Crankshaft worn

[Blurredman]

Hi guys,

Lately i've been investigating a clutch slippage problem. And maybe by happy coincidence I happened to realise my main seal behind the clutch was on backwards. Now I knew this was the case years ago, but I have actually rebuilt the engine twice and was sure I replaced or rectified the issue, but apparently not. Or maybe I put a new one/same one in backwards yet again? Who knows.

Anyway, I removed it with the standard screw in and pull out method (works very well!) as shown:




But as you can see, there is some sort of metal behind there. I initially decided to just clean it up but after many uhming and ahing and advice I decided to remove the metal plate that sits between the bearing and the seal. There is evidence of heat here. The original plastic caged bearing which the clutch rides on was burnt to the rear most washer and the washer indeed had signs of heat on it as it had turned blue. The seal itself also had burning marks on the inner side.

Heat on the seal and washer:


Clutch bearing:


The metal found behind seal:




Upon removing the metal plate I inspected the bearing and there is the lightest dusting of metal on the cage, but additionall (see video) I discovered that the inner race moves around the crank. When turning the crank the crank slips within the race so it's not completely lose but yes there is play there and you can spin the race around the crank.

I will have to dismantle the engine obviously. It's a possibility that bearing retaining compound might work- but i'm not sure. This is the original crank for the engine at 35k. Crank was re-built and engine re-built with new bearings at 26,000 miles (November 2017).

[Puffs]

Sorry to hear this...

Some comments:
- the metal grit/filings are behind the seal, so should come from the crankcase area, rather than the clutch. It's always good practise to test if it's aluminium or steel with a magnet, and look at them with a magnifier.
- The video shows the main bearing has a steel cage. And that the inner race is loose from the crankshaft - that's not good, and I wouldn't know how that could have happened. They are normally quite tight on it (vaguely remembering things like heating & freezers). Such play could only develop if the bearing seizes, so that the inner race doesn't turn? Do you see any damage?
- Maybe electro-plating a thin layer might work, on either the shaft or the bearing. I wouldn't trust any plastic or glue there.

[Blurredman]

It's not what I want when I have so much more things to do


I don't see any damage, and whilst turning the inner race I feel no problems. It's a smooth bearing! By my calculations, this has occured anywhere between now (obviously) and about 700-1,000 miles ago when I replaced the main driven gear in there (have to remove clutch for that). No signs of over-heating/melting then.

Maybe electroplate could work. I will have to see what it's like when I take the engine apart (when I get time). I do think though that I might try bearing retaining compound (I have the green Loctite stuff) and just see how it goes. But my decision will obviously only bear on me after I take the crank out and start investigating. The surface the inner race rides on on the crankshaft might be a lot worse than at first we think, esspecially considering the metal shavings, and apparent heat that has built up in this area... The problem with the glue is that if it DOES work, then it's going to be a pain to take the engine apart in the future without the bearing puller (to take off crankshaft). Thankfully in all my MZ engine rebuilds, the bearings has always stayed with the crankcases.

But... the question is.. Why has this happened!?

[Guesi]

First of all, according to MZ , the oil seal on the left side of the engine has to be installed with the full side(the side where the letters and numbers are written) showing towards the crankshaft.
Not like shown on your pictures.
Second: The metal parts shown on the oil seal seem to come from the needle bearing of the conrod.
So the crankshaft is defect, you have to change the crankshaft...or rebuild it.

[Kruh]

First of all, according to MZ , the oil seal on the left side of the engine has to be installed with the full side(the side where the letters and numbers are written) showing towards the crankshaft.
Not like shown on your pictures.

What is the reasoning behind that? I'm very curious as to why the seal would be installed like that (sealing lip facing clutch).
The only reason I can think of, why MZ would recommend installing the seal that way is ease of installation. It prevents any chance for the lip to flip on the edge of the crank while installation. Whereas if installed "the wrong way around" the lip can twist on that egde if not carefully installed.

With that being said, I don't think that the way Blurredman installed the seal is wrong. The vast majority of 2 strokes (even ones from the time like jawa, simson, etc.) have the seal installed with the lip facing the crank. The idea being that the pressure inside the crankcase helps with the sealing of the lip of the seal. Some MZ racers install them like that as well.

