What size packing should be used for a pump with a 4" stuffing box bore and a 3 1/4" pump shaft?

The appropriate size packing for a pump is determined by the diameter of the stuffing box bore and the pump shaft. In this case, with a 4" stuffing box bore and a 3 1/4" pump shaft, the packing must effectively fill the space around the shaft while allowing for proper sealing and operation.

Choosing packing that is too small can lead to leaks, while packing that is too large may not fit properly or may create excessive friction, leading to premature wear and potential mechanical failure.

A size of 3/8" packing is often used as a standard for applications with a 4" stuffing box, as it strikes a balance between sealing capability and accommodating the shaft diameter. This choice allows for enough compression to provide an effective seal, while also ensuring that the packing is not overly restrictive around the 3 1/4" shaft.

It is also common to consider the design of the packing gland and the manufacturer's specifications to confirm the best choice. In summary, using 3/8" packing for this setup provides a suitable compromise for an optimal seal with reasonable wear characteristics, making it the correct answer.