Piston Rings
A short course on selecting piston rings:
Material
Three general types: cast iron, chrome and molybedenum.
Cast Iron: cheapest, quite acceptable for a daily drive that doesn't getting a good thrashing and engine/combustion temps are nothing to write home about. They have a porousity that helps retain oil and lubrication where they contact the cylinder walls. Not so great in a heat and dirty environments. Iron rings are probably best used on re-ring jobs where the cylinders haven't been rebored. They will accommodate imperfections of bore taper and out of roundness much better than Moly or Chrome. These are the rings that need to be bedded in by loading up the engine on the initial drive.... chrome rings hardly wear over the lifetime of an engine and moly rings aren't too far behind. If the second ring (scraper) is cast iron it will need to be bedded in by loading the engine in the initial drive.
Chrome: much harder than cast iron with a higher scuff and scoring resistance (twice that of Moly). In environments where there is a lot of dirt and dust, chrome rings have the advantage in that they are not porous, so therefore tend not to hold dirt that would otherwise be abrasive. They will tend to expel foreign matter via the exhaust ports because of the low adherence. The chrome has a lower melting point than moly and can weld to the bores and cause bore scuffing. Chromed rings, like moly, are lapped in the factory for true fit in properly bored cylinders and will not seat properly in unmachined bores.
Moly: Highly scuff and scoring resistant. It has the highest melting point and is well suited to high reving, severe load and prolonged high temperature conditions. It has a porous face and therefore holds oil well to inhibit scuffing against the cylinder wall. However like cast iron, a dirty or dusty environment will encourage adhesion of abrasive foreign matter that will shorten the ring life and wear the bore. The molybedenum is plasma sprayed into a ring groove to allow the moly to grip to the ring substrate (moly doesn't stick well). Like chromed rings, moly rings are factory lapped and require a properly machined bore.
Rings Misc
Ductile Iron Rings: or modular iron rings are well suited to high temperature applications. It describes the substrate and they are invariably either chromed or faced with molydenum. Twice the strength of cast iron rings. Used mainly for the compression ring.
Oil rings: are generally a three piece chromed assembly. They are designed to restrict oil consumption during the power stroke to between 0.0001 and 0.002 of a drop. Oil consumption tends to increase by the power of 3.5 as engine speed increases e.g a car doing 100kph will use about six (6) times the oil than when travelling at 60kph.
Ring Gap
Your engine specs should tell you what the maximum accepatble wear on the bores is. But just incase you don't have them available the following info should give you a rough guide.
When the ring is in the bore, the ring gap should not be less than 0.0035mm per mm bore size. Maximum ring gaps are in the general range of 0.25mm and 0.55mm for average sized car pistons in the 75mm to 100mm range..
And for bore taper the wear should not exceed the lessor of 0.003mm per mm nominal bore size or 0.03mm at any point, else you need to oversize the bore , pistons and rings.