Replacing The Rear Main Seal
1. Remove the transmission. Look at the Audi A4 Clutch Replacement Guide for steps on how to remove the transmission.
2. After the transmission has been removed remember to drain the coolant before proceeding further. If you forget this step, coolant will come gushing out when you start removing the sealing flange.
3. Remove all the 10mm bolts from the sealing flange, should be 15 total + 2 from the bottom of the flange.
Old sealing flange.
3. Here comes the tricky part. The inner lip of the seal will get in the way when you go to install it and will end up curling (this is bad). To prevent this, I got the Felpro PTFE Rear Seal Kit for a 1998 Dodge Neon for $12. This kit comes with a plastic ring that makes installing the seal a breeze. I cut some of the plastic ring off (it was a little too big to begin with) and slipped it on the Audi seal. Look at the pic below and you will understand more clearly. Also, don't forget to apply RTV gasket sealer to the bottom of the sealing flange.
4. Before installing new parts, remove all the old gasket material and clean all the contact surfaces. Install a new paper gasket first and then install the new flange. Torque all the bolts to 7 lb-ft using a star pattern, starting from the inner bolts and going out.
5. Reinstall the transmission by following the final steps of the Audi A4 Clutch Replacement Guide.