Assuming the car you're speaking of is the RS4 18SS in your profile the exhaust manifold screws are 3x30mm which means they're 3mm screws that are 30mm long. I can't recommend a screw size because I don't know how wide the engine is and whether or not you can fit a locknut on the other side of the engine. However if you bring the car to your local hobby shop or hardware store I'm certain they'll be able to help you out.