Yeah, I remember our discussions about that and I wanted to let you know where I am at this point...
I only put one piston back in the block, and it was a PITA. Like you, I found the band style compressor to be very finicky. I'm assuming it gets better with experience, but it took me three tries to get the first piston back in without catching one of the oil rings on the block deck. Third time it snicked into position, but I'm just not happy with the compressor. I've actually got two of them and I'm not completely enamored with either of them.
So I decided to try something different and made myself a tapered installation ring tool.
Bought some steel tubing of appropriate size. Here's what I started with, Band tool on left, and thick wall tubing on the right:
I chucked the tubing up in the lathe and bored it out to be a precision fit over the piston:
And then I tapered one end with a gradual taper and polished everything up so the rings slide easy:
The concept is you can slip a fully ringed piston into it and the rings will compress as you push the piston down the tapered section:
And once the rings are all compressed, the theory is that I can just slip the pistons into the bores from here:
And why does this pertain to ring gaps? Because since you're pushing straight down, the gap positions don't migrate. They stay where you put them.
So I'm jumping the gun here a little since I haven't actually used it to put a piston into the block yet, but it worked great on the bench. What could possibly go wrong? go wrong? go wrong?