I have a 55 2nd production engine that I completely rebuilt in 1973 -- crank ground, cylinders bored, cam bearings etc. It has pinned upper main bearing shells like the earlier blocks. I installed a rope seal and rolled it flat with a hammer handle. As soon as I installed the seal the crank no longer turned freely. It was scary tight but I proceeded with the main cap shims per the book and set the mains clearance per the book. The engine ran leak free for many years but eventually developed a leak that I couldn't find. I pulled the flywheel to check the cam plug when I had it apart for a clutch. That wasn't it. It would leak intermittently. Eventually, I changed the rear main seal for a neoprene one. It has leaked all the time since and eventually got worse and soaked the clutch. I put a full set of bearings in it (now with 150,000 or so miles) and used tapered shims on all the bearings to adjust for crank wear of about 0.002 (still round). With the old bearings, the rear main plastigauged at 0.0045, which is why I checked all the bearings. I felt that this was enough clearance to flood the rear seal and might be the source of my leak so I bought a set of bearings. The rod bearings were out to about 0.004 and the number-two, upper-shell came out in two pieces. The rods never knocked, go figure.
Anyhow, I shimmed the mains per the factory procedure but I set the rear main tight at 0.0015, feeling that less oil would flood the seal. That appears to have worked. It still leaks but not very much and it has pulled many times without complaint at full throttle for 15 minutes at a time over one of our many mountain passes. It's beginning to burn a little oil so its due for an overhaul soon. But I can't complain after nearly 200,000 miles.
