My 3.79 crank needs to be turned the mains or .010 now and the rods are standard both need to be turned down to clean it up probably 10 on the rods and maybe down to 30 on the mains it’s already been balanced or should I just put a new one in it motor is a low 9 second motor looking at putting a 200 shot of no’s on it also

Many stock 440 cranks have had .125 taken off the mains to install in 400 blocks and taken a beating.
I'm not sure another .010-.020 would make that big of a difference from where you're at now.

Turn it down to whatever you can get bearings for.

This!

That makes sense to me. Wonder why for decades a .010-.030 turned crank was so frowned on.

Because back in the day, like before my time most likely the mid 70's at the latest undersized bearings all had the same shell thickness and the overlay was made thicker to get the next undersize. As the overlay became thicker (I've heard it was anything more than .020 under) it would start to migrate and then flake off. It became worse as the undersize was more and the HP/RPM went up.

Now, they all use different shell thicknesses and they use the same thin overlay.

Grind it down to small block Chevy large journal if you want. Makes bearing choice much easier. I did that on every Chrysler crank I did. The customer would [censored] like crazy until rebuild time came. No one wanted to grind a new crank, but I did it anyway.

I don't think it was frowned on, when I bought reman cranks I wanted .010 .010 cranks because both sizes were the same, and some cranks are nitrided or hardened, and it only goes so deep into the metal, so the less you turned it the better chance you had of a hard journal surface. and with both jornals the same size its just easier to order parts and not make a mistake. Actually as you reduce the size of the journal you reduce the bearing speed which is a good thing on a race engine. NASCAR for example has rules for what size journals can be for this reason.

The rods are already 2.100 Chevy size just wondering how deep the hardness goes I’m sure I can get the bearings without a problem thanks for the input

I cut a 360 iron crank to 340 size to get some cheap cubic inches. Worked fine. Broke the motor, but it wasn't the cranks fault. Offset grinding to get a smaller pin + more stroke is common. My level of comfort would depend on the material.

It’s a 4340 crank it’s a molnar 3.79

No data w/r/t MoPar cranks, but factors on regrinding other makes:
1. the drilled oil transfer hole is slanted in the journal, and is uncovered with a big undersize - lost OP
2. the grinding didn't use the correct cheek radius, too large (not enough metal removed) will bind the shell
3. crank stiffness is reduced by both the extra stroke and smaller journal, typically small enough not to matter. Read more in my article: http://victorylibrary.com/mopar/journal-c.htm
I wrote an .xls spreadsheet to compare the actual (intersecting lenses) overlap area vs. nominal (diameters compared) area, e-mail for a free copy.

I've seen a hundred BBF 460 factory cast cranks offset ground from 2.5 to 2.2 that most made 750 to 800 hp NA as a 514CI then watched a bunch of them get the daylights sprayed out of them as well. So much for the hardness issue people always talked/worried about.

I won one of the mopar muscle engine challenges years ago with a stock mopar crank turned .04 . Kept burning it up trying to see how little oil it could live on. Made like 650 hp with all junk parts on pump gas, and no CI's had to turn it 8500rpm to make power. after it got enough oil, it was never a problem. Also cut the counterweights down in the lathe instead of drilling it to balance.

If you can get bearings for it, fine...Never seen -.03 though. I have gone as far as -.020 on one of my race engines. Its still running mid 10's in another car after almost 800 passes since it was first built.

I will tell you one thing though, I am not having any more cranks welded. They always spring on me...I hate that.

I believe King makes a .030 under main bearing.

Was thinking years ago they made .060 under for rods & mains

Maybe for some other brands and industrial applications
I can't remember the name of a company in SO CA that specializes in old car parts but I seem to remember them offering under size bearings for some old motors in .040 and maybe smaller
I have seen .040 listed years ago for early Poly 318 motors and the same for the 273,318 and 340 LA motors

Have found some main bearing for .030 if it had to go that far thanks for your input