And make sure you find all of it .
The very first job I did was replacing that retainer .
Go to the Hydro Gear web page & download the repair manual for your tranny
It is a easy fix but it will take you all wekend to do it properly
And as Star has said, the tranny has to be clean.
I blow away the loose stuff, degrease , pressure wash , soda blast then hot water wash & blow dry before I start to pull them apart.
And any time you leave it open cover with a damp low nap cloth .
They are assembled in a dust free factory for a very good reason .