Crankshaft worn

[Blurredman]

Hi guys,

Waiting for some equipment to carry on on one of my other vehicles, and not wanting to lose out on time when I have a little bit of it available I decided my interest in this problem had grabbed me.

So yesterday the engine came out and other than 1 screwhead that I had to drill out of the crankcase, the engine split went very well and quickly too.

Foremost, before the split occured I could inspect the little end bearing and you know- it was perfect, it wasn't all smashed up or had rollers missing etc. So onwards I went.

As expected, it went easier/quickly partly because there seems to be no interference fit between LHS inner race and crankshaft so it just came apart. The RHS of the crankshaft was very secure in the RHS bearing.....

But this is what I found....





What is this !? It feels like common green grease. It could be just an accumulation of oil residue left behind from petrol? It isn't gritty but it appears to be embedded in the bearing too as I expect it might be if it was in the crankcase, but oddly enough it is only on the LHS bearing. The RHS bearing is completely clear of this gunk and so is the RHS crankcase.. There seems to be little specs of rust on the crankshaft itself on the edge of the shoulder where it enters the bearing, and also a little bit on the inner race but also some possible witness marks of water being evident at the bottom of the crankcase (oil/water line)- Hmm..... But other than this gunk, I can't see any reason for failure per se (other than the slipping bearing inner race- but is this gunk a reason for that, or just coincidental? Or simply byproduct of use?). Engine was last apart @ 7k miles ago. Perhaps indeed, the clutch slipping has just overheated the crankshaft pinion maybe..

I suppose the metal I found behind the seal initially in this thread was the bearing cage/balls wearing? From the inner race seizing, or just failing to stay interference fit with the crankshaft so that the crankshaft produced this metal by simply spinning in the race? I offered up some old bearings to the crankshaft, some of them did just slip on, others needed force to get on and off. I measured the diameter (although with a basic vernier caliper and with restrictions of access) and found the bearing in the crankcase had inner diameter of 29.94-29.96mm and the crankshaft pinion had 29.88mm varying by 0.01mm depending on placement.


Additionally I found another issue however. I noticed on taking off the stator rotor that the seal seemed to be leaking:


It looks like the spring of the seal on the RHS has somehow come off (or inadvertantly incorrectly been installed which is most likely- and which therefore most of this wear probably occured in the first few thousand miles) and has not only therefore allowed a slighy weepage leak, but also worn a groove on the pinion. It isn't too obvious in the picture due to poor focus but it is actually quite pronounced.. I have however offered up a spare seal and it seems that with the seal facing in either direction it doesn't look like this will be in issue for the sealing surface that the seal actually rides upon...




So, basically... My plan, is buy a new bearing for LHS crankcase, I might need a replacement seal for RHS. Depending on how the new bearing fits on the crankcase I might go down the route of bearing retaining compound... It's likely I could install the RHS bearing with spring/lips facing towards the stator.


SNH bearings (https://www.snh-europe.com/en/): on the face of it, all the bearings that are installed (SNH) do not have any play that I can detect and all feel good.. So...





I have been in discussion with my friend RE plastic cage vs steel. He said the only manufacturer he could find was SKF with C4 clearance and plastic cage. Though with a price of £65 per bearing (?????) Seems a bit steep. Anyway, he mentioned that although the plastic cage is most ideal in regards to minimal wear and something to do with backpressure against crankcase (??) But, later Saxons apaprently just used metal cage so...... In all honesty, I don't think I need to read into this too much. Just get a new c4 bearing (although, i'd need 2x, seeing as the RHS bearing is also C3 currently).



I would like to know why Ost2Rad supplied C3 mains bearings https://www.ost2rad.com/MZ-Spare-Parts/ ... Z-250.html and actually seems to (maybe lost in translation) promote them over what is stated in the manual. Anyway, I can only imagine how hot that pinion got in order to let the race loose, because certainly it was a tight fit originally (I of course remember).


