kevlar

i have an 01 with a 6 spd. i was going to change the tranny fluid until i noticed i did not have the proper tool(assuming it would be 3/8 or 1/2 square). my is an allens of approximately 3/4. i called the dealership to get a size so i could buy the right wrench, but 2 mechanics there swear it should be slightly larger than 3/8, but still a square hole. they say my plug must be other than stock. i have never changed my fluid so i don't it has ever been off. it could possible been exchanged at a quick lube type place, but it doesn't look to be touched. does anyone know what size allens wrench this is?

bulabula

17mm