nrv@ulysses.UUCP (N. Richard Venditto) (12/13/84)
First, I would like to thank all the people who responded to my plea for help. No one, however, wins the contest. The noise in the engine was caused by something not mentioned in any of the replys. For those new to the story, my Subaru had been making a faint rapping noise at idle, which sounded suspiciously like a worn rod bearing. The noise also went away when the spark plug for cylinder #3 was disconnected. The rod bearing was just fine though, as were the wrist pin, piston, etc. With all new bearings in the engine, it still made the same noise. The solution to the mystery awaited the second tear down of the engine. A problem common to the air cooled VW engine was the cause of the noise. The Subaru is basically a water cooled version of the same flat four cylinder design. The problem was harder to spot in the Subaru though, since the tolerances seem to be more critical. The problem: The main bearing journals had expanded in size a couple of thousands of an inch, probably from the pounding they took when the original main bearings had worn out. The worn main bearings were the only obvious problem with the engine noticed the first time it was apart. I was only able to discover this by measuring the bearing clearances with the new bearings in place, using plastigage. I really should have done this the first time out but as I said in the previous article, I was in a rush. Anyone planning to rebuild an engine in the future should stock up on plastigage. The engine bearings will not be crushed down to the proper size when bolted in place if the journals are too big. As installed, the bearings had from .002 to .003 inches too great a clearance. To get an idea how critical this was, the allowable clearance for the center main bearing (which carries the most load) is only 0.0000 to 0.0008 inches. Why the noise went away when cylinder #3 was disconnected must have something to do with the way the load is distributed on the crankshaft as each cylinder fires, with #3 being the most likely to cause a pounding noise being one of the two closest to the center bearing. As for today, the car is back on the road as quiet as when new, and I hope it stays that way. Rich Venditto (ulysses!nrv @ Bell Labs in Murray Hill)