When you install piston rings, make sure you don't expand them more than absolutely necessary to put them on the piston.
Some people expand them fully, to the limit of the pliers, and they don't like to snap back round when you do that.
This is ASSUMING you put each piston in the cylinders and checked them for cylinder to wall clearance BEFORE you installed them on the rods!
Once you know what piston fits what cylinder the best -- and every cylinder in every engine is different, along with every piston, so move them around until you get the best, most uniform fit.
Once you get the pistons pressed onto the correct rods (each piston top should be marked for the cylinder it fits in) you are ready to fit the rings...
What I would do If I were you,
1. I would make a piston ring depth gauge.
This doesn't need to be fancy, but it MUST square the rings up in the bore for test gauging.
Mine are usually home made, although there are 'Plastic' versions you can buy that don't cost very much.
An old piston top mounted on a flat plate works well.
Some guys turn the piston up side down with a rig in the oil groove land and use the piston up side down in the cylinder to square up the ring in the cylinder.
It works, but you must make sure that ring doesn't snag in the oil groove or you will cock the ring you are gauging in the bore.
The idea is to put the rings in the cylinder, one at a time, then use the tool or a piston to square up the rings.
Measure the gap with feeler gauges,
If it's too large or too small, you move the ring to another cylinder to see if it fits better there.
Keep the rings for each cylinder together, so they don't get mixed up again once you figure out what ring fits what bore the best.
I use a board with pegs for the rings/piston-rod assemblies numbered for each cylinder,
2. Once you find out what rings fit what cylinders correctly,
Or you have clearanced the rings to fit (Grinding rings is sometimes required to make a good fit for all in a set),
Then you can install the rings on the pistons.
Clean the piston grooves, pay particular attention to sharp edges or places where the piston is deformed.
It's rare you won't find a machining chuck mark on the ring area of the piston, so some times you will have to file or cut out the groove to clearance the rings during install.
I often file the ends of the rings on the VERTICAL surfaces, just break the edge over,
This keeps burrs and sharp edges from hanging up in the soft aluminum as you install, and it keeps burrs from digging in once they are installed.
Sometimes the ring end burrs will be so sharp and misaligned, they will dig into the piston and the ring won't rotate in operation, which is NOT good.
If you are using 'Molly' coated rings,
Be VERY careful about expanding the rings, you can pop the molly coating right off the ring surface if you expand it too much.
With plain ductile iron rings,
Make sure you are installing the rings with the CORRECT side facing up, and you get the rings in the correct places on the pistons.
The last thing you want to do with brittle ductile iron rings is break one taking it on and off two or three times during the install.
I've seen a TON of rings installed up side down, and on the wrong grooves, so it's easy to do.
Same with over expanding the rings and getting them out of round, which leads to scoring the crap out of the cylinder walls and chipping the crap out of the ring edges.
DO NOT FORGET to use some emery cloth and 'Break' the top edge of the cylinder!
If you have a completely square edge, or worse yet, machining/honing burrs on the top edge of the cylinder, you WILL hang a ring up when trying to insert the pistons.
LUBE YOUR RINGS WITH ASSEMBLY LUBE BEFORE YOU INSTALL!
This makes them come out of the ring compressor MUCH easier, Keeps the dissimilar metals (aluminum, ductile iron, cast iron) from corroding when they come into contact with each other,
And if you take your time putting the rest of the engine together, installing it, ect.
There WILL be plenty of time to corrode/rust before you fire the engine up and get some engine oil in there.
I would cut some long pieces of fuel line,
Slip those over the rod bolts so the rod bolts don't get into the crank journal surfaces when you install the pistons/rings.
A little rubber hose saves a LOT of cussing in the long run!
When you crank the engine over to put the rod cap on the rod,
Remember to BOLT SOMETHING ACROSS THE CYLINDER, OR USE DUCT TAPE!
The piston/rod WILL fall right out of the bore when you flip the engine to bolt the caps on the rods!
(Ask me how I know that!
I ruined an $800 off set & barrel ground Mahle blower piston just about 5 months ago, and I've been doing this for about 35 years... So it DOES happen...
And no matter HOW careful you are about keeping a hand on the piston when you flip the engine, sooner or later that hand will try to catch something else, or reach for rod nuts, or drive tool, and the piston WILL hit the floor!