Final answer:
The proper tap drill size for a 1/4-28 thread in steel is a #3 drill bit, which corresponds to a diameter of 0.213 inches.
Step-by-step explanation:
To determine the appropriate tap drill size for a 1/4-28 thread, which is a fine thread designation for a 1/4 inch bolt, you subtract the number of threads per inch from the bolt diameter. In this case, with 1/4 inch equating to 0.250 inches and 28 threads per inch, you perform the following calculation: 0.250 inches - (1/28 inches) = 0.214 inches. The closest drill bit size to this measurement is a #3 drill bit, which is 0.213 inches in diameter. This size will allow for the correct thread engagement and strength in steel without compromising the material.