Rebuild LM7...rebalance?

[N9nefingers]

I've got two Gen 3 LSx motors...one LM7 that has good crank and rods but water sat in the block and rusted a few cylinders. And a 4.8 that spun a two rod bearings and ruined all bearings, ruined the crank and cam, and put metal all throughout the block. Question is...on a rebuild how necessary is it to rebalance the rotating assembly? On a standard ring, bearings, pistons and gaskets rebuild is it a must? Are all motors balanced by GM at the factory? Seems the cost of balancing would push the rebuilds into out of range for the standard in home garage guy who's just trying to put a nice motor in his old cruiser.

[Warrens69GMC]

I would rebalance.

[N9nefingers]

Care to elaborate?

[SunSoaked]

I have rebuilt and ran many SBC engines without balancing and never had an issue. Just depends on if its worth the cost. On a daily driver with quality name brand parts, probably not for the average joe.

[N9nefingers]

I have as well...I've never had anything balanced and they all run well. What I've read about LS platform though is that its internally balanced as opposed to externally balanced SBC. There is a harmonic balancer on the LS motors, but it's not as forgiving looking as a SBC balancer in my mind. I have so many questions...

[LS short box]

No question I would rebalance. Can't be that much cost wise. Of a bigger concern is all the metal chunks floating around the inside of that block.
Will a jet wash of that block remove all the bad stuff? I have no idea. Will it clean all the oil passages?
Can the rusted block cylinders be cleaned up with a hone? The reason I ask is that it doesn't have metal in the oil passages.

[Warrens69GMC]

What is it? $150? I will tell you if you check the balance of current condition, then your new engine balance numbers, the old engine would be way off optimum.

So you will spend $3500 to have it rebuilt, but you won't spend 4.28% (150) to make sure its balanced?

[Joyridin]

If you are replacing pistons, you need to rebalance the rotating assembly. You have no idea how much of a weight difference there is between the old pistons and new, and the wrist pins.

Personally, if I were doing all that work, I might just dig around and see if you can find a 6.0 liter. The crank and rods are the same, and you are replacing pistons etc. No replacement for displacement.

[truckdude239]

on a stock build id say its not as important as a performace build

[A1971Blazer]

balancing is never a bad idea or a waste of money...
from the factory....they are balanced generically....the same across the board....it's pretty good but not perfect...
I have every engine that I build with any new rotating components, balanced.

[burnin oil]

If you have the matching rods and pistons you can use a cheap scale to lighten the new piston. This will give you a matched set. Now balancing the rods at home would be a big trick.


The factory balances everything by color code. Then they assemble the motor to that color. Its a ball park assembly system but good for low RPM street motors. You have to remember that the rod stress isn't linear with RPM but exponential. The harder it spins, the more stress is envolved.

[N9nefingers]

Ballpark balancing makes sense. My plan was to reuse the crank and rods and just put new pistons on it and try and stay out of a machine shop as much as possible. It's been my experience that machine shops get expensive real quick. In the end it's not going in a race car, just a cruiser.

[crazy longhorn]

If you are running, the same rotating assy, from the 5.3 & just replacing the pistons, you should be fine on a daily driver. That said, if you have ever had a properly balanced engine......you can hear/feel the difference! Personally, I will not build an engine, & skip on the balance longhorn

[burnin oil]

Totally agree with you longhorn but I would just lighten up a replacement set of pistons in this application. Gram scale and a dremel tool should do the trick. Every set of replacement pistons I haver ever come across was slightly heavier. My one motor is balanced to some real tight tolerances but shakes like a washing machine due to the cam.

[RustyBucket]

Originally Posted by N9nefingers

What I've read about LS platform though is that its internally balanced as opposed to externally balanced SBC.

Not to be argumentative, but my recollection is that traditional SBC's were internally balanced until the externally bal. 400 was produced. 454 BBC's were(are) also externally balanced. The external balance weight is welded on the flex plate or cast into the backside of the flywheel... they are interchangeable 400SBC to 454BBC. They have a bored hole that aligns them with a dowel pin on the crank.... just had another dim memory flash, I think the harmonic balancer had balance weight cast into the inside on externally bal. engines.

[A1971Blazer]

Originally Posted by RustyBucket

Not to be argumentative, but my recollection is that traditional SBC's were internally balanced until the externally bal. 400 was produced. 454 BBC's were(are) also externally balanced. The external balance weight is welded on the flex plate or cast into the backside of the flywheel... they are interchangeable 400SBC to 454BBC. They have a bored hole that aligns them with a dowel pin on the crank.

This is correct....I once put a 400 harmonic balancer on a 350...needless to say...it vibrated a right smart....

[Tom]

If you don't balance the new assembly then you aren't actually rebuilding an engine.
Not sure why you think its so expensive either.

[MySons68C20]

Once you have driven a vehicle with an engine out of balance you wouldn't be asking this question.
I put a "crate engine" in my 77 K10 years ago and it had a harmonic that drove me nuts......thanks Motor Works of Spokane.....I won't be back!

[Joyridin]

Originally Posted by MySons68C20

Once you have driven a vehicle with an engine out of balance you wouldn't be asking this question.
I put a "crate engine" in my 77 K10 years ago and it had a harmonic that drove me nuts......thanks Motor Works of Spokane.....I won't be back!

This statement is correct. It really sucks when you have to tear apart the engine because your teeth are chattering when driving down the road.

I had a BBC that was out of balance because somebody tried to balance it internally and used the factory flywheel. It felt like the engine was going to blow apart at 70 mph.

A little money now, or a lot later.