Let's recap. The bottom ring (an oil control ring) has a locating lug in the piston groove. When the ring is in the groove, it can rotate quite freely. The problem I'm having is compressing the ring into the groove whilst keeping the locating pin aligned with the gap in the ring (or, keeping the gap in the ring aligned with the locating pin). The ring compressor I have will compress all 4 rings with ease. Neither end of the bore has a taper (or lead-in) so the rings need to be compressed fully before the piston can be inserted into the bore. That's correct Cobbler. Conceivably yes, but that wouldn't be my preferred method. The problem is I haven't found my preferred method yet! Fitting the piston from the top of the bore would give me better access but wouldn't help with the fundamental issue I have which is compressing the bottom ring whilst keeping the ring gap aligned with the locating pin. It may well be that I need a different type of ring compressor but I'm not aware of one that will allow me to compress all the rings, keeping the bottom one aligned properly AND fit in the crankcase from below.