Given by some of the discolouration of the steel washer between crankshaft pinion and clutch carrier bearing, which was blue in places the temperature of that pinion could have been anywhere between 500-600f (250-315c). Definitely temperature at which the race could let go. But again... Why? As mentioned it wasn't long ago that I looked in the area very recently and found no evidence of issues. And clutch slip has constantly been a problem for me on this bike for the last few years.

[Puffs]

Busy man!

So it is not the small end. On reading back what Guesi suggested, he called it "the needle bearing of the conrod", which may mean either big or small end, but I presume the big end is also still fine (?).

On your "What is this !?" greasy muck with speckles, is it possibly the result of an over-heated oil? Which oil was used for the premix? Only on LHS --> that was the side with the race clearance (the source of heat? or was that the slipping clutch?), maybe that seal leaked --> gear oil? Did you find similar in the gearbox? Note that with that race/shaft clearance, the LH seal was likely leaking.

Maybe some ingress of water might cause your issues, but I wouldn't know how that would work. That rust may have been there for years.
With water, oil may give a mayonnaise-like emulsion (often seen in rocker covers of cars), but I do not see that (and the water would also evaporate).

On the source of the metal filings, that started this thread, the only sources I can see are:
1) piston --> you should see that, and they would be non-magnetic
2) rings, cylinder bore --> you should see that.
3) small end bearing --> inspected & OK. You would see scoring on the inside of the conrod.
4) big end bearing --> minor wear on the needle cage might be difficult to spot, but normally big-end failure is quite obvious.
5) the LH main bearing. Was it a C3, or a C4? Yes, it might come from the metal cage, and it would still be very difficult to see. I have had a main bearing where the balls & race were all still fine, but the cage had worn through. In your pics I cannot see any evidence of something like that, but I would look very well (magnifier), and still be most reluctant to re-use these bearings, particularly because of the poor fit on the crankshaft.
6) a wear product from between the inner race and the crankshaft pinion. You should then see wear marks on the shaft, which would be the softer of the two. It is conceivable that somehow that bearing seized, being heated from the inside by a slipping clutch, if the race metal would have a bigger thermal expansion than the shaft metal. A bit tentative and you did not mention any visible wear marks.

In a way it's a pity you did not find any clear source, but absent of that, to me #5) then seems most probable.

Comparing inside & outside (ID/OD) measurements with a vernier calliper may not be reliable. A modern digital calliper has a readout which is more accurate than the measurement itself, and note you use different claws, and also a different preload, for an ID & OD measurement. Nevertheless, that race is too loose.

FWIW, I measured the same race on my old crankshaft, and found 30.05mm with a modern digital calliper, and 30.00mm with a conventional high quality Mitutoyo, which I trust more. It's outside the range of my micrometer.

Comparing values measured with different tools introduces another inaccuracy, but 29.88mm for that crankshaft pinion may indicate it's too small. I have no experience with any bearing retaining compound, it's easy, but it may require a leap of faith yet I'm not sure on the long-term reliability. Chroming/electroplating 0.12mm on should be doable, might be more reliable, yet is a non-standard method, and will be more hassle & more expensive. But the only alternative is a replacement crankshaft. So please keep us posted how the bearing retaining compound works!

Right, on to the RH side. I think what I see there is, seen from the inside, the bearing, the oil guide, and the spring from the seal, which has partly come off. Very odd. And ground a groove in the shaft, just inward of the black ring indicating where the rubber lip ran. Some bother, but it shouldn't impact sealing. Yes, I'd invert the new seal, if it's a single lipped one. New seals on both sides. Will you install C3 or C4 bearings?

Sorry for your misfortune & hassle...

Edit: following your edit on the plastic cage and the silly price of £65 per bearing: the plastic cages will take more volume than the steel ones, hence marginally increase the pumping efficiency, just as inverting the seals does (only even less). Maybe that's what he meant by 'back pressure'. Small fry. Note the earlier pic of an SKF C4 with a steel cage.

Yes, use C4. Using C3's might have caused it???

Yes, I'm afraid a constantly slipping clutch can get & make things very hot...

