This little bushing doesn't look like much, but it's critical! It rests in the back of the crankshaft, and provides alignment and support for the transmission input shaft. If your engine and transmission are going to be separated, make sure you have one ready!
Usually, if you need this, you will have the clutch apart - be sure to inspect the different components and replace the clutch if necessary while there! You may also want a clutch alignment tool while at it.
