-- 4.6mm thickness metal around the square hole. Nomore cracked when you try to twist it hard. -- The oil filter removal tool can bear tremendous pressure and won't slip nor crack.The welding of the joint is strengthened to make sure its wear resistance while cheaply made oil filter wrench cracks easily when you torque. (Hint: put a rag or sandpaper inside the wrench will help to grab filer tightly)