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Main bearing clearance

 
Old 04-11-2018, 05:07 PM
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Default Main bearing clearance

I'm ***** deep into my build and just got very accurate measurements of crank diameter and bearing clearance. The clearance is supposed to be 0.8-1.4mils (OEM recommended) and the absolute max is 3.9mils. My gap is measuring 2.7-3.3mils...

As I understand it, a few hps can be found by loosening up that gap. I'm going to guess 2.4mils. But as you get too wide a clearance you lose oil pressure and run the risk of bearing cavitation damaging the bearing surface.

Are there any engine machinist that can shed any light on this? Should I stop worrying and just button her up?

(also, this is a crank that is machined down .02", so I can't change that. And I believe what happened is that my machine shop line honed the mains a little too much.)

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Old 04-11-2018, 06:01 PM
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You could get oversized bearings to close up the gap, no? I run my clearances (at least when assembled) at .002 for rods and mains, using Retella T6 and a Boundary Street/Strip pump with no shims. No problems here. Blackstone oil analysis at every change says so as well
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Old 04-11-2018, 06:41 PM
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when i had my block machined they set rods and mains at .0025 and i will run a 20w-50
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Old 04-11-2018, 08:38 PM
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Anyone out there have any anecdotal evidence of a bearing failure due to over clearance? (Doubt it..)
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Old 04-11-2018, 10:15 PM
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I used .0018 for mains and .0020 for rods. My crank was also cut 10 on mains and 20 on rods.
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Old 04-12-2018, 12:09 AM
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Did you measure .0018? Using a dial bore gauge is not trivial. Without some 2 and 3" standards you can't hope to get any accurate results. And you also need decent micrometers 2-3" and 3-4".

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Old 04-12-2018, 12:56 AM
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Old engine builder saying.

Set your clearances slightly too loose and you will know it.

Set them too tight.....and EVERYONE will know it.
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Old 04-12-2018, 01:17 AM
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The Shop measured before cutting the crank.And they measured the mains with a dial bore indicator. He cut the crank to the bearings. I verified it after with plastigauge. The crank shop I used only does cranks and is backlogged due to demand for his services. Crank spins with about 5lbs of force (rods attached)
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Old 07-11-2018, 12:15 PM
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Too button up this thread (I HATE dead end threads),

Turns out I got the crank machined down (lots of scratches on it) by .020" and got the block line honed. I didn't know this at the time (of course, what I don't know would fill an ocean), but you can't get bearing that are both .001" oversize (due to the line hone) and .020" undersize (due to crank work). You can't do both, you have to choose...

options:
1. You can shim your bearing with .001" strips. Terrible idea. Don't do it.
2. You can get another crank that hasn't been turned down or touched.
3. You can get another block.
4. You can try and convince the machine shop to slightly cut down the main bore halves. But this should be followed with a main bore hone to ensure alignment. And you can't make the width any narrower, just the height of the bore.

Good luck, whoever is reading this.. I'll report back with what I decide to do.
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Old 07-11-2018, 01:23 PM
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Buy another crank they are cheap
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