Plain bearing failure

[kangurada]

I've got an engine that I was told had failed shortly after being rebuilt. Taking it apart clearly something about the plain bearing has failed but I'm not sure what or more importantly quite what is salvageable.

I've pictured the bushing in the crankcase, the crankshaft, and a ring that went over the crankshaft journal. The ring is a fairly tight fit on the crankshaft but loose in the bushing. Its outer edge i.e. the edge that faces the crankcase is chewed up. The bush isn't seated completely flush against the crankcase, it appears to be square to it but about 0.5mm away. The movement in the crank took out the stator!

Any advice on what might have happened or what minimum needs doing gratefully received. I've read the previous thread on roller conversions but can I reclaim what I've got?

many thanks!

[3potjohn]

Hideous!
I've only seen my bottom end once so by no means an expert.As I understand it there should be a plain main bush fitted into the bearing carrier. Is this ring in fact the main bearing bush? I just went through my boxes of bits to see if I had one kicking about but it seems not.Where was the engine rebuilt? I have a bearing convertion on my crank,following a seizure 3 years ago, the latter induced by the size of the bill.I guess the punchline is what's the state of that journal?

[buzby]

I,ve got an engine in pieces with the same problem.The stator was destroyed by the play in the crank.
Hopefully the experts on here will have the answers.

[EVguru]

When the engine design was converter from ball race to plain bearing, a housing for the shell bearing was fitted into the original ball race recess in the crankcase (125 single and 250 twins retained the original setup). You can see that housing with it's oil drain channels in your third picture.

Smearing of the bearing like that looks like lubrication failure. The oil pump drive is only by friction, probably to save the crank cases in the event that the oil pump siezes and it's not unheard of for the drive to have come loose. On the other hand, maybe the bearing siezed due to insufficient clearance and then spun in the housing (that's really still lubrication failure). What do the big ends look like?

It's just possible that the crank journal could be polished to remove the smears of white metal, but really it's a regrind job. You'd have to check that the right undersize main is available. If not, then the you'd either have to get the crank metal sprayed, or go with the roller conversion. Depending on who is doing the grinding work, there may not be much difference between costs.

I'd want to do as much as possible to find out why there was a problem in the first place. Try and measure the journal where metal isn't transfered and see if it's already been ground and whether it was to the correct size. I know of one engine shop who very carefully reground crankshafts to precicely the middle of the manufacturers specified range, but they hadn't checked their micrometer and it's test piece for many years and they were a few thou off! Take out the sludge traps (they'll have to come out anyway) and see what's in there.

[kangurada]

Paul that's really helpful and I'll see what the micrometer says.

The bigends move freely and have no discernible play except for a lateral movement of about half a millimetre from side to side on the crank.

What I find confusing is that if there was insufficient lubrication it would result in such looseness of the bearing in the bush rather than overheating and seizing together of parts.

Thanks and I'll report back after next time in the garage ...

[EVguru]

What I find confusing is that if there was insufficient lubrication it would result in such looseness of the bearing in the bush rather than overheating and seizing together of parts.

Morini used 'white metal' or 'babbit' material for the bearings. It melts at quite low temperature and makes an actual siezure unlikely. The shell then probably srunk as it cooled.

The bigends move freely and have no discernible play except for a lateral movement of about half a millimetre from side to side on the crank.

They're going to have to come off anyway, and you want to see what scuffs, scratches and dirt have found their way into the bearings.

[kmamac]

For what its worth that crank looked like my 500 crank I bought in bits. NLM ground the crank down and fitted a new bush in the ring thing. I'm sure fitting a 'proper' bearing is the best way but it seemed a bit pricey when I inquired. I cannot recall the exact price but it was not unreasonable and significantly cheaper. If you want I could dig out the price but it may be quicker to just give NLM a call.
Regards
Keith MM