Crankshaft questions

QMFB

QMFB

Fastest Welder in Texas
Joined
Apr 23, 2016
Messages
1,061
Alright gentlemen, I'm in the middle of rebuilding my '04 and am trying to sort out all the modifications I want to make in the bottom end... So, Crankshaft.
What is the consensus on lightening, champering (knife edging), or even complete removal of the counterweights ?
I know this is all common practice in the performance gasser world, but are there benefits in a Cummins ? Drawbacks ?
Experience is great. But, as always, all opinions and discussions are valuable.
 
Having seen a few broken cranks on stock CR 5.9 engines, I'd be hesitant to do a whole lot to one to try to lighten it.

Mark.
 
Wonder what caused them to break ? They are pretty stout pieces... I'm curious what the high HP trucks are doing ? 1000hp, 2000hp, 3000 ? I know not too many guys are dropping the coin on custom billet crankshafts like Winburg.
 
No clue, though one I know for sure, the crank cracking across a rod journal was the cause of the engine coming apart.

I have a 6.7 crank from a very low miles 2012 engine that broke on the crank arm between 5 and 6.
It's not the only one I have heard or seen do it, there have been others.

Mark.
 
Wonder if lightening would reduce failures, or increase them ? A lighter crank would create less torsional load across the crank on the compression stroke, would it not ?
 
The biggest thing is making sure there are no stress risers.
These cranks are so hard on the surface that it's like cracking tempered glass, it can take a hard hit, but once it's fractured, it comes apart.

The hardening on these cranks is a lot of the reason many crank "rebuilders" will not weld and regrind a damaged B series crank.
Most won't even touch them if they show any sort of heat related discoloration on the journal or arm.

Mark.
 
So possibly polishing the rough casting areas between the journals would help relieve cracking. Like is common practice on OEM forged rods.
 
Anything you can do to smooth it certainly is a possible gain.

Keep in mind, though, the rough part of the forging/casting is hardened along with the rest of it and the hardening only goes so deep, so if the forging/casting has any deep flaws in it, you can add insult to injury by correcting it.

You can smooth the appearance, but don't try to correct it. Make sense?

Shot blasting it after polishing/deflashing a part is common practice and alleviates minor stress risers from the tooling.

Mark.
 
:thankyou2: Thank you for the insight mark.
 
That would still leave the counterweights free for lightening / champering since they are outside of the torsional stress line between main journal and rod journal ?
 
I see no harm in modifying them, but realize that re-balancing could be an issue.

Mark.
 
Yeah, I might need to talk with my machinist about that, lol. Seems like if you removed equal amounts off of the counterweights on a pre balanced crank, then a final Ballance it could be done... But I'm no machinist.
 
Well my crank is sitting on a pallet in my garage right now, so not much hassle... But cost to have it done, IDK ?
 
There is a thread on here about the Nordic machinist cutting the weights off his crank and he was driving it around in a suburban.


It's my opinion that the cranks that break between the last rod and main journals got rammed into something in manual trucks.
Just an unproven opinion.


Sent from my XT1575 using Tapatalk
 
You got a link to that thread biggy ? I've searched and searched. Couldn't find much info on it one way or another.
 
Thanks ! Wonder why that didn't show up in my search strings ....
 
biggy238 said:
There is a thread on here about the Nordic machinist cutting the weights off his crank and he was driving it around in a suburban.


It's my opinion that the cranks that break between the last rod and main journals got rammed into something in manual trucks.
Just an unproven opinion.


Sent from my XT1575 using Tapatalk
Click to expand...

I've seen quite a few broken cranks in mainly stock trucks. Probably near 10 in total and only two have been in automatics so I too believe operators are tearing them up.
 
There are guys on this board that have ran with lightened crankshafts. I have one but have not used in in a build yet. Another one just like it is living in a higher hp truck.
 
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