Crankshaft Grinding

Hi there, why grind one one single big end jorunal undersize and leave the remainder standart size would cause a disaster when taking the engine apart and overlook this?

In this example was considered using Standart bearings?

I understand the bearing clearance of the big end that was grinded will be different to the others, but why the engine will fail after the freshen up of the engine overlooking this and not after building the engine this was for the first time? Is there a specific reason? Thanks in advance.

If I understand your question correctly, you are thinking of an engine someone ground a single (1) bearing journal undersize, and used an undersized shell for it, and stock journals and bearings for all the others.

This was, and may still be, common when people would do a very cheap "repair" for a damaged journal, and I know of one instance where a single cylinder was "repaired" with a sleeve and a SMALLER piston was fitted - it was NOT a nice find!

There are two problems that may arise - some crankshafts are significantly weakend if the regrind is done poorly, and if one is doing a careless "rebuild", especially for a stock engine, they may not check every journal* and use the wrong bearing shells on the undersized journal.

*taking that further, MANY years ago I ran a big end - MY fault - and had ALL the crank' big ends ground -0.030". I didn't get around to re-assembling the engine and sold everything to another party - car, stripped engine, full gasket sets, etc, AND a spare set of STD big ends I had lying around. He, in turn, on-sold it to a third party. Some months later the third party came round wanting money from me because his ëngine builder" put it together and, of course, killed the big ends immediately because he DIDN'T check, ignored the tag stating the big ends were reground, and that the bearings were STD. I pointed that out to the aggrieved third party and told him the problem wasn't mine, but his builder and, maybe, the person who sold him the car and engine.

Hi Gord how are u? what do u mean when saying a poorly regrind? Grind it just a little bit and the protective layer removed? or grind too much or just a bad machine shop job?.

I think I get what u said. u sold a crank grinded -0,030 with std bearings and others things, the person who bought sent it to a builder that didn´t blueprinter the engine or didnt check clearances, in that case the clearance was so big, right?

Thank u for answering all my questions, do u have a car shop? I would like to follow u on social media to learn more. Thanks in advance.

All three can be a problem - some crankshafts are given a special treatment to harden the journal surfaces and this can be ground away as it's very thin, some crankshafts have a minimum diameter and someone may have taken them down too far* and weakened it, some 'shops may not correctly radius the journals, or otherwise do a poor job that weakens the crankshaft.

Yes, that's exactly what happened.

Just someone who, a long time ago, was an Automotive Engineer, but who has continued with it as an interest in retirement.

Thank u Gord for the help