1. When the thermostat is replaced (97 A6QA), does that drain all the coolant, or only the coolant in the engine, or only part of the coolant in the engine? Since I had my thermostat changed earlier this year, I am wondering if I can count that as a coolant flush.
2. Last time I changed the coolant, I reused the rubber O ring on the engine coolant drain plug. If I do it again I want to replace that O ring. But I cannot find the O ring in ETKA. The closest I can find is a gasket on a drain plug that (in the ETKA diagram) is horizontal (not vertical) and the gasket is 11x3, which looks like the right size. Can anyone give me a part number for that O ring?
I also have a box of rubber O rings at home, and wonder if I could just use one of those, assuming I have one the right size.

1. When replacing the thermostat, its probably only a partial flush. The best way to do it is to remove the block drain, and then drain the radiator.

2. Check link below for part numbers. ETKA shows the drain as a horizontal bolt, which seems strange, i know.<ul><li><a href="http://www.mnsi.net/~natwhite/Audi/Coolant%20Flush%20&amp;%20Fill%20Procedure.htm">http://www.mnsi.net/~natwhite/Audi/Coolant%20Flush%20&amp;%20Fill%20Procedure.htm</a</li></ul>

The seal part number you gave me is the one I thought it was in the Etka diagram. The parts guy at the Audi dealer told me that the Etka diagrams are not always accurate!! the rubber seal is $2.50