At the end of the day both ways of installing the seal work. I don't see any obvious right or wrong. It seals just fine in either orientation. Personally, I prefer my seal to be installed with the lip facing the crank (like blurredman installed his). But I'm quite interested if there are any major reasons against installing it "backwards" like that.


As a bonus, that way of installation did help with the screw in and pull out method. Leaving ample area between the seal and oil plate, so the screws don't damage the plate.

[Jay Bee]

First of all, according to MZ , the oil seal on the left side of the engine has to be installed with the full side(the side where the letters and numbers are written) showing towards the crankshaft.
Not like shown on your pictures.

What is the reasoning behind that? I'm very curious as to why the seal would be installed like that (sealing lip facing clutch).
.

I recall asking the same question with reference to my ETZ 125 when prepping for an engine overhaul. (OK, different bike, different engine, but same principles applying).
My understanding is that the seal is fitted with this orientation to prevent oil from the primary drive migrating into the crankcase, and this is confirmed in the Mz Repair Manual where a cross-sectional view of the engine shows the seal fitted with the open/"garter" side of the seal facing the outside of the engine.

[dave47]

I agree with Kruh about the oil seals, but the subject has already been much discussed on this forum with a complete lack of agreement, and is not really relevant to bm's problem.
If the problem started when you refitted the clutch, is it possible you made a mistake? Would fitting the inner chamfered washer the wrong way round be a possible cause?
Is the loose bearing a separate issue? I have nearly always found the bearings stay on the shaft when the cases are split (annoyingly), so perhaps your bearing was always loose. You have not fitted the type recommended for ETZs, ie plastic caged C4 bearings. If it turns easily by hand with a cold engine it is likely to be free with a hot engine under load.
I mean free on the shaft. Its hard to see how this would not damage the crankshaft over thousands of miles.

[Puffs]

An interesting topic!

Firstly, BM will know pretty well what to do, finding the time is the challenge though.

- On electroplating to fill a loose bearing race: to my knowledge this is not a standard approach. But it could work, on the condition that the shaft is smooth & undamaged. I have never done it, but neither have I ever had the issue that the race is loose from the crankshaft. BUT, assuming it wasn't so loose when it was fitted, something has happened to create that clearance. Something to do with replacing that driven gear, 700-1000 miles ago, seems plausible, with the new set maybe giving too much radial load on the main bearing, along the line as Dave said?
Replacing that crank pin would be the conventional way to fix it, in combination with new suitable bearings.
Anyway, they are normally quite tight, as also suggested by this:

Bild 30. Abziehen der Kugellager 6306 von der Kurbelwelle.jpg (18.16 KiB) Viewed 162 times

If it is caused by anything with the clutch, best look at that side too.

- On the orientation of the clutch side seal: the text in the manual is clear, as it the picture:

Bild 3. Explosivdarstellung Motor EM 250.jpg (22.14 KiB) Viewed 162 times

And BM already acknowledged this in his thread starter post. BUT: the common way of installing seals is with the lip side towards the higher pressure, to prevent flow in that direction. That way the pressure helps the seal to do it's business od sealing, and without looking it up, I think I've seen a few engines with the lips on both seals pointing inwards, towards the crankshaft. So I too wonder: why does MZ advice to fit the clutch side seal with the lip pointing outwards? Does it have anything to do with the clutch (here on the crankshaft!) generating oil pressure when rotating? Is the gearbox vented? I made a vent pipe in the oil filling plug donkeys years ago, because I couldn't find a vent. And obviously when the gearbox temperature rises, the air in it expands, and without a vent, pressure will build up inside. (I know the clutch cable will normally leak a bit, but I closed that off better.) Has it anything to do with that? Fitting issues, as Kruh suggested, maybe - but you should be able to see that.

- On the source of the metal filings: as said, because it's behind the seal, it should come from the crankcase side, so: main bearings, big end, small end, rings, piston. Why, Guesi, do you feel it should come specifically from the small end? True, I once had a small end failure in my 251, and it probably less noisy than the alternatives - but still: why that suspicion?
But you are right, if the small end is gone, as well as the clutch-side crank pin, a replacement crankshaft might be cheapest. Yet to press out the crank pin, the big end has to be spit anyway.

