

[Blurredman]

Okay,

So in the end I opted for the recommendation the shop normally use for air cooled cylinders. That being 2 thou (around 0.05mm) for both.

I am unsure where these pistons originate but they look like they are cheap and would expand faster and possibly more than the original piston. It may seem a lot larger than the manual, but I think it is for the best. Esspecially considering one of the original pistons was 0.4mm undersized for the bore (I have been riding around on that for many years )

What is bore clearance measured? Is the 'piston/wall cleanrance' measure with the piston touching the opposit side of the barrel you are measuring or with the piston as central as possible?

I've used this piston machining shop before in the past, but i've merely given them the barrel and piston and said "here do this". They must have used 2thou clearance on that if it is their standard size tolerance.

[Guesi]

ETZ 250 and 300 with original piston have 0,05 mm clearance.
TS 250 have 0,04 mm
ES 250/2 have also 0,04 mm.

As you don´t use the original piston I cannot say what clearance you need. For Almot pistons some people here in germany use 0,06 mm or more.
Which means that the pistons make noise when the engine is cold.

And it does not matter where you measure the clearance.

On the piston there is the size printed (for example 69,48 ) .
So you add the clearance (for example ETZ 250 + 0,05 mm) and tell the machining shop to drill the hole 69,48 + 0,05 = 69,53 mm.
That´s all.....

[Blurredman]

That's interesting.

Because the Original 1981 service manual, haynes and indeed the 1989 engine maintenance video tape on youtube say 0.04mm for ETZ 250 piston clearance


300 piston is 1st oversize, the 250 piston is 4th oversize


Quick edit, the video actually does say 0.05mm.. Interesting.,

[Guesi]

Because the Original 1981 service manual

As far as I know, there was no ETZ 250 in 1981.
The ETZ came out in1983.

So 1981 was the TS 250/1 (Supa 5).

And for this engine 0,04 mm is right.

But ETZ has 0,05 mm (as you mentioned about the video).

[Blurredman]

Here is the manual for ETZ.

I assumed it came out in 1981 because that is what is printed on the manual 2nd page


ftp://mattswebsite.ddns.net/Vehicle_Documents/MZ_Documents/MZ_ETZ250-1981-Official_Service_Manual.pdf

[Skorpion]

Here is the manual for ETZ.

I assumed it came out in 1981 because that is what is printed on the manual 2nd page


ftp://mattswebsite.ddns.net/Vehicle_Documents/MZ_Documents/MZ_ETZ250-1981-Official_Service_Manual.pdf

This manual also gives a mounting clearance of 0.05mm under picture Fig 48.

Back in the day I had two 300 barrels rebored, 0.05mm was the clearance used.

[Blurredman]

Okay, Not a problem.


What is the manual talking about in section 3.4.3.1.C ? What is this clearance?


Either way, I feel a lot happier about my choice. Thank you guys for also correcting me. It's been a great help.

Haynes obviously didn't get its bad reputation for nothing..

[arny]

Factory manual Section 3.4.3.2. specifies the clearance as 0.04 mm, but also the wear limit of 0.09 mm. Obviously you can try with any clearance in that range if unsure (I guess it wouldn't be wise to go for 0.09 though ) and 0.05 is well within tolerance.

[Puffs]

Indeed, the clearance required very much depends on the piston. Just asking for 'a clearance' is a little bit like asking 'what size shoes does a man need?'. Some piston manufacturers indicate the desired clearance on the crown (for instance as "SP 0.07", meaning 0.07mm Spiel = play = clearance).

More specifically, the required clearance depends on the thermal expansion characteristics of the alloy used for the piston, and on the shape of the piston. Under use, the piston is much warmer than the barrel, and the crown is warmest, and therefore expands most. A piston that is too cylindrical will require a higher clearance. It is also correct that it does not matter where you measure the diameter of the bore, but if you are measuring clearance (with a feeler gauge) you have to measure it at the skirt - the widest point of the piston.

The other important factor of course is the size (=bore). All other things being equal, a bigger thing expands more, hence needs more clearance. I have never understood why a 300 should have less clearance than a 250, but I know that some sources indicate so.

I would have loved to try an Almot (which is K20 alloy) with 0.06mm clearance, and that is what I asked the shop for, but they let their own opinion prevail and made it 0.07mm. That works too & is less sensitive to running in, but of course it's equivalent to having run some 5k miles with it. If you want to make many miles before your next piston + rebore, best choose a top quality piston with a tighter tolerance, but that will require careful running-in.

The tighter tolerance of 0.04mm as indicated for instance in Haynes pertains to an original piston, is IMO probably correct, and requires very careful running in. But I'd suspect that if you give a Berta or a RAM such a tight tolerance, it'll seize on you.

edit:
... but maybe not - just saw this thread is actually a year old & started before I joined....