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I had a spun main bearing due to low oil, the crankshaft journal is pretty scratched, no dents just looks spun on the surface.

enter image description here

To the inexperienced diyer it's hard to imagine this would be a huge issue if you put on a new bearing yet everything I read says that any markings on the journal will ruin your replacement bearing asap, I think I have that correct.

So is this right that crankshafts will only work if they are like factory new polished surfaces, nothing else will do?

Does anyone have any experience with trying it without grinding, I know you can use Emery paper and try and polish it a bit but it won't be 100%.

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  • Looks pretty good to me, use fine emery cloth to remove the bearing lead from the journal. Looks like you caught it before journal damage started.
    – Moab
    Jul 19, 2020 at 19:12

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The bearing material is softer than the crankshaft, so any imperfections will chew up the bearing.

There is no way you will get an accurately cylindrical shape with the correct diameter and surface finish using emery paper, even if you think you have polished out the scratches.

Any imperfections in the shape will also tend to destroy the oil film in the bearing. You don't want a high spot scraping the oil off the metal surfaces at every rev of the engine!

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  • Can you give an estimate how long a new bearing would last with this journal damage?
    – Andy S
    Jul 19, 2020 at 18:53
  • Can you give an estimate how long a new bearing would last with this journal damage?
    – Andy S
    Jul 19, 2020 at 18:53
  • SolarMike's comment above is the correct response. In English: there's no way to know.
    – DavidRecallsMonica
    Jul 19, 2020 at 19:38
  • It will last till it fails again. Sorry if that isn't a useful answer - but note that it implies it will fail again. The point is, you won't get enough oil to lubricate properly where the clearance is too small, and you won't get enough oil pressure to lubricate properly where it is too big. The actual bearing surfaces are precision engineering, even if the rest of the crank component looks fairly crude.
    – alephzero
    Jul 19, 2020 at 22:49
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TLDR: To do it right and not have to do it again in a month (or possibly right after start up), the only way to fix this is to either pull the crankshaft and have it turned, as well as check the connecting rod, or replace the engine (might make more economic sense to do this, depending on the engine).

There's a whole other part to this nobody here is seeming to understand or is communicating. There are two parts to what you're looking at: the crankshaft and the rod (not to mention the bearing itself). When you spin the bearing, there has to be an issue with either (or both) the crankshaft journal and/or the big end on the connecting rod. If there is too much friction on the crankshaft journal, it can grab the bearing and spin it, which damages both the journal on the crankshaft as well as the big end of the connecting rod. If a bolt stretches too far on the rod, or it gets out of shape, it can allow the bearing to spin, which damages both the journal on the crankshaft as well as the big end of the connecting rod.

Many engines use multi-layered bearings (not knowing your engine, I wouldn't know ... newer engines use solid aluminum bearings). The backing on multi-layered bearings (the part which faces the connecting rod) is steel. If this spins inside of the connecting rod, it will cause damage. You have to have the connecting rod inspected to ensure it isn't out of round or beyond tolerance. If you don't, the same thing will happen to you.

As someone else alluded to here ... you can either do it right, or you can do it again. If you expect the engine to last longer than a few miles down the road, you need to take things out and have them inspected and machined.

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  • Good answer thanks. Apparently you can also polish the journal by hand with Emery cloth. The end cap bolt actually popped out of the socket and ended up in the sump. This resulted in about a 0.5cm^2 hole right at the very bottom of the cylinder bore. How would this affect everything, I can't see it needs to be an issue as it's right at the bottom. I was thinking of buying a used piston and new bearing, polishing the journal and hoping for the best but now I notice this hole in the cylinder.
    – Andy S
    Jul 19, 2020 at 23:45
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    @AndyS - Yes, I just answered your other question. If the bolt popped out of the cap while it was running, you're lucky things didn't find its way out of the side of your block. Also, if the bolt came off while the engine was running, then the rod cap is almost definitely deformed. Machine it or replace it.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Jul 19, 2020 at 23:51
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If you bodge it then you get to take the engine out a second time - good for practice...

So, mic up the bearing surfaces and if it is not perfect then get it machined.

If you want an idea of what the bearing has to do, take the bearing diameter, multiply by pi and then by 2000 or 3000 to get an idea of the linear speed. If the surface is slightly damaged then it won't take long...

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  • Can you give an estimate how long a new bearing would last with this journal? i.ibb.co/hHfBx3m/IMG-20200719-120243.jpg
    – Andy S
    Jul 19, 2020 at 18:55
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    @AndyS yes, sure seconds, minutes, hours, days, weeks... - take your pick but you will guess wrong.
    – Solar Mike
    Jul 19, 2020 at 18:56
  • You must have some idea/experience?
    – Andy S
    Jul 19, 2020 at 18:59
  • @AndyS There is no way you can predict how long that bearing may function - some can continue for years but sound horrible, others seize and rip the engine apart. If you want that engine to run trouble free then build it properly. When I have rebuilt engine I have replaced all the bearings - crank, cam etc as a matter of course and then I can expect a good life out of them. You should measure the clearance with plastigauge or an equivalent.
    – Solar Mike
    Jul 19, 2020 at 19:01
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    The one time I did a half ass engine rebuild , it lasted about 45 minutes. i decided engine rebuilding was not my thing and concentrated on college.
    – blacksmith37
    Jul 19, 2020 at 21:43
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The picture is a rod bearing . Push the rod up and hand turn the crank so you get emory around the journal. I think you can get some level of repair by running emery cloth over the journal. If the mic shows it still in tolerance you may get lucky with one rod bearing replacement. However , because you say it was caused by low oil, all the bearings saw low oil so chances are there is other damage. Anything more than a half -ass job will require pulling the crank ,which almost certainly means pulling the engine.

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    Who has replaced main bearings without pulling the crank ? I thought it might only be me. As a boy I worked part-time in a transmission shop ,for a smart guy. The GM Hydromatic plant burned down in about 1953 ; No Hydomatics for Cadillac and Olds. So GM put in Dynaflows. Buick engines had very tight control of crankshaft end-play, Cad and Olds did not. After years of service the end play in the Cad and Olds engines tore up the front pump on the Dynaflow. So the rear main bearings in those engines needed to be replaced ( the only bearing that controlled end-play.).So I replaced several .
    – blacksmith37
    Jul 19, 2020 at 21:58
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I totally mangled my crankshaft and threw one set of bearings from another car I found in a junkyard, add thicker oil because why not and my engine has been running like a champ for n3 months. The machine shop isn’t necessary

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