[Puffs]

Further comment on OstZweirad's offer: yes, the the daddy of inventions must be raising his eyebrows, but I cannot see how to get this text in German. What is shown might be the original. And 'SNH-Europe GmbH Leipzig' is doubtlessly just the EU distribution centre for their products made in China, but once again: that's not necessarily bad.

And if there aren't any marks on the part of the shaft where the bearing is fitted, I still do not understand how it can have reduced (that much?) in size. Surely it can become hot due to a slipping clutch, but that part is not under tension, and I cannot see it becoming so hot that it would easily deform. That tempering colour chart should not be confused with an incandescent colour chart - it won't get 'glowing hot'.

Nevertheless, something appears to have happened. Apart from the accuracy of calliper measurements, some bearings fitting easily on that shaft shows it's too thin. Not good. And if such a shaft can get thin, I wonder how effective that bearing retaining compound will be.

[Blurredman]

Hi,

Yes a little progress here. I have some parts, but have yet to buy the main bearings. I can get a pair of Koyo C4 for a good price.

Interestingly I tried to save the seals if I could, esspecially the clutch side one as that had not long been installed. I used a pull-hammer from the inside and got the RHS one out okay. And was able to pull out the bearing, seperator plate and then simply pop the seal out. Unfortunately this one can't be re-used anyway as it has been leaking because of the mis-assembly on my part. The spring went back to it's proper position okay but all that time of the lip being a bit deformed I wouldn't want to put it back, at least not on the clutch side (because it's more of a pain to replace!).

The clutch side bearing just didn't want to come out. I don't have any source of heat (except the oven but then i'd end up potentially melting the seal rubber) so in the end I just punctured the seal anyway and then easily drifted the bearing out from that side. So, two new seals I have now got ready to go in (single lipped, and Nitrile).

I have Loctite 609 which if the spec sheet is to be believed that can withstand up to 150c so should hopefully be fine for this application.

I tested a bearing on the crankshaft and interestingly although the inner race was easily easily rotatable when the engine was assembled, it was actually a lot more difficult to do it when everything was disasembled. But was essentially easy enough to remove (it didn't fall off though)..


But, as it is my plan is:

Install 2x new bearings (koyo c4)
Install 2x new seals (nitrile, single lipped), both side to be installed with the spring facing outwards of the crankcase.
Loctite LHS inner race/lhs crankshaft shoulder.


One thing to note, whatever the preconceptions of SNH bearings, they felt really really good before removal. No descernable play, no noise etc... I'd trust to use the RHS one again actually but for the fact they were both removed with a pull-hammer (well, the LHS wasn't, but i certainly tried- and therefore damage probobly occured), and new bearing will be C4 instead of C3.. I couldn't see anything like a crack, or a broken cage etc in the bearing, and actually as mentioned before my spare old bearings which I had lying around also fitted onto the crankshaft in the same way- but I suppose they might indeed if metal has been removed from the crankshaft pinion.... I think the metal dust on the bearing is just from the same source as wherever the swarf behind the seal came from (crankshaft pin?).


The small needle bearing is good and conrod is not scored in that area. The large one is more difficult to see, but once again there's no play, what I can see looks shiney and perfect.

Piston: Looks the same as it was last time I checked it. A little scored but absolutely fine!


Still also a bit confused as to the metal swarf (it looked like a lot, but actually a lot of swarf can become of a small thing like this- It's all gone so I can't see if it's magnetic or not). And, yes there could have been scoring on the bearing race/crankshaft but they're smooth as they should be- but why wouldn't that be the case I wonder? If it was so close a fit then the wear might have been so circular that any witness marks that point to the obvious would only be apparent if the crankshaft was so loose in the bearing that there would be play and a hot spot I think. There's nothing decernable to see there at all..



I think using the same crankshaft but new bearing + retainer compound is worth a go! I'm not really losing much doing this. Who knows, the new bearing might not even need the retaining compound... - we'll see.

