Question on balancing a 5.9 Mag with KB107 pistons

[Justin Angle]

I'm running a 5.9 Magnum. Stock polished crank with 2-ICR6123-2124 rods and KB107 pistons. I had the engine built by a shop but I do know the rotating assembly was balanced. This was all 6-7 years ago. The car has never been the smoothest at part throttle highway speeds, which it does have solid motor mounts. Full throttle is smooth but part throttle is not.
Looking at the flexplate and balancer it does have a B&M LA externally balanced flexplate and harmonic balancer on it, which is how it came back from machine shop. Convertor is internally balanced (At the time I didnt know the LA and Magnum harmonic balancer/flexplate balancing differences but I now I do.) But I'm not familar with the balancing process machine shops use so...

On an externally balanced engine do they balance the rotating assembly with or without the flexplate and harmonic balancer?
Seems it'd only make since if they did it with.

And does the Magnum vs LA balancer/ flexplate application go out the window when a magnum rotating assembly is balanced after pistons and rods are changed? So you can use a LA style damper/flexplate on a magnum if rotating assembly is balanced to that?

Just making sure I'm going up the wrong tree
Thanks

 

[Professor Fate]

Depends on your machine shop and what they did with your particular engine.
Almost ANY combination of parts can be balanced to work together.
Some will balance it with the flywheel or flexplate/TC, others don't require it as long as they know which pieces you'll be using. But they all should be balancing with the harmonic balancer in place.
And they could theoretically have balanced it any of the 3 ways:
Neutral (internally) balanced
Externally (LA360) balanced
Externally (5.9 Magnum) balanced
At this point only the machine shop can answer these questions, and after 6-7 years it'll be a crapshoot if they still have the info on your motor.

 

[TT5.9mag]

Perfect answer. @Professor Fate

 

[TT5.9mag]

If it’s a street car and you’re running it down the highway put rubber mounts on it.

 

[Justin Angle]

Depends on your machine shop and what they did with your particular engine.
Almost ANY combination of parts can be balanced to work together.
Some will balance it with the flywheel or flexplate/TC, others don't require it as long as they know which pieces you'll be using. But they all should be balancing with the harmonic balancer in place.
And they could theoretically have balanced it any of the 3 ways:
Neutral (internally) balanced
Externally (LA360) balanced
Externally (5.9 Magnum) balanced
At this point only the machine shop can answer these questions, and after 6-7 years it'll be a crapshoot if they still have the info on your motor.

I suppose it is correct and was LA360 balanced then since it had the LA 360 balancer and flexplate on it but I will try calling, odds are they won't remember. Worth a shot though. Thanks

 

[Professor Fate]

Also make sure your ignition is performing up to snuff- an idle/cruise speed misfire can be easily masked at higher rpms, appearing to be a vibration issue.

 

[oldkimmer]

Maybe they thought the LA 360 and magnum 360 had the same balance. Otherwise the added too much weight with the LA360 balancer and the B&M flex plate. Must be a lot of Mallory in the crank. Kim

 

[Bobzilla]

I'm running a 5.9 Magnum. Stock polished crank with 2-ICR6123-2124 rods and KB107 pistons. I had the engine built by a shop but I do know the rotating assembly was balanced. This was all 6-7 years ago. The car has never been the smoothest at part throttle highway speeds, which it does have solid motor mounts. Full throttle is smooth but part throttle is not.
Looking at the flexplate and balancer it does have a B&M LA externally balanced flexplate and harmonic balancer on it, which is how it came back from machine shop. Convertor is internally balanced (At the time I didnt know the LA and Magnum harmonic balancer/flexplate balancing differences but I now I do.) But I'm not familar with the balancing process machine shops use so...

On an externally balanced engine do they balance the rotating assembly with or without the flexplate and harmonic balancer?
Seems it'd only make since if they did it with.

And does the Magnum vs LA balancer/ flexplate application go out the window when a magnum rotating assembly is balanced after pistons and rods are changed? So you can use a LA style damper/flexplate on a magnum if rotating assembly is balanced to that?

Just making sure I'm going up the wrong tree
Thanks

Tell us exactly which harmonic damper is on the front.