[Kruh]

I'm sorry I went a bit off topic. But I'm always curious about this specific topic.

I think I've seen a few engines with the lips on both seals pointing inwards, towards the crankshaft. So I too wonder: why does MZ advice to fit the clutch side seal with the lip pointing outwards? Does it have anything to do with the clutch (here on the crankshaft!) generating oil pressure when rotating? Is the gearbox vented?

From working on a few engines, to doing some reaserch online, seal installed with the lip facing away from the crank are quite a rarety. I can't name any modern engine that would do it like that.

Yes, the gearbox is vented, so there should not be pressure inside. The vent is the rubber fill plug, it has a hole in the midlle of it.

My theory is, like I mentioned, ease of installation. When the seal is installed with lip facing the crank, if you're not careful, it is possible for it to catch on the edge where the clutch washer goes (marked on photo below), an turn in on itself. As MZ was a factory that mass produced these engines, it is quite benefitial that there is no potential for failure while assembly.

Other manufacturers use special tools (sleeves) for installing seals to prevent this issue.

[Puffs]

Thanks for the reply Kruh, and yes, you may be right.
I have an old EM250 crankshaft (the one with the small end failure), and here is the clutch side crank pin:

EM250 crank pin clutch side.jpg (26.75 KiB) Viewed 162 times

That edge is sharp-ish, but not excessively sharp. On this one at least, others might be sharper. You can see where the sealing edge ran, indeed with the lip pointing outward, as installed by the factory.

I cannot remember how I installed the new seal on the new crankshaft, as it was 20 years ago. But I might have installed it with the lip facing inwards, as it shouldn't be too difficult that way too, with a bit of lube. Unfortunately I do not have an old seal to try.

If need be, the seal could be installed with the help of a guide, like here using the slick side of a sticker backing to lead a dust cap, seal & Teflon-coated slide metal over the very sharp edge of the recess for the piston metal, in a YZ fork:

Install Dust cap, Seal & Teflon slide metal over sharp edge for Piston metal.jpg (15.35 KiB) Viewed 162 times

The recess with the sharp edge is below where the paper has wrinkled; a factory will make a more professional tool for it.

Something like that shouldn't be necessary for that crankshaft seal, nevertheless, I can imagine if you assemble 1000's of engines a day, you'd want it as easy & fail-proof as possible, and then letting the lip go with the flow makes it easier. But not better.

Also, pointing inwards the seal should still fit fine on that shaft, well before that tapered section towards the bearing seat.

If ever I need to replace my seals, I'll probably install them both with the lip pointing inwards, as that should be better. And that way it could have a fresh track, at least on the clutch side. Well, unless someone could give reasons why the MZ way is better...

[Blurredman]

Guesi, In the post I already mention that I knew the seal was on the 'wrong way' (according to MZ gods), and one reason for replacement was the scorch marks on the seal (and melted plastic cage on clutch carrier bearing), and thought it would be a good time to install a new one with the MZ orientation.

I won't go into the seal orientation discussion myself much here, let it be said though that I did 7k miles with the seal outter face towards the clutch as in the pictures. Up until I replaced some components in the area only recently I saw no problems here. The only thing I would like to comment about the seal is... Does it have to be an R23.. Would not an R21 suffice?? And if not, why? The secondary lip seems a bit negligible here as it's quite a bit smaller and thinner than the primary lip...

It's true, it could be a small end is at fault here with providing the metal shavings, and I suppose it's a possibility some got in the main bearing and seized it and that resulted in the crankshaft spinning within the inner race of the bearing (although the bearing seems clear to move now with no resistence, at least from this slow speed). At this moment in time that is the only reason I think these two problems (crankshaft loose in the bearing) and the metal have in common and how this has developed. It must be said that the engine sounds completely normal before I took the clutch off. I would have had no idea this was a potential failure in the making if I didn't have clutch issues.

Dave47- I'm not sure whether these are C3 or C4 bearings from memory. I do believe I bought them from Ost2rad. Actually they are SNH which although is a brand I never heard of, I don't think the bearing itself is the failure here. You're right though C4 is stated in the ETZ250 factory service manual.