[Puffs]

Ta for you reply BM, some quick comments:

- Koyo is fine IMO.
- I wouldn't re-use main seals anyway.
- Your plan sounds fine, though I'd prefer Viton seals for the better high temp reliability. The only problem is that the source of that metal grit has not unambiguously been identified; there may be a gremlin lurking in the shadows.
- When assembled, a big-end is difficult to assess visually, often all you can do is feel it, for play or rugosity.
- But if the metal grit were to come from the big-end, you should have found it on other placed too. Only finding it between the LH bearing and the LH seal suggests it come from that area. The bearing's metal cage need not be worn through, yet it might still come from the inside of that cage.

Good luck!

[Blurredman]

Okay, it's been some time but that's because things have not gone to plan exactly.. plus all my other projects.. I only decided to investigate my problem now because i've stalled on my other projects.


So.. the engine went together fine with the new bearings. The crankshaft was put in place on the tight race side, and then green bearing retainer material was used on the other side of the crankshaft and a bit on the inner race of the bearing. Anyway, all went to plan and all very nicely too.

So about 15 or so hours later on that night I installed the bike in the frame and I got it started and it ran well. I thought, wahay we're all good to go. Great, about half an hour testing things all sounded good. It was however getting late so decided to call it a night and planned to go out the next morning.

The next morning arrived and I finalised some things (like attaching clutch cable and stuff like that) and I got ready to go, but soon after starting the bike, about 20 seconds or so I heard a metalic 'dink' sound and the cylinder sounded very wrong, I turned it off.

Cylinder is out as is the barrel and here is what has happened. The top piston ring on my 298cc piston has broken. Not sure what happened first whether the ring broke because it had twisted around and interfering with the exhust port, or whether the break occured first and that allowed the twist. However, looking at the piston ring retaining pin, it is quite deeply recessed. It is my opinion the ring was allowed to turn (because the pin has worn or managed to walk too far into the piston) and then that made the damage. Barrel is usable I think still. Piston however will need replacement.

[Puffs]

OK, so then it's something similar to what happened to my YZ... From what I can see, the damage to your cylinder wall seems very little (if any) - much better as what I had. So it looks like just a new piston & rings will sort this problem out.

For completeness, a pic of a normal locking pin in a 250cc ETZ:

250cc ETZ - top lock opposite exhaust, locks from side.jpg (24.38 KiB) Viewed 61 times

In this engine, the rings lock onto the pin in a sideways manner. See the sideways recesses in the rings. But surely, if the pin goes too deep into its hole, that will allow the ring to rotate, and stick with an open-end into the exhaust port, which is the largest of the holes in the borehole wall (at least over the path of the rings). The top ring locks opposite the exhaust, the other 2 at roughly 120°, over a closed section between exhaust & overflow ports. But obviously the 300cc has only 2 rings.

Another common way to lock rings, in a 2-stroke, is from below. The rings then have recesses on the inside, allowing better coverage onto the borehole wall. Here in said YZ250:

YZ piston - rings lock from below.jpg (21.42 KiB) Viewed 61 times

Sorry to hear of your misfortune & hoping for an easy fix. Check that you accounted for all the broken ends of the ring.

[Blurredman]

I have a new piston ready and waiting to be installed. Measurements seem to check out- just need to remove the small burrs that occured on the exhaust port, and to find the missing piston ring (probably in the exhaust but it could still be in the crankcase, you never know).

[Blurredman]

I finally had time to install the thing. All went well. Bike sounds good too (albeit sound on video isn't the same as real life). Also apologies for poor camera quality, you can see I tried wiping the camera a few times. But it was a hot and sweaty day, what with the other things I had already tasked before tackling this ETZ298.

video-1661102846.zip

(3.23 MiB) Downloaded 889 times

Started very easily after assembly and a quick timing adjustment/check. Now for careful running. Old random picture for fun.

