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]

Removed

[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]

Removed

[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]

Removed

[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]

Removed