assuming a bore is round, paralell and on size, is it necessary to hone
when installing new pistons and rings?
It's necessary to provide some texture to the cylinder walls to enable the piston rings to properly seat. It'd not be necessary for pistons, assuming that the original rings were retained. If the bores have no taper using your methods of measurement, the cylinders could be readied with a whirlybird hone, or a dingle-berry hone if you prefer. Both have small balls out on the ends of many small flexable wires. If you are careful to not allow it to stay in one location, you can "dress" the cylinder walls sufficiently to make the rings seat properly. I don't want to mention this, but in emergencies, these dingle-berry hones have been used to clean the walls in engines that still reside in the car, crankshaft and all. You absolutely must protect the crank and bearing areas so no particles from the cylinder walls or the hone are washed down. Plastic and duct tape work pretty well for this. The honed cylinders also need to be cleaned very carefully prior to assembly. For a quick and dirty job, no pun intended, make a wall scrubber from a reshapped coat hanger with several layers of BOUNTY paper towels taped to the hanger at the upper end. You can use some carb and choke cleaner at first, then do it with hot water and soap and finally with WD 40 on the hanger towels. Use new towels for each step, so you may need at least one big roll. Bounty is the best towel, so don't fool around with anything else, especially the blue "mechanic's" towels. All that you are doing in this operation is removing the glaze that builds on the cylinder walls, so don't go wild. Another little tip is to make sure that you duct tape the old headgasket to the deck. This will prevent the little balls from "polishing" the area around the bores. If a drill motor and a good Sunnen hone is available with the correct grit stones for the ring material , go with it. Remember that almost constant fluid flow is mandatory with this type of hone, so be prepared to use a tub to collect it. In the 60's, we put a small fuel pump and an oil filter in the tub and the lubricant flow was continuous. Remove as little material as possible and remember that keeping everything clean is mandatory.