The plan is to come back to it in 1,000 miles. I'm going to take the clutch off and inspect for any discernable issues like melting seals. I might even remove the seal and inspect the bearing (replacements can be just a couple of pounds). I will also be looking into a different issue concerning the kickstart at that time. Who knows, maybe i'll finally properly remove the oil pump too- It's been pumping the same little bit of oil in a loop system for years, and it's easier to install the clutch cover without the locating drive key in the way!).

[Puffs]

Congrats! Compression feels OK?

I normally find that in recordings, running engines often sound harsh & sharp, which is probably because of the quality of the microphone in common phones/video cameras (not a large diaphragm). Nevertheless, it sounds fine! Yet like me, you probably assemble with ample lube, so it might get a little rougher after it's warmed up & that's gone.

Yes, remove the oil pump - that's easy. Just make a plate:

[Blurredman]

Finally had the opportunity to-day to wear in a bit that piston/rings. Unfortunately I didn't get far.

About 2 miles in I decided the idle was a bit high, and at the 5 mile mark I actually did something about it (no traffic/stopping you see). And well, an old problem came back, it was a bit missy, but ultimately it stalled at one point and the engine stopped momentarily and then it carried on going, but the rev counter was not going. I've been here before says I. I ride home.

The problem is that the engine is running backwards! Hence the rev counter not registering. I had this occur from '17-'18 and I did all sorts to try and rectify the problem, from re-building the engine (putting a different crankshaft in it), to putting electronic ignition on it, and lots of things in between but ultimately it was a desperate moment when I decided to ditch the electronic ignition at the time (I later sold it on) and put it back to points. And the problem never occured again. I must say though It ran backwards with points before back then, which is why after many part swapping and replacing I decided to try electronic ignition (the only reason I bought it at the time). So, to re-iterate, I still had the problem and reverted back to points and for years it never ever ran backwards ever again, until now!

Obviously I've timed it many times during the next process so I think I was spot on. But after I got back home from realising the above, I checked the timing (was fine). Changed points, changed condensor, and after each item was swapped I gave it a test. My test is basic but ultimately most useful. The test is to start the engine (so far has always started whilst rotating in the proper orientation-rev counter working). Then when it's idling just turn the key off and turn the key back on before the engine stops completely. about 2/5ths of the time the engine would come back to life but backwards.

Put the original points and condensor on. Taken rotor and alternator off and made sure surfaces were clean and clear.

The annoying thing is I don't even know what to look for, as in 2018 it seemed to just be a fluke!

For experimentation I decided to put the timing adjustment on it's furthest most advanced and it's furthest most retarded, but my test results are the same. A backwards running engine.

I unfortunately don't have another cam I can use, unless I temporarily steal it from my brother's MZ 30 miles away, but otherwise I'm not sure what else to do.

To be sure on timing, you get the lobe to actuate the points to their highest point (at TDC in my case, always has been) and gap to 3mm, then turn the engine backwards to 2.5-3mm BTDC, and then either by taking the spark plug out and keeping it in the cap, or using a timing lamp, or even just listening/watching for the spark you can with the ignition on move the timing plate. The points should not open at any other time then approaching TDC I'd say. The difference with this bike compared to other MZ's is that the points don't seem to stay open for very long, once the contacts start to close just beyond TDC they close immediately- is it possible my cam is not allowing enough dwell? Aren't points stay open for about half a rotation on a single cylinder? Or am I wrong in thinking that? I know obviously the longer they are closed before they open gives the ignition charge more time.


But here's the annoyance- I haven't changed any timing components on this engine in it's recent downtime! Gah! Help? The damn thing is fighting me all the way lately.

EDIT: checked my Trophy and the points opening/closing happens in the same way there so not sure that's an issue, now. Scratching my head!

[Puffs]

The 3mm gap is probably a typo, but the points gap is 0.3-0.4, I tend to use 0.35.

