Flex shaft- for easy maneuvering. One end fits "Double-D" screws and the other 4.5 mm hex heads. Fits idle mixture screws Thexton 350 GM carburetor adjusting tool is for the use on all 1979 and up GM carburetor. The flexible shaft tool makes reaching and adjusting idle mixture screws easy. One end fits "Double D" screws and the other fits 4.5 mm hex heads. So actually, you have two tools in one! Sliding handle fits securely in either end of tool.