ORIGINAL: ptulmer
Well, I can tell a story about two of our friends. Flyswatter had an AME that wasn't turning up like it should and the case would get REALLY hot. He spoke to our own infamous Larry Driskill, and shipped him the engine. My grapevine understanding is that Larry chucked it up in his lathe and took some material out of the center of the crank using sandpaper. That bad boy turns UP now! Maybe since I've prodded their memories, one of 'em can give us details or correct me. I don't know how common this is, but it seems to me the same thing could be accomplished with a drill press if you are very, very, very careful?
Infamous? Infamous? Wow, except for the fact that good old ptulmer had seemed like such a good a good guy and positive contributor here, his throwing out identifiers like "infamous" rather than "clever, sought by the babes, and talented too" would make me think he might really be the corpulent attack dog Ted Kennedy using an alias.
When we started using AMEs they would often seem to go lean soon after launch. But when you picked up the plane (after the engine died from the seemingly over lean condition) the bladder fed needle would still be putting out fuel at the normal rate. But the engine would be very hot - just like you would expect from a lean run.
After it cooled you could fire it up and the needle setting would seem fine, even though you had not changed the needle. Launch again and you would again get the few good seconds of run and then . . . lean again.
In spite of "clever, sought by the babes, and talented too", I was slow to solve the problem. The engines usually got better after a whole bunch of tries and flights, if you stuck with it. Then I realized that the engines were overheating from the bottom end vice from being too lean. The shafts were fitted too tight in the crankcases and when the engine began to warm up the clearance decreased and friction increased causing more heating and it was all downhill until the shaft seized in the case. I finally figured it out when I checked out a shaft and found aluminum on it where it had galled in the case.
I cleaned off the aluminum, lapped the shaft in the case with Bon Ami and oil and viola! Problem solved.
The next step was to put a shaft in the lathe and use emery cloth, followed by 400, 800, and 1200 wet / dry paper with oil to remove a tiny amount of material from the center portion of the shaft. The surface of the shaft behind the port opening and the front 3/16" was left alone. This reduced drag between the case and shaft, but left the front and rear bearing surfaces untouched.
And so . . . especially with plain bearing engines, it may well be that a careful fitting followed by an equally careful bench break-in to the point where you are sure the engine will hold a good, top end needle without overheating and stalling is good practice and will help prevent unwanted dead stick landings. If the engine does not hold a setting on the bench, it probably will not be any better in the air.
Burma Shave