But yes, we have been over this before. In principle a conventional battery/points/coil ignition will also give a spark if it runs in the reverse direction. If the points close, the primary side of the coil charges, and then when the points open you get a voltage spike in the secondary side of the coil, and a spark. You don't need any rotation for it, just the points closing & opening. Only: normally the points stay open for a long time, and only close near BDC. Yes, that's Bottom Dead Centre. So if the crankshaft runs backwards, the points will open near BDC, you get a spark, the points stay open, and then, at 2.5-3.0mm ATDC they close again, and the coil re-charges. If all is perfect & ideal, but in practise there might be some cheerful dancing of the points near TDC, when they close and/or when the gap is small, and that might cause a brief charge (dwell) followed by a spark. In the attached document by Lothar this is further explained.

Nevertheless: what to do? Mine never did this. Surely it kicks back a bit, if I kick it wimpy, but it never runs backwards. Yet, as said, a little while back my brother had a Hva 250 that did this too (which is very awkward at the starting line), so it definitely is possible. It must be something in the shape of the cam, or the cam follower, or the spring of the points, or the hinging of the points, something that causes the point to vibrate & make a contact near TDC, when running backwards. It might even be play/vibration in the crankshaft's main bearings. But an EI should not be susceptible to that (although that could still give a spark near TDC). Replace items one at a time to diagnose.

But on the up side: it sounds like the crankshaft is OK now, and the piston is fine too! (Right?)

[Blurredman]

Yes, okay, interesting. Thank you for the document. Interesting reading.

As mentioned I'd already replaced the points and various ignition components including back timing adjuster plate etc and stuff when I got home yesterday after the quick test ride to see if I could locate/diagnose the problem, whilst I was still prepared to work on it. I myself also don't think it's a timing issue. The problem is, when this last occured in 17/18 I didn't really look into it other than replacing parts and getting myself into a bit of a rut as is what happens sometimes. We've all been there I am sure. I guess when I put the points back on in '18 I might have set the contact breaker gap a bit large, or maybe the cam or timing plate itself ended up seating in a slightly different position (perhaps from a slight bit of grit or something alike), or something stupid like that- and it simply eliminated the problem and made me lucky! And I carried on for almost 5 years with no more problems but ultimately without ever actually understanding WHY.


Yes after the recent rebuild I did set the contact breaker gap to 0.3mm (not 3mm!).


Now looking into it, it does seem very plausable that there is just points chatter, and it explains how it has come back despite no electrically mechanical item being changed in the bike's downtime, other than the fact that it has simply been disasembled and re-assembled.


What I'm going to try to do is put a low resistence in line bulb between the contact breaker + and earth. Spark plug out, put bike in 5th gear. And see if that light lights at any other time other than 2.5mm BTDC. And do the same in reverse. We'll see what we get. It might not be sufficient enough, vibration wise to do anything there, but it may or may not be caused by vibration, it could simply be an anomoly that could be adjusted out.
So yes we'll see what that gets me and then regardless of outcome I suppose I'll indeed put some greater gap on the contact breaker (0.4 maybe yes). And then do my further physical testing (ignition cut out/in) and gauge the results.

[Puffs]

OK, so you'll just get a light indicating when the points are closed. For the entire dwell period, typically 180° or so, very useful for setting the timing. But I forecast you'll get exactly the same lighting pattern, only in reverse, if you rotate the engine backwards. I suspect the chatter / bouncing that causes the spark requires more speed and/or the vibes of the running engine.
And also, a very short period of contact might be sufficient to generate a spark, but too short for an incandescent bulb to react. A LED between the coil's primary + & earth would react quicker & might overcome that issue - but not the former one.

I'd try to polish the cam.

[Blurredman]

Yes it's true, I was also considering an LED. Potentially, going very slowly might prove something, which is obviously much easier too with leverage rotation of 5th gear.


How would you polish the cam?

I'll have a think and ponder tonight.
Certainly there is a washer between my cam and the stator rotor- it's possible that removing the washer may provide a different riding surface for the shoe of the contact breaker, for the washer itself is roughly 1-1.5mm in thickness.


Interestingly as a side note, i've been looking through posts/PM's and it seems like I had already tried my brother's cam in 2018 when this occured and that didn't resolve the problem back then.