Petrol tap adapters. Available in two sizes, the large one will reduce a 3/8 petrol tap thread to 1/4. The small one will reduce a 1/4 petrol tap thread to 1/8. Please choose large or small from the dropdown. The gas threads often cause confusion. The largest size is the 3/8 which has a thread diameter of 5/8 inch. The next is the 1/4 which has a thread diameter of 1/2 inch, and the smallest is 1/8 which has a thread diameter of 3/8 inch.