on the SR its a 10mm. I suggest buying an allen socket instead of the normal L shaped allen keys or T handled ones. I picked up a set of Craftsman metric allen sockets for $30 and they are on sale quite often for $20. Get a 1/2" breaker bar, 1/2"->3/8" adapter, 3" 3/8" extension bar, and the 10mm Allen socket. Make sure the socket is seated firmly and is perpendicular to the head bolt and put your back into it. Its easier if the engine is in the car, but if its on the stand make sure you can brace the stand from moving while breaking the bolts loose.