So let me interpret reading between the lines.
In order to have determined that the cylinders had some taper, one would have to have removed the cylinder and checked with a dial bore gauge since one could not check the cylinders with the head off and the cylinders still in the bike. Then you suggest that the previous owner decided to carry on and run with the original pistons and rings. That being said, he probably just cleaned up the pistons and rings and then reassembled. And that's why you have oil consumption. The cylinders have a small measurable taper due to wear. and the piston rings would have worn to fit that wear, so the rings are now not round. Now the previous owner cleaned things up and reassembled. This is where things went wrong. It would be impossible to clock the used rings into the same previously installed position so that the rings sat in the bores in the same place as they had fitted themselves to the worn bores so now you have high points on the rings running on high points in the bores and the worn taper in the bores having no contact with worn areas on the rings. If he had torn the engine down to check for oil usage and then reassembled the way I think he did, then the oil consumption probably went up not down.
So what to do now - The proper fix here would be to tear the motor down and send the cylinder out to a machine shop that has a precision hone like a Sunnen vertical cylinder honing machine. (see
) The honing head on this machine has 2 guide shoes and 2 honing stones. It will remove the taper with minimal material loss. (trust me, an Licenced Automotive Technician and also a millwright with 30 years experience in a nuclear power plant and I've spent 100's of hours operating one of these precision hones performing my trades) Then you're now working with a true round cylinder. If the fit tolerance is close to the service limit specs, you could source and replace the piston rings with new ones and get away with the stock pistons. The critical fit is the piston rings to the cylinder walls. So what I'm saying here is that if the pistons are outside of the service limit by a couple of thou I'd run with them rather than a total replacement of pistons and rings (just to save the cost of new pistons).
You did mention that the leakdown was < 10 % before teardown? or was that after rebuild and then reseating the rings? Less than 10% is considered a tight well sealed engine unless racing, then the goal would be < 5%. At 10 % leakdown, oil consumption should be minimal. I would have assumed that if the engine was <10% and was consuming oil, that the oil consumption was attributed to valve seals. That would be evident by carbon build up on the back sides of the intake valves. If you had a bore scope and pulled the carbs, you could see the back of the intake valves to confirm.
I'm impressed that leakdown testing was performed and Kudos to whoever did that. The interpretion of the leakdown testing is the second half of the testing. Understanding what the testing is telling you will save a lot of time and expense. The leakdown tester is a $50 tool. The knowledge of understanding the results- Priceless !!
Hope these thoughts help with your problem.
KB