2. Test Fitting2. Take out the two sizes of push-fit fasteners and do a test-fit of the size that will work for you. If you start to put the small one in and it just falls in or out—just put the small ones aside, you won’t be using them for this project. If the tip of one just starts to grab the first threads, you probably have the right size. DON’T PUSH IT ALL THE WAY INTO THE HOLE YET, they can be very difficult to remove.