401 crank is reground, does it need rebalance

I can call King and see what can be done. I have a contact there that I call when I need to confirm/locate bearing for various engines I build. Right now I think the problem will be getting items in stock due to the whole virus BS.

I'll find out in the next day or two what the response will be.

If your pressure is acceptable at cruise and under acceleration I would run it. I have seen low pressure engines run for many trouble free years

Specializing in dyno services, engine building, and cylinder head porting

rbjracing.com
Phone Number 518-915-3203

Did i miss something...The engine was modified without turning the crank? So when the org work was done(Piston replacement) was it balanced then????

68 AMX 390 4 Speed,68 American,64 American 2 Door Wagon Altered Wheelbase,78 Concord Build 360,727,8.8

jcisworthy wrote:

I have a set of .001" mains

jcisworthy wrote:

If your pressure is acceptable at cruise and under acceleration I would run it. I have seen low pressure engines run for many trouble free years

Do you have a part number for those? Would you be willing to part with them? Not sure if I need them yet, but if I do need them, I may hit you up, if that is ok.

OK, roger that. My factory electric gauge shows about 36'ish on the freeway but my Mechanical gauge is under the hood and I cannot see it when driving. I will have to move it so I can verify it while driving.

FSJunkie wrote:

King's catalog shows .001" undersize rod bearings for AMC V8's, but not the 390 and 401, and they show no .001" undersize main bearings for AMC V8's period. That sucks.

You might be able to call them. They might have a creative person on the line who can figure out a bearing that will work.

Time to start calling bearing manufacturers.

I have to verify what I need first. I am gathering all the goodies to pull my engine and keep my downtime as low as possible (gasket set, coolant, etc.) and hope to pull my engine in mid October'ish.

bbgjc wrote:

I can call King and see what can be done. I have a contact there that I call when I need to confirm/locate bearing for various engines I build. Right now I think the problem will be getting items in stock due to the whole virus BS.

I'll find out in the next day or two what the response will be.

Awesome! I would buy you a beer if you were local, to say thanks.

Kreep wrote:

Don't waste your time with plastigauge. All it tells you is you are in the ballpark.

You might want to do some of the following oil mods while the block is torn down. (It worked for me along with tight main and rod clearances).

Re-drill the main bearing oil feeds with a long 5/16" bit.

Install a plug in the driver's side oil galley with a 0.200" hole.

Reduce the four rear camshaft bearings oil supply holes to 0.150.

All I have is plasti-guage at this point.

Copy all the suggestions. I'll see what I find when I open it up. I do not plan on doing a full tear down so I am hesitant to do any drilling on the block unless it is absolutely an "issue" that needs fixing. I say that because I feel if I do any drilling, I feel like I need to completely strip the block to make sure there are no metal shavings that get left in oil galleys/passages.

I hope to not have to pull the pistons out or the cam. If I do need to, I will do what I have to, but I am trying to avoid it.

jblue1555 wrote:

have a friend who bought a jeep cj with a 304. it had less than 10 lbs on the guage when hot from day 1 and carried 35 lbs at speed. It ran that way till he sold it and new owner ran it till he replaced it with a 401. this was a new 1979 jeep .

Copy that. Makes me feel a little better. If I had a 360/304 I would run it til it puked but I get really scared of damaging this 401 crank. I would rather spend the $500 now doing a bottom end, then have to find another 401 crank later because I torched this one.

one bad rambler wrote:

Did i miss something...The engine was modified without turning the crank? So when the org work was done(Piston replacement) was it balanced then????

Nah, when I did the original rebuild, the machinist measured my crank and said it was within tolerance and only needed to be polished and balanced. So it was polished and balanced without being reground.

garland performce wrote:

Also before you tear it apart check the oil pump end clearance

they make a few that are thinner or thicker

If you have any questions give me a call at the shop Monday -Friday 816-547-5695

John Garland owner of Garland Performance

Will do, John. I JUST had it blueprinted when I rebuilt the engine but will check it anyway.

************************************

So how do I check to see if my lifter bores are OK? I am thinking when the engine is on the stand, I will pull my intake, spin the oil pump and see if it is squirting out from around the lifter. Is that a valid test? Is there a better way to check it?

Also, just pulled out my receipt from the machinist. My mains were at .0023" plus or minus .0003" and rods are .0026" after polishing. I have never found metal shavings in my oil filters but I know I had really low pressure for the first 200 miles or so I may have worn those bearings a little more than that, too.

Last thought, I have ARP hardware on my rods. I did not chamfer the nuts so this may be impacting me as well:

SC397 wrote:

A flanged style rod nut will distort the connecting rod unless you chamfer the nut like this one.

If the rod is distorted, it may plastigage ok in one direction but not purpendicular to that measurement. I am not saying that what the issue is but, it doesn't help.

79 Cherokee, 401

As the original question was about balancing...

Balancing an engine is about the relationship between weight and its location to the centerline of the crankshaft. The further a "weight" is from centerline, the more centrifugal force acts on it as the crank spins up in RPM.

So as cranks are cast or forged, the metal density changes throughout the structure of the crank. The manufacturing tolerances for pistons is pretty wide and densities vary in them. If you want the crank to spin vibration free, you have to balance it and the rotating elements. You balance it by either removing the excess weight that is hanging too far out from centerline or you add weight to the opposite side of centerline to "compensate or even out" the weight. Or you may add weight or drill the flywheel or harmonic balancer. It all depends on the location of the "imbalance" or the offending weight.

The crank has "throws" or big chunks of metal opposite from the rod journals to offset the weight of the rod and piston. Drilling holes in the throws removes weight and adding slugs of "Mallory metal" adds weight.

The throws are drilled or weighted based on weights of the pistons and rods. Ideally, each rod, piston, wrist pin and ring set would all weight exactly the same. "Dummy" weights are then added to the throws, the flywheel and harmonic balancer are installed and the crank and weights are spun up and the machine checks for vibration. The throws are then drilled or weighted to compensate and provide for a vibration free engine...within a tolerance of course. The tighter the vibration tolerance desired, the more critical the removal/addition of weights is and hence cost goes up. You wouldn't balance a factory cast crank and rods to spin above 12K RPM like an INDY motor...

If you grind the crank .010 under on the MAINS what have you changed in the balance equation? Nothing. You removed .005 from each side of the crank journal. Nothing changes. Weight of the rods/pistons/piuns/rings FROM CENTERLINE didn't change. This holds true for the MAINS.

If you grind .010 from the rod journals, you will have removed a portion of metal that is offset from centerline and you will need to rebalance. I believe it was mentioned in a post above that it was miniscule and it is. Your balance may not change enough to justify spending $$$ to rebalance. Now, I say that for a street motor. If you are on the strip and spinning into 8K regularly, I recommend re-balancing.