The Difference Between a Plain Bore and Finished Bore

Posted on Sep 06, 2019

Plain Bore Vs. Finished Bore

There are differences in both the appearance and advantages of plain bores and finished bores.

When looking at a component with a plain bore (sheave, sprocket, gear, pulley, etc.), it will have an untapered bore drilled through its center. Other names for the plain bore are MPB (minimum plain bore) and SB (stock bore). Occasionally, part numbering systems will use "MPB" to signify that the component contains a plain bore.

Plain bores provide more options as to how to mount a component to a shaft. Because of its ability to be opened to an increased diameter in the bore, a plain bore can later be manufactured into a finished bore or designed to contain screws and pins.

A component has a finished bore when it has a plain bore with set screws, a keyway, or both added on. Part numbering systems will frequently use an "F" or "BS" in the product number to indicate that it is a finished bore.

An example of a finished bore part number would be 35BF48x1-1/4 or 35BS48 1 1/4. This is a roller chain sprocket designed to pair with a #35 chain, a B-style hub, a finished bore, 48 teeth, a 1.25" shaft, a standard keyway, and set screws.

Finished bores help prevent the shaft from slipping. The component will be able to carry increased load weights with higher precision. However, if used in the food and beverage industry, the set screw holes in the finished bore can fill with surrounding materials or ingredients in instances where the drive is near areas with large amounts of foreign debris or contaminants.

finished bore

What are keyways and set screws?

Components sometimes have slots cut into them to take a key that works with a similar cut in the shaft to help prevent the motion or rotation of both products. These slots are called keyways. Keys joining shafts and hubs are often used in no-slip systems.


Set screws go into the component to secure it to the shaft and reduce slipping. There is commonly one set screw over the keyway and one at 90 degrees to the keyway. Set screws commonly assist larger products in moving increased loads.