Turning a 340 crank, How much is to much?

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604

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Can any one tell me what the max safe amount you can turn a foraged 340 crank? I found a nice looking foraged 340 crank for cheep I am thinking about picking up for my "spare parts" pile. But want to make sure it still has some life left in it. I have been looking one line and in my books but can not find the min dimensions for some reason? Thanks for any help!!!
 
Bearings usually come in .010, .020, and .030 under. I don't like going more than .020 under. The machine shop will let you know. tmm
 
Can get Clevite bearings in, mains in .010,.020,.030,.040 under and rods in .010,.020,.030,.040,.050,.060 under. Personaly i dont like to go any more than .010/.010 on a stick car and .020/.020 on an auto car. JMO, I know people that have gone .030/.030 and even .030/.040 w no problems.
 
hi, they will run and last at .030 or .040 under.some motors have a smaller dia bearings and run just fine.one of the tricks, was to turn chev crank rod journals to 1.88 which is honda size. smaller dia has less bearing speed therfore less friction =more power. just food for thought.BTW, my stick car is at .020 under, hhmmmm
 
perfacar said:
hi, they will run and last at .030 or .040 under.some motors have a smaller dia bearings and run just fine.one of the tricks, was to turn chev crank rod journals to 1.88 which is honda size. smaller dia has less bearing speed therfore less friction =more power. just food for thought.BTW, my stick car is at .020 under, hhmmmm
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The guy @ the machine shop I use drilled the .010/.010std and .020/.020 auto in my head. He builds mostly drag motors. Other machine shops have told me that he was being to particular (Anal) and I would be absolutly fine turning them as much as .030/.030 or even more They even made referance to the chevy cranks w/ honda rods.
 
Unless you're really going to thrash on it, I don't see any problems with going to .030" under, especially with a forged crank. Mopars definitely don't have the smallest journal sizes out there, lots of engines around that start off with smaller journals/bearings than a 340.

One thing you might consider is having the crank nitrided though, the more you turn the crank the deeper into the hardened journal you'll get. It still probably won't be an issue, but for piece of mind you could have the journals hardened again.
 
Food for thought. I have a fresh .020/.020 forged 340 crank ive been debating on using or not.
 
hi, nitraded hardening is a surface chemical process, its usually about .0003 thick, once you polish the crank its gone. the crank doesn't get soft by turning it!! if that was the case, couldn't turn any crank !!!
 
Thanks every one for the input!
Just found out the mains are already 30 under, and the rods are 20 under. So I am going to pass on it. I am in no big hurry, just the price was right at $50.
 
perfacar said:
hi, nitraded hardening is a surface chemical process, its usually about .0003 thick, once you polish the crank its gone. the crank doesn't get soft by turning it!! if that was the case, couldn't turn any crank !!!
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Sorry, but that's not entirely accurate.

Most nitriding actually gets you a hardened surface about .010" deep.

Most production cranks have induction hardened bearing surfaces, which can be turned without issue. But not forever, they aren't hardened all the way through. If you're cutting a big amount, say .030 or larger, you may run into areas that aren't as hard as the original surface was.
 
Whatever size the bearings are available in is a safe limit. People have been turning cranks the max sizes since the beginning of the internal combustion engine. As long as the machine work is quality, you could probably go .100" on the rods and mains.
 
StrokerScamp said:
Whatever size the bearings are available in is a safe limit. People have been turning cranks the max sizes since the beginning of the internal combustion engine. As long as the machine work is quality, you could probably go .100" on the rods and mains.
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Another fine and accurate post by StrokerScamp.
 
i, I know its not a mopar, chev cranks are turned .100. the trick is to turn big journal crank down to the small journal size, 2.100 to 2.00. less bearing speed,= less friction= more power. these are race engines in stock and super stock, most run 7500 to 8800 rpm. another trick was to cut them down to 1.88 honda size, for less friction. just food for thought.
 
anything over .010 for a high torque high reving motor using heat treated federal mogel bearings. The crank will be destroyed quickly. For a normal performance 340 -360 I seen .030 with no issues. Soft bearings are easy on the crank but don't like hard loads. Hard bearings with heavy loads eat cranks that are turned through the hardened surface. I chose a Callies super hardened stroker crank and real hard bearings. Not the way to go for the street but the strongest for my foot. If you load a soft bearing it squashes in the rod. This causes it to free up the lock and spin. If it spins it usually sticks fast . At high rpm's this causing the rod to to break. You know where this is going. There is a choice of parts for every build. Make these wisely or you'll pay on the big end. If you can take a bearing and squeeze the end together easily they are for non performance motors. I wouldn't use them in any motor. But they will survive in a .030 cut crank under light loads????? A good bearing you can stand on and not squash them flat. But you need a real good hardened and polished crank to use them. Just some words from someone who's been there and paid the price for not listening to the wiser.
 
race engines are gone through quite frequently, street engines are not.
sofer bearings have in mind the debris that makes it past the filters and the possibly infrequent oil changes. soft will eat the dirt instead of the journal... other way around with a hard bearing. Im pretty sure those babbit bearings are a lot less forgiving to the journal surface when the misfortune of say the oil film going away.

use hard azz race bearings on the street without changing the oil/filter every 2 weeks month and u'll more than likely have more wear on the journals than if the other way around.jmo

there is a diff between race and street.
If hard babbit bearings were the thing dreams were made of..then all these production engines going 400k have got to be absolute miracles.
 
So, does that mean my forged 340 crank that was turned 30/30 is worthless?
 
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