I'm doing a head gasket on my 951 and I have some scoring on #3. I'm guessing it's from either:
a.) coolant in the cylinders from the bad head gasket
b.) metal shavings could have gotten in the cylinder when in a previous project I had to drill out an intake bolt.
c.) there was about 2 weeks I was not getting oil pressure for 5 seconds or so on startup from sticking OPRV (only used the car sparingly here)

All pistons have similar rock in all directions, this is only in the lower side of #3. Is it possible to use a feeler gauge to measure the distance from the piston to the cyl wall to make sure my ring is not completely worn?

 

It won't really be possible to do what you're thinking. I'd just do a leakdown test after the head is reinstalled and see where you're at.