Actually yes,
this is the kit I bought at the time:
https://www.ost2rad.com/MZ-Spare-Parts/ ... Z-250.html
Additionally with this small end bearing:
https://www.ost2rad.com/MZ-Spare-Parts/ ... Z-250.html

But the C3 vs C4 discussion has been well discussed before as per thread: https://wew.mzriders.com/viewtopic.php?p=52036

The bearing set had a mix I think- i'd have to check on the bearing itself when it comes out as to whether it is c3 or c4. That said, I'm not sure if it really makes a difference. After all this time, I don't think the bearing itself is to blame. The metal fragments are. As far as we can speculate at the minute with the engine still assembled, it seems the fault here (and potentially the blame for the loose bearing) is either the big end or the little end. The crankshaft was rebuilt and balanced by Burwins at the time, and when I re-built the engine last the bearing certainly wasn't loose around the crankshaft pin. That would have definitely had me thinking red flag. I think i've just been lucky that the bearings always stayed with the crankcases in 4x engine re-builds now. But yes, the crankshafts themselves certainly did not simply drop out without a lot of persuasion, in all cases. As for the clutch and the washers, they were in correct orientation and order- I always double and triple check those!


I'm not actually even sure when I'm going to get around to taking this engine apart.. I'm a busy man.. Obviously we can all discuss this topic in the meantime, but with 8 vehicles I have a lot to do without a little discovery like this turning up. The bike is in the corner of the garage for now until I find the energy and time to look at it bearing in mind I have other 'work in progress' situations. You'll have likely seen my other threads.. But please carry on the discussion i'd be interested in your replies regardless!

Additionally, it's worth mentioning that MZ do have a special tool for installing the RHS seal... In two parts it is a special punch and a sleeve.

https://youtu.be/6ptXHE2i0x0?t=2599

[Puffs]

I knew the seal types, but not their names, so: https://www.bearingboys.co.uk/Oil-Seals ... R23--119-a
Both will probably work, yet the un-sprung lip on the double lip R23 could help if installed the wrong way round, and I think double-lipped Viton seals are advised. The 2nd small lip acts more like a dust seal, I'd guess, useful on the ignition side, and on the clutch side it might help???

Viton is better resistant to various chemicals than common nitrile rubber, and resists higher temperatures: ~215°C for Viton, vs. ~108°C for Nitrile (= NBR). On the low end Nitrile is slightly better: ~-40°C for Nitrile vs. ~-28°C for Viton. Viton is far more expensive, yet for crank seals the better choice, IMO.
Edit: Hydrogenated nitrile butadiene rubber (HNBR) has a better high-temperature resistance (typically -40°C up to 165°C, with minimal degradation over long periods of time), and is also used for seals and O-rings, and those are suitable for main bearing seals.

Your analysis (small end failure --> grit --> main bearing temporarily seized --> inner race spinning) is one of the possibilities. It is also possible the bearing's inner race ran too hot (due to the load by the clutch? due to axial load?), and that caused the bearing to seize temporarily, which caused the clearance on the shaft. And the main bearing's metal cage could also provide shavings, that's another. Take the barrel off to see if it's the small end.

That loose race will give the shaft more clearance, so that any seal will have a more difficult time sealing, + of course the risk on more damage.
https://www.nskbearingcatalogue.com/c4-bearings/. From that link you gave, it is not entirely clear if the EM250 needs modern-day C4's, or that modern C3's are fine too - a DDR C4f might be equivalent to a modern C3.

Well, I'd guess you should have heard it if the big end failed.

[dave47]

I see that Ost2rad offer the 6306 bearing in 3 versions, all described as suitable for all the 5 speed MZs.
SNH metal cage C3, 10,50E
SKF plasic cage C4, 22,90E and
SKF Plastic cage C3, 21,60E.

It may be that the SNH bearing is entirely blameless, but based on BMs experience, SKF C4 might be the safest choice for the ETZs if you want to pay the price.

If the bearing did run hot, the inner race would expand making the bearings tighter and the grip on the shaft looser. friction between the shaft and bearing would lead to yet more heat etc etc. This might explain the burnt clutch bearing, but not the slipping clutch.

However I don't want to unfairly malign SNH bearings. There is most likely a specific problem with BMs engine.

