I have done this "fitting piston into barrel" task many, many times. This thread is rejuvenating some old memories of how it went. First, dismantling the piston from the rod: Unless the engine has been kept meticulously clean wilth ashless oils there will be a ridge of "shelac" or baked on dry oil on the pin between the rod and the piston. This ridge will make it difficult or impossible to press or pull the pin out of the piston with your fingers. With a suitable pin puller that misses the circlip it could be done but not likely.
Once the engine is at operating temperatrue the pins are full floating but cold the fit of the pin in the piston is a hard "thumb press" fit. I have encountered pistons (not Hepolite) whose pin bores were a press fit. In this case once assembled, the piston had to be forced to move on the rod with a good bit of wrist action. These pistons took a trip to the Sunnen Pin Fitter for some quality time until they were a thumb push fit. Pins, of course, should have NO radial play.
All this is to say that the pistons should be removed from the bores, the pin fit exmined, the skirts examined for scuffing, the ring-to-groove clearance checked, the ring end-gap checked and the bore taper and piston-to- cylinder-wall clearance checked. All this can be done with a set of quality feeler gauges. You don't need expensive bore gauges, though if you do much of this work you'll need them. When you have the barrel off check the base for flatness. It it's not flat, get it that way or it will continue to leak; likewise the cases. Reassembly is not that big a deal. Heat the pistons with a hair dryer if your thumbs are not strong. If you must use much heat, the pins are too tight. Use the plywood support method and good ring compressors made for that purpose. Also note that the bottom of the bore has a chamfer to help guide the rings in.
Don't rely on the previous shop's thoroughness or expertise. Even mechanics who know what they're doing make mistakes-- maybe the phone rang in the middle of something and they lost track or where they were in the process, maybe they had a bad day, or a headache but had to press on because they had several more bikes waiting in line. Use patience and attention to detail. As they say, it's not brain surgery. Novices do it successfully all the time.
