Whether or not that ring is incorporated into a new set will depend on the manufacturer of the new rings. DO NOT add it if the replacement ring set doesn't include a part like that. You're right on the borderline of needing a rebore job, so honing and re-ringing the engine is an option, especially on a vehicle that won't see a lot of use. I'd strongly recommend knurling the piston skirts to get the clearance right, if you can find a machine shop that still dies that kind of work. At one time, when an engine would routinely get 2-3 ring jobs before getting bored for bigger pistons, piston knurling was simply part of a ring job- - - -all the good quality shops did it. If you were on the south side of the border, I'd suggest that you box up the pistons and rods and send them to me. I can knurl piston skirts on my lathe by chucking up the piston and turning the machine by hand against the knurling tool. Any machinist worthy of the name can do the same thing, but finding one willing to think outside the box like that is virtually impossible these days.
You can measure piston skirt clearance with a feeler gauge by sliding the piston into the cylinder upside down and inserting the feeler gauge at right angles to the pin. If there's more than .0025"-.003" of clearance on a cold engine, there's probably going to be a little skirt rattle present during warmup. I used to have a set of feeler gauge blades 12" long and a spring scale that we used for piston fitting, but it can be done with a regular-length gauge blade if you're careful.
Jerry
