Rear Covers : Standard Bore vs Big Bore

The #1 cause of pump downtime is mechanical seal failure. While there can be many causes of seal failure, creating an optimal sealing environment at the procurement stage will increase the chances of extending the life of the seal, other things held equal.

Pump Downtime

One of these decisions is whether to use a standard bore or big bore rear cover (a.k.a. stuffing box). The stuffing box is the round plate behind the impeller, and behind the plate is the chamber where mechanical seals or packing are installed to control the leakage of a pump. There is also a third type of bore called the taper bore where the bore gets larger in diameter towards the impeller side.

What's the difference between a standard bore and big bore?

The circumference of a standard bore is much smaller than the circumference of the big bore, as illustrated by the highlighted gap comparing the 6" STX standard bore and big bore rear covers. The gap for an STX standard bore pump is only 5/16" around the shaft. In comparison, the big bore gap is 3/4" around the shaft for the same STX pump.

Standard Bore vs Big Bore

The extra room afforded by the big bore allows more process fluid to enter the chamber, which increases the lubrication between the seal faces and does a better job dissipating the heat from friction of the seal faces. More and more new pumps are ordered with big bore or tapered bore stuffing boxes because the larger open areas reduce heat and solids build-ups, which can increase seal life.

Should I always get a big bore stuffing box?


A standard bore stuffing box is required to use packing. There is no such thing as packing for big bore or taper bore stuffing box. You will notice that STX pumps use 5/16" packing, which is exactly the size of the gap between the standard bore and shaft.

If you currently use a mechanical seal for your application, you may also want to check your spare mechanical seal inventory before making the switch to big bore. A standard bore mechanical seal will not fit a big bore stuffing box.

Price is also a consideration. Big bore rear covers often cost more upfront, but may be more cost effective for the life of the pump.

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