[Puffs]

Yes. If the heating-up happens slowly, shaft & race increase in temperature simultaneously, so stay roughly at the same temperature, and with roughly the same expansion coefficient that won't cause the race to become loose. But if the heating-up is quick, the race will have a higher temperature than the shaft, and that will cause clearance. And then, when the bearing gives more friction, the contact between the poorly lubed shaft & inner race will start to rub & indeed give more heat, and might be the cause of the metal? Or maybe the clutch caused the shaft to heat-up so much, that either the of the two deformed, to cause the clearance? Maybe the inner race has a hairline crack? Who knows. There are many ways things can go wrong - or not. For now it's all speculation.

Ta for the prices, which suggest there's not too much difference between C3 & C4, but that, at least with that vendor, the Swedish SKF is considerably more pricey than the Chinese SNH, which indeed may not necessarily reflect a quality difference.

Btw, the SKF 6306 is available in clearance classes C2 to C5, and https://www.skf.com/us/products/rolling ... uctid-6306 suggests they all have a metal cage. I know, "Image may differ from product.", but also the technical document shows them like that. And Bearing King also shows the C3 (https://www.bearing-king.co.uk/bearing/ ... x19mm/4357) with a metal cage, for £8.56, while wrt the C3, https://www.qualitybearingsonline.com/6 ... 72-x-19mm/ explicitly says: "Cage Type: Steel", for $15.47. And actually, also https://www.ost2rad.com/MZ-Spare-Parts/ ... 15169.html is explicit on the C3: "Lagerkäfig: Metall". Maybe at some stage the C4's were produced with a plastic cage, and those now command a price increase? The MZ manuals prescribe a C4f, with a plastic cage. Or did SKF stop producing them with a plastic cage because it gave problems? Again: who knows...

SKF 6306C4.jpg (21.6 KiB) Viewed 151 times

I wouldn't know which to favour, a steel cage or a plastic cage. I think the steel cage is more common nowadays.

[Puffs]

On the question is it better to fit the clutch side seal with the lip pointing outwards?, I browsed a bit on the German forum, and found the following arguments that are valid IMO:

a) A modern seal works equally well in either orientation, but if you fit it with the lip pointing out, the volume of the crankcase is reduced, thus enhancing the piston's breathing.
Reducing the 'dead' volume in the crankcase was, at some stage, a prime aspiration of 2T tuners, because it increases the difference between upper & lower pressures in the crankcase. Of course it's still beneficial to keep that volume as low as possible, but it appears that in later high-power 2T's the focus was more on improving flow (at lower ΔP's). Nevertheless, reducing that volume remains desirable.
I found an old 25x72x7 seal (single-lipped) and measured the volume reduced by having the lip point outward to be about 10-11cc, so yes, that would help, a bit, and it would come easily. But it raises the question: why not do both sides & double that volume reduction? The fact that on the ignition side the outside is dry & dusty should not matter for the seal orientation, as the sharp end of the seal (the bit that touches the shaft) is symmetric. However, if it's a seal with an extra dust lip, that may impact the lubrication of the sealing lip on the ignition side, so inverting a double-lipped seal on the ignition side may cause quick wear of the sealing lip.

b) While a seal is commonly installed with the lip towards the side with positive pressure, or towards the direction from which you want to prevent leaking, in a crankcase the pressure goes both up (when the piston moves downward) and down (when the piston moves up)

Generic port timing diagram of a 2T engine (with piston-controlled intake)

Generic port timing diagram 2T engine.jpg (27.16 KiB) Viewed 146 times

Generic port timing diagram of a 2T engine (with piston-controlled intake)

That is largely true. While +'ve pressures might be slightly higher than the -'ve pressures (with a tuned intake length), the difference is probably quite small. So, as long as the seal is supple & in good condition, it shouldn't matter much how you install it. (As evidenced by MZ's recommendation.) Note that the outside arrows pertain to crankcase pressure, and that on 2T's with reed induction this does not apply; there the +'ve pressures are far greater then the -'ve ones and the breathing cycle is asymmetric.

So I think that the next time I fit (single-lipped) seals in my MZ, I'll fit them both with the lip pointing outwards, to reduce some 21cc of crankshaft volume.