Adapter rings for attaching automotive clutch assemblies to different types of flywheels are disclosed. A single adapter ring may be installed on different types of stock or aftermarket flywheels. In one embodiment, a multiple-disc clutch assembly may be mounted onto the adapter ring. The multiple-disc clutch unit may fit inside a stock bellhousing to enable the user to utilize the stock clutch release mechanism and simultaneously increase the holding power capability while maintaining light pedal effort. Aftermarket replacement OEM clutch units including the adapter ring may be used with higher horsepower engines in automotive vehicles with manual transmissions.