OK, I feel a little dumb asking this since it's something I should know. I've bought several TU clutches over the years and they've all been great. They include a plastic tool to align the disc, and also a small round plastic piece that I can only imagine gets installed into the end of the crankshaft as it fits tightly into the crank's flywheel flange and the center is the perfect diameter for the plastic disc alignment tool.

What is this small round plastic part for and is it necessary? I've never seen this included with the Luk clutches I've used in the past. Is it optional? Is it only to aid in aligning the disc? Has anybody ever installed one and had it fall out?