ETZ 250 Main Crankshaft Bearings

[nice2day]

Hi Chaps (sorry I'm on here again) Just one quick question please. (Oh and a bit of conjecture at the end )
I understand that the bearings are the 6306 type. From the MZ videos they are semi shielded (inner side) Are the shields the rubber ones please or the metal versions?

One little side issue which I am SURE will cause some differences of opinion. I will be buying standard clearance types, not C3...My opinion is that C3 bearings are used mostly when the centre shaft runs hotter than the case such as that of an electric motor. The crankcase of 2 stroke motor runs reasonably coolly and the cases are heated along with the crankshaft to a very similar temperature so there is no extra slack required when running as the bearing heats uniformly too...On top of that a reciprocating piston engine creates lots of harsh push and pull forces in the vertical plane at TDC and BDC, so unlike smooth running electric motors the extra slack of a C3 could create some hammering (IMO). Many people are misguided that C3 bearings are of superior quality...this is totally wrong they are identical in quality, just that the C3 is looser.

Les

[Blurredman]

Hi Les,

The bearings aren't sheilded. This is well discussed within the MZ community of those who are confused vs those who know.

Those aren't sheilds you see on the one side of the bearing. That is the nylon cage that holds each ball in their repspective position. You are free to use standard open caged bearings.

As for class of bearing, I don't know to be honest, in reality I imagine even a 'standard bearing' (is that C4?) would last longer than you have the bike.. Unless you intend to keep it 30 odd years more and put another 50k miles on it?

I've used C3 on both my recent engine builds (within 2 years). I've only put 4-5k miles on each of those engines though so couldn't say how well they have done... All I know is 4-5k isn't much..

But then, just adding up the bearings I bought (all but clutch bearing(koyo) were SKF) it only amounted to £40 inc delivery...
Though I have purchased SNH from Ost2Rad - Apparently a relatively unknown company. But they were more than the SKF's on e-Bay. Engine rebuilds really aren't much in the grand scheme of things.. People pay £40 for 2 litres of 2t oil..

[nice2day]

Many thanks for your great answer Blurredman. My first reaction is one of annoyance as I know I should have waited until I stripped the engine down, but I'm in the mood now to urgently carry on where I left off nearly a year ago so ordered 2x SKF 6303's on Ebay. Then I took a refresher by looking at the MZ rebuild video's where the apparent "shielded" side of the bearing is mentioned. I have previously rebuilt a 301 engine but couldn't remember seeing them but anyway I changed the order to single shielded type (more expensive too ). If you think about it, the shields will be right up nearly flush with the flywheels. I can't remember the oil route for the ETZ engine, could you remind me please?.. As far as I know the oil is fed to the main bearing which escapes from them and then is flung up to the big-end? If this is correct the shields would block this passage of oil. Anyway too late to cancel, But as far as I know shields can be removed fairly easily which hopefully I will be able to do.

The C3 business always leaves me with some concern as to what is correct. As said and explained on my first message, I consider that standard (Normal clearance) bearings would be better and one other thing to add by me is that less slop will give the oil seals an easier time. Certainly the mileage I'll be doing will amount to less than a thousand a year (if that) so whatever I fit as you quite rightly suggest is not going to be much difference.

Very much appreciate your answer Blurredman and any chance of reminding me of the oil route for the pumped oil version on these engines?
All the best ..Les.

[Puffs]

On the shielding I'm with BM. I never saw the film, but I'd say the bearings need some lubrication, which will come from the premix or oil injection, and shielding the main bearings from that would prevent lubrication. That said, this applies to the 250cc ETZ (EM250 & EM251), and I understand there was at least 1 earlier engine wherein one of the main bearings was lubed via a channel to the gearbox oil.

On bearing class & clearance: yes, I believe the differences are not in the quality of the material (they are all the same alloy), but simply in the clearance. 'High-speed' bearings have more clearance, which will indeed allow the balls & the inner race to expand more (due to heat, either from running fast, due to high load, or from another source) before they seize.

I hear you on the temperature & relatively small temperature differences (which is what counts) in a 2T engine, and on the possibility of 'hammering'. Yes, maybe you are right - but maybe not. Maybe the balls & inner race heat-up & expand when exposed to the pressures under full throttle? I don't know, but these engines have a few years of history, with millions produced, I would just go with the factory recommendation.

For completeness, the following from the official manual (Kurbelwellenhauptlager = crankshaftmainbearings):

Technische Daten EM250.jpg (32.48 KiB) Viewed 53 times

And where these are:

EM250 (3G).JPG (22.14 KiB) Viewed 53 times

(Zoom to see the suggestion of open bearings, which may also follow from the name code.)

Edit: sorry, didn't see your latest post when I posted the above.
IMO the oil from the premix precipitates in the overflow ports & flows down to the main bearings, helped by those oil guides which are right next to them. The rest is via splash lubrication & more precipitation.

Premix comes in small droplets into the crankcase, droplets hit the hot metal, fuel evaporates & cools the metal & oil stays behind. With an oil pump it must be mainly splashing that is responsible for the distribution of the oil.

A final comment: bearings seizing has nothing to do with extensive use, but rather point load. You can have a seizure in 10 miles.

[nice2day]

Many thanks for your reply Puffs. I have never seen (or maybe overlooked) the C4 recommendation. C4 is even looser than C3. CN is normal and C3 is just one step looser (C2 is tighter than CN) So maybe C3 is the best compromise then. Ah well I'll have to re-order 2x more bearings....22 quid wasted.....
Les

[Puffs]

Yes I was a bit surprised to see C4 there, and just went downstairs to have a look in my box of old bearings. There are no C4's in there, only C3's, both bearings & boxes. Of various sizes, for various engines.

