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I am rebuilding an M52 engine, and would like to give it as much longevity (or more) than it left the factory with. Towards that goal, I would like to put all the important oil clearances as close as possible to the lower end of the factory tolerance ranges given in the specs. But I am left with a difficult choice regarding the crankshaft’s connecting rod journals. The journals show no signs of wear, but if I put in new bearings and go with the standard BMW “red” ones (or any of the aftermarket equivalent bearings – Glyco eg.) my oil clearances will be 0.0022”. This is right at the upper end of the factory tolerance range 0.0008” – 0.0022”, given in the specs. If I go with all BMW “blue” bearings, that will bring the clearances down to 0.0018”, still in the upper half of the tolerance range. The only other option I can think of is to go with the +0.25mm bearings and have the journals turned. This way I could have whatever clearance I want, but this may have a drawback. I have read that the crankshaft may have had some sort of hardening done on it – and if so, perhaps this hardening is responsible for the almost undetectable wear on all the crank journals. If I have those rod journals ground I will lose that hardening (right?).

So my choice for the rod journals is – oil clearance at the sloppy end of the tolerance range – or loss of factory hardening. Does anyone have an opinion on this matter?

It bugs me to have all the other important clearances in the lower half of their ranges and then this in the upper half. A chain is only as strong as its weakest link, right?
 

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I'm not sure that having a thicker oil film on the bearings is a bad idea. I guess it comes down to cost/benefit, as having the journals turned will cost.

The next question is: if you have the journals turned and get the appropriate bearing set, do you have any assurance that the clearance will be less? After all, you're talking about a factory spec. The replacement bearings also can be made to factory spec, so i would assume there will be factory variability in their tolerance as well.
 

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Discussion Starter #3
Thanks for your reply!

I am assuming, and correct me if I'm wrong, that you can have the journals turned down to whatever diameter you please.

It may well be that there is an optimal oil clearance for minimizing wear and that it may be somewhere in the factory tolerance range. But why not start at an acceptable clearance smaller than it ?

Thanks again for your reply.
 

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Thanks for your reply!

I am assuming, and correct me if I'm wrong, that you can have the journals turned down to whatever diameter you please.

It may well be that there is an optimal oil clearance for minimizing wear and that it may be somewhere in the factory tolerance range. But why not start at an acceptable clearance smaller than it ?

Thanks again for your reply.
My engine rebuild days are a few years back, but my experience was that bearings come in pre-determined sizes, like .030 under. Crank journals would be turned to fit the bearing sizes available, which are not user-definable. For BMW I would doubt that there are many options in turning the crank journals. And as I point out, when you have this done, the bearing are made to fit that size, and I doubt that their tolerances are any closer than factory specs. There will be variation in bearings, too.

In short, unless you have journal damage or wear that requires turning the journals, I think you should take the best set you have at present and use it.
 

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+1 Leave the crankshaft journals alone. The fact that journals are in perfect condition is evidence that the factory clearances are fine. The primary reason for going for close tolerances in the bore is to achieve as close to perfect gas seal as possible. Working toward the low end of clearance tolerances all around is not a good idea as you also need to be conscious of surface finish to ensure that an adequate oil film is available at all times. Remember that when the engine starts, the coefficient of expansion varies significantly between metals - white metal bearings will expand more than a steel crankshaft or conrod, as will alloy pistons vs cast iron block.
Going too close can lead to engine siezure when it gets hot and the close tolerances will also test the performance of your engine oil to the max. I have seen engines built "tight" that were incapable of being cranked by the starter alone (i.e. had to be clutch started with starter assistance) and in spite of this, did not achieve any significant benefit in terms of either performance or engine life.
Remember that internal friction is directly proportional to how tight a fit you achieve between running components and that higher friction = more heat generation and greater wear.
Take into account also that the crankshaft is both pressure and splash lubricated (unless you are runnng a dry sump), so I would be very happy with the clearances you can achieve running the available bearing shells.
 

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+1 Leave the crankshaft journals alone. ~~~
~~~ I have seen engines built "tight" that were incapable of being cranked by the starter alone (i.e. had to be clutch started with starter assistance) and in spite of this, did not achieve any significant benefit in terms of either performance or engine life.
Remember that internal friction is directly proportional to how tight a fit you achieve between running components and that higher friction = more heat generation and greater wear.
Take into account also that the crankshaft is both pressure and splash lubricated (unless you are runnng a dry sump), so I would be very happy with the clearances you can achieve running the available bearing shells.
Too tight a bottom end can lead to premature wear on startup ..
Too tight on the bore can lead to slapping skirts.
Better to stay somewhere closer to the middle of the tolerances [dont forget 'stacking' penalties apply.]
 

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Discussion Starter #7
For what it's worth

Here is a quote from the King Bearings web FAQ:

"Engine bearings should operate with the least amount of oil clearance as possible. Tighter oil clearances produce less peak loading on the bearings and the engine runs smoother with less vibration."
 

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You may also like to consider the fact that grinding down the crankshaft journals increases the pressure on the bearings (from a mechanical engineering standpoint, Pressure = Load/Area). More pressure = more wear. I don't think King Bearings FAQ is meant to cast aspersions on normal manufacturing tolerances, more as a warning as to symptoms of a sloppy bottom end. You would see a far greater performance benefit by dynamic balancing of all of your engine components than super-scrutinising tolerances. After all, engine blueprinting is only ensuring that all the manufacturers tolerances are to specification.
 
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