Definitely not 3/8ths. I never paid any attention to whether they're metric or otherwise. I find either 8mm or 5/16ths will work. When a 12 point wrench slips I use a 6 point wrench or socket. A 12 point is more convenient because the wrench can be positioned with more options than with a 12 point. Sometimes the bleeders have been re[placed with non-original type. I worked on a 240 recently that required 3 different wrenches to cover all 8 bleeders. -- Dave