For a #4 rivet, which drill bit size is recommended?

The recommended drill bit size for a #4 rivet is indeed the #30 drill bit. This pairing is based on the standard specifications for riveting, where the drill bit size must accommodate the diameter of the rivet to ensure proper fit without excessive play or interference.

A #4 rivet has a nominal diameter that matches well with the #30 drill bit, allowing for an appropriate hole size that provides strength in the riveted joint. This sizing is crucial in maintaining structural integrity. If the hole is too large, it can lead to a weak joint, while a hole that is too small can cause stress during the insertion of the rivet, potentially damaging the surrounding material.

In contrast, other drill bit sizes like #40 or #35 would create holes that are too small for a #4 rivet, making it difficult or impossible to insert the rivet properly. A #25 drill bit would yield a hole that is excessively large, compromising the fit and overall effectiveness of the rivet. Hence, the #30 drill bit is the correct choice for drilling holes for #4 rivets in aircraft metal structural repairs.