When I rebuilt my ETZ 251 engine (because of small end failure), that was a while back, and I did not have that German document. I just put back what I found in there, and those were C3's, apparently. It has done many thousands of miles since.

For the interested a PDF on bearing clearance attached.

Attachments

Bearing Internal Clearance and Preload.pdf

[nice2day]

A very informative attachment Puffs. One very important point is the interference fit aspect....I can only assume this has a bigger effect on running clearance than I imagined:..I feel happier choosing C3 sizes now...Les

[Blurredman]

Hi Les,

The video shows the bearings:

https://www.youtube.com/watch?v=6ptXHE2i0x0

at 37 minutes exactly..

What you see there is not a sheild. That is a plastic cage to retain the ball bearings.. I suppose it's just cheaper to make them that way. It seems differing depending on the year or particular model of ETZ but some seem to have them, others have a more 'traditional' metal ball cage.. I suppose they just went with the cheapest production of bearings at the time..

The new bearings that go in shouldn't be sheilded. They should be open. Both sides. I don't recall what my 250 had, but my 251 had the plastic cage. Both bikes have had normal open metal caged bearings as replacement.

It is also seen in a picture here:



I am sure there is a more documented topic on this as I raised the question myself!

As for oil injection.. I removed it from both my bikes..

[nice2day]

Thanks Blurredman. I have the full picture on all the criteria now. I will be ordering 2x 6306 C3 unshielded FAG bearings. Thanks for your help.Will be splitting the cases quite soon...well, in between completing the bathroom refurbishment

Les

[Puffs]

Well, the main bearing in that pic (which is not the one in the red circle) sure looks like it is shielded... But OK, if you tell us that's a plastic cage to retain the balls, that's possible too.
All the main bearings I know just have a metal cage, which would, in my mind, give better access of oil to the balls & races.

For completeness, I too removed the oil injection pump from my 251.

[Blurredman]

That red circle is incidental. It was nabbed from another topic. I only linked it because of the main bearing.

I do believe I still have all the old bearings in a pile on the floor in my parents garage.. when I go there next I will take one and show you the retainer cage!

[Kruh]

Well, the main bearing in that pic (which is not the one in the red circle) sure looks like it is shielded... But OK, if you tell us that's a plastic cage to retain the balls, that's possible too.
All the main bearings I know just have a metal cage, which would, in my mind, give better access of oil to the balls & races.

The one in the picture are the original bearings. I had the same ones. Yes, that is just a plastic cage, not a shield.

Some modern bikes use polyamide cages (plastic), its supouse to reduce friction and be better for higher rpms.

But for the low reving, low performance MZs steel cages are fine

[parrbd]

What is the reason for using C3 bearings when the engine designer specified C4? Do you think there will be some sort of improvement?

[Kruh]

There are no meaningfull improvements or losses to be had.
C3 bearings are just more widely avaliable and more affordable.
Most newer bikes use C3 anyway, even high performance machines, so there is just no need for an old MZ to use C4. In fact people use C3 in their old 2t racebikes.

Also, I've heard that the standards back then in DDR weren't the same as the ones today. So a C4 bearing back then is equivalent to a modern C3. Maybe someone can confirm this? Seems likely.

[Puffs]

parrbd wrote:
What is the reason for using C3 bearings when the engine designer specified C4? Do you think there will be some sort of improvement?


Yes parrbd, good point. Originally I was thinking along the same lines, + that they have been making engines like this, in Zschopau, since the 1920's or so. As said, at the time I did that work I never looked at any original manual, as I didn't have any. And it looks like what came out were C3's, and I just put back the same class bearings.

But see that that original German spec for the EM250 says C4f. That might be some DDR classification? I am not certain C4f is the same as what you nowadays get when you ask for a C4?
In an attempt to clarify, I had a look at the "Reparaturhandbuch MZ ETZ 125, 150 & 251.pdf", which is dated 1/07/'89, so just when the wall was about to fall. I think that's the latest official manual there is on these engines, but still in DDR times. The EM250 engine and the EM251 engine are largely the same (with some differences in the barrel). There I find:

Technische Daten EM251.jpg (19.55 KiB) Viewed 53 times

C46??? What is that???
They show open metal-caged bearings:

EM251 - 6306 is open.jpg (43.65 KiB) Viewed 53 times

But elsewhere I saw a similar plastic caged bearing as BM showed - I guess it varied with time?
And they seem to have a certain predisposition for these C4f's - they also seem to use them in the gearbox...

EM251 - gearbox also has C4's.jpg (32.03 KiB) Viewed 53 times

I agree with what Kruh says, and I'd put C3's, as I think I did many thousands of miles ago (something like 20k miles).

Not yet entirely satisfied, I finally had a quick glance on the German MZ forum, where I see written that:
- modern-day C3's are commonly advised for the main bearings.
- C3's should also be considered in the gearbox where the manual specifies C4f's. The extra clearance should reduce the chance of jamming.
- The C4 thing with even more clearance came from the higher production tolerances in DDR times.
- C3/4/4f has nothing to do with the cage type, and yes, plastic cages were sometimes used. They are supposedly equivalent to open metal cages.
- the 'f' addition may have something to do with being a low-noise bearing. In DDR times.