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401 crank is reground, does it need rebalance
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rang-a-stang View Drop Down
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    Posted: Sep/18/2020 at 10:04pm
My 401 rebuild is pretty fresh; about 8k miles. It is only slightly warmed over: .030 over, forged pistons, 9.5:1, hyd flat tappet, iron heads, etc. When the machinist measured my journals on my crank he said they were good enough to just polish and run standard bearings. But now, I just don't have the oil pressure I should (probably about 8-10 PSI hot idle with 20W50 at 650RPM). Rick (SC/397) blueprinted my oil pump so I am confident that is not my problem. My oil system is stock (no bypass, main bearings only have an oil channel on the upper half). 
I don't have my receipt in front of me but if I recall correctly the clearances my machinist documented on the mains and rods were within spec....for a small block Chevy... but at the bleeding edge for a warmed over 401. I have a feeling I am going to need to pull my crank and have it reground to .010 and replace the main and rod bearings.

If I do have it re-ground, do I need to have it re-balanced? I had it dynamically balanced during the first rebuild and I really don't want to have to pay for that again... 
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Points: 5479 		Post Options Post Options   Thanks (0) Thanks(0)   Quote SC397 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/18/2020 at 10:14pm
If they only take 0.010" more off, you will not have to get it re-balanced.
Have the machinist measure the bearings as installed in the block and then have him check the crank to get the actual clearances.  Some machinists don't want to do this but will for a price.  (Some machinist just measure the housing bores and call it good enough)  King bearings run on the big side compared to Clevite. So a good machinist should be able to hit the clearance that you want.
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Points: 5808 		Post Options Post Options   Thanks (0) Thanks(0)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: Sep/18/2020 at 11:29pm
Sounds like you need somebody to measure and check all bearing clearances thoroughly. Not sure if you can still get 0.001 undersize bearings, but that could be an option.
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Stop. Don't. Come back. 

You do not need to machine your crank.

If the oil clearances are a little on the high side currently with standard size bearings, there is absolutely nothing wrong with replacing them with 0.001 in. undersize bearings to tighten the clearance. If 0.001 in. is too much, you may mix standard size bearing halves with 0.001 in. halves to adjust your clearance by 0.0005 in. so long as you are consistent with placing all the bearings in the same position, such as all standard bearings in the block and all undersize bearings in the caps. It's a perfectly acceptable procedure. AMC did it themselves when assembling these engines. They select-fit the bearings. 

You have the opportunity to really get those clearances exactly where you want them, without doing any further machine work or balancing. 
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Points: 5479 		Post Options Post Options   Thanks (0) Thanks(0)   Quote SC397 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/19/2020 at 7:31am
Good luck in finding 0.001" undersized bearings for a AMC engine..
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Are Your lifters AMC or mopar? The mopar ones may have an affect on pressure.
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You guys are all awesome! Thanks! 
Originally posted by SC397 SC397 wrote:

If they only take 0.010" more off, you will not have to get it re-balanced.
Have the machinist measure the bearings as installed in the block and then have him check the crank to get the actual clearances.  Some machinists don't want to do this but will for a price.  (Some machinist just measure the housing bores and call it good enough)  King bearings run on the big side compared to Clevite. So a good machinist should be able to hit the clearance that you want.
I am going to check myself with some plastigauge before anyone touches my motor. I will then take it to a different machinist and have them check the clearances as well. That way I have 2 sources. 
Currently I have Clevite 77's in there. When I was re-assembling after the first machinist did his work, I saw remnants of plastic gauge on all my bearings so I know he measured. 
Originally posted by WesternRed WesternRed wrote:

Sounds like you need somebody to measure and check all bearing clearances thoroughly. Not sure if you can still get 0.001 undersize bearings, but that could be an option.
For sure. I suspect mains and rods are the problem but I will also check the cam bearings and lifter bores. 
Originally posted by FSJunkie FSJunkie wrote:

Stop. Don't. Come back. 

You do not need to machine your crank.

If the oil clearances are a little on the high side currently with standard size bearings, there is absolutely nothing wrong with replacing them with 0.001 in. undersize bearings to tighten the clearance. If 0.001 in. is too much, you may mix standard size bearing halves with 0.001 in. halves to adjust your clearance by 0.0005 in. so long as you are consistent with placing all the bearings in the same position, such as all standard bearings in the block and all undersize bearings in the caps. It's a perfectly acceptable procedure. AMC did it themselves when assembling these engines. They select-fit the bearings. 

You have the opportunity to really get those clearances exactly where you want them, without doing any further machine work or balancing. 
Originally posted by SC397 SC397 wrote:

Good luck in finding 0.001" undersized bearings for a AMC engine..
I guess I'll have to see once my motor is on a stand and measured. I hope a) the clearances can be tightened up with a slightly undersized bearing (if that is my problem) and b) that I can find undersized bearings that will work.
Originally posted by BDCVG BDCVG wrote:

Are Your lifters AMC or mopar? The mopar ones may have an affect on pressure.
OOoohhh... Good question. I bought a Comp Cams kit. I think the kit has Comps lifters CCA-822-16 which comp shows as AMC/MOPAR. I did not check my lifter bores when I had my engine apart. I do not remember there being any "sloppy" feeling lifters when I installed them but I also do not remember checking. When the engine is out this next time, I will spin the oil pump and see if I see any leaks in the top end (including around the lifters). Is there a better way to check lifter/lifter bores?
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I thought someone had posted that you could tighten up the standard bearing clearance with Pontiac bearings.  But I could be mistaken.
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That was me using the Pontiac bearings. CB-758HN are the same as a 401 rod and the CB-758HN-1 are 0.001" undersize. You have to grind off the tangs and locate them on assembly. Bearing crush is what holds a bearing in place and NOT tangs.
These bearings are a little wider but there is no interference with the stock crankshaft chamfer.
These are rod bearings only, have not found 0.001" or 0.002" undersize mains.
I will also mix and match on the bottom of the connecting rod to get within 0.0005" for my rod clearance.
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I have a set of .001" mains 
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