What happens if you spin a coupler? When my engine was balanced the coupler was marked to go onto the flywheel in a specific location.

The reason I am asking this is, I am thinking of assembling a spare engine. What would happen if I have that engine balanced without a coupler then add one later?

Couplers are neutral or "zero" balanced from the factory so you could mount it any way you want without affecting your motors balanvce,if you want to really be precise take a new coupler in and have it balanced perfect as a spare for your spare motor,Smitty

I replaced my coupler and just bolted it on. Never even thought about balancing it. I figured it was like bolting up a clutch or torque converter in a car.

My current engine was balanced, and they even drilled holes in the coupler.
Would they do that to get it neutral?

The coupler came at neutral but could be off a gram or 2 and what they most likely did was balance it perfect to a tenth of a gram. You'll see this sometimes on harmonic balancers too,other than the real new stuff,the 454 and 400, most other chevy balancers are "neutral balnced" but when you get everything balanced they'll spin it up and drill some real slight holes in it to make it perfect. It's remotely possible, but no engine builder would normally do this, that they used your coupler to help achieve corrective balance for the "off balance" your motor needed on the flywheel if it was externally balnced but it would be the dumbest thing I ever seen and there would be serious holes drilled in it or weight welded to it,Smitty

My harmonic balancer also has small holes drilled into it.

So let me get this right, they are making these components neutral?
I never pay attention to the flywheel. I don't remember seeing any drilling there.
I never have really understood the steps in balancing an engine, just paid for it

Exactly,they were "neutral" to start with by mass production standards but brought to "perfectly neutral" by a tighter standard,Smitty

Ok Last question.
My coupler was marked with a big YELLOW stripe along with the flywheel.
Does this mean they where balanced as one component??

I'm not sure of the answer on that. They might have been balanced together,did you get a balance sheet,Smitty

coupler is neutral. balancer and flywheel (on a 454 and 502) are not neutral.

quickie version:
When you balance, you equalize pistons to the lightest one, then you equalize the little ends of the rods to the lightest one, the big ends of the rods to the lightest one, then the neutral components are balanced alone. Then all the parts are put together and the entire rotating assembly is balanced.

That's sort of a real quick explanation.