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).

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[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.

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[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.

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[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

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[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.

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