Problem: I have a 350 small block Chevrolet engine in my street rod with a 700R4 transmission. I rebuilt the engine completely with all new parts. The transmission was also rebuilt. When starting the engine, I noticed a distinct scraping sound when the transmission was placed in reverse. The scraping sound only happend when I started the engine after sitting for a day or more. After running the engine for a couple of minutes, the scraping sound goes away. In fact, if I start and run the engine any other time during the day, and place it in reverse, there is no scraping. The scraping only happens in reverse, when starting the engine, after having it sit for a day or more. And it only last several minutes. What could the problem be.
Solution: At first I thought that the scraping sound was coming from the flywheel ring gear, scraping the inside of the dust cover of the transmission. However, after doing some checks on the inside of the dust cover, there was no evidence of scraping. The thing that confused me the most was the timing of the scraping, (only during cold starting and only in reverse). After thinking about it for awhile, I know that when in reverse, the axial forces on the crankshaft are reversed as compared to being in a forward gear. Also, I know that the thrust bearing on the crankshaft allows the crankshaft to move axially. This is called crankshaft end play. On this engine, it is between .002 to .006 inches. So changing from forward gear to reverse gear, the crankshaft could be moving this distance.
At this point, I decided to remove the oil pan and take a closer look at the end of the crankshaft. I found two unusual things. There was a scraping mark on the back end of the oil pan, near the last oil pan bolt. This is shown in the photo below.