Engine balancing question.

Hi,

If you are rebuilding an engine, but re-use the original crank and rods, and grind the crank for new oversized main and rod bearings, do you still need to balance it all again? Or, would it maintain its original balance from the factory, since it's the original parts? 

Thank you. 

I would have it balanced again, especially if changing pistons.  Factory wasn't always as accurate on balancing as it should be. 

Not an issue.  The amount of material moving (it's not removed, just moved from the crank to the rod) is so minimal that it's well under what a balancer can work with.

Pete. (l33t FS) said:

(it's not removed, just moved from the crank to the rod) 

Mind Blown.

Side issue-has anybody seen/developed a way to do assembly balancing at the DIY level? 

For my challenge car I fabricated a jig to balance both ends of the rods, but didn't see anything to do the flywheel/crankshaft individually or as an assembly.

Pete. (l33t FS) said:

Not an issue.  The amount of material moving (it's not removed, just moved from the crank to the rod) is so minimal that it's well under what a balancer can work with.

Never actually thought about that before.  That is really cool.  I would take it further and say that the material removed from the crank is replaced by the thicker bearing and is moved by the bearing to crank gap. Not exactly true because the weights of the two metals are different but the difference is so small that the change is extremely small.  
 

@The op.  If you have the pistons and rods apart I would put them on a scale and even the weights out. A minor thing but every little bit helps.  

You can balance pistons and rods at home with just a scale and machine tools to remove weight but the equipment needed to balance the rotating parts would be difficult to build at home. 

When I had my crank ground, I didn't do any balancing.  Worked just fine for me!

APEowner said:

You can balance pistons and rods at home with just a scale and machine tools to remove weight but the equipment needed to balance the rotating parts would be difficult to build at home. 

If it's an inline engine, that's pretty much ALL you can do.  When you balance a crank, you spin it on a machine that measures the imbalance at both ends.  You measure the connecting rod weight at both ends and then the piston and rings get added to the small end's weight and a percentage factor is applied and that plus the weight of the big end and bearings gets you the bobweight that you attach to the rod journal.

The thing is.... with an inline crank, the bobweights all cancel each other out on the crank.  So to balance an inline, you make the pistons the same, the small ends the same, the big ends the same, then you balance the crank all by itself. 

Technically you should alter the counterweights if you make the pistons and/or rods lighter or heavier, to even out the bending couples across the crank mains.... but how do you measure how much?  You can't, really.  And probably most inline engines don't have enough counterweighting to accomodate even the stock pistons and rods.  Some four cylinders don't even have real counterweighting anywhere! 

Pete. (l33t FS) said:

The thing is.... with an inline crank, the bobweights all cancel each other out on the crank.  So to balance an inline, you make the pistons the same, the small ends the same, the big ends the same, then you balance the crank all by itself. 

Does balancing a 4 cylinder inline crank where the crank throws are 180* apart from each other really require a complicated machine? 

Because all of the weight is along one plane aren't you just making sure that the weight from the counterweights for the 1 and 4 side of the crank are equal to the weight from the counterewights on 2 and 3 side of the crank?

Oooor is is also important that the counterweights 1 and 4 are equal and counterweights 2 and 3 are equal?

In reply to CrustyRedXpress :

Balancers will tell you the imbalance at each end.  You can have a crank that is statically in balance but dynamically way out of balance.  (Think of putting a huge weight at each end of the crank 180deg out... it will be statically in balance but dynamically waaaay off)