The Sonnax 72978-01K 4C retaining ring and shim kit includes a custom G-Loc ring to prevent the factory retaining ring from coming off. The clutch backing plate thickness has been changed to .078 in. thick (.010 in. thinner than the original) to allow more choices when setting up clutches. The spring pack shim allows 4C clutch adjustment for a quicker release, resulting in firmer shifts when setting the clearance to the higher end of specifications.