This page provides imperial dimensions for square taper keys, gib-head keys and their keyways to the legacy British Standard BS 46 Part 1:1958. It includes shaft diameter ranges, square key sizes, shaft and hub keyway dimensions, tolerances, corner radii and gib-head dimensions.
Reference standard: BS 46 Part 1:1958 – Keys and keyways and taper pins. Specification. Keys and keyways.
Square taper keys have equal nominal width and height. The tables below give the corresponding shaft and hub keyway dimensions and the dimensional data for taper and gib-head keys contained in the RoyMech BS 46 reference.
All dimensions in the following tables are in inches.
Use the shaft diameter range to identify the corresponding nominal square key size W × T. For these keys the nominal width W and height T are equal. The table then gives the dimensional limits for the shaft and hub keyways together with the specified keyway radius.
The numerical values below reproduce the legacy BS 46 dimensional data published by RoyMech. Where a component is being manufactured, inspected or replaced against an existing drawing, the original drawing and applicable standard should take precedence.
| Shaft Diameter | Key | Keyway – Shaft | Keyway – Hub | Keyway Radius r |
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| D | Width / Height | W | H | W | h | ||||||||
| Over | To / Inc. | W & T | Min | Max | Min | Max | Min | Max | Min | Max | |||
| 1/4 | 1/2 | 1/8 | 0.124 | 0.125 | 0.072 | 0.078 | 0.125 | 0.126 | 0.039 | 0.045 | 0.010 | ||
| 1/2 | 3/4 | 3/16 | 0.187 | 0.188 | 0.107 | 0.113 | 0.188 | 0.189 | 0.067 | 0.073 | 0.010 | ||
| 3/4 | 1 | 1/4 | 0.249 | 0.250 | 0.142 | 0.148 | 0.250 | 0.251 | 0.094 | 0.100 | 0.010 | ||
| 1 | 1 1/4 | 5/16 | 0.311 | 0.312 | 0.177 | 0.183 | 0.312 | 0.313 | 0.121 | 0.127 | 0.010 | ||
| 1 1/4 | 1 1/2 | 3/8 | 0.374 | 0.375 | 0.213 | 0.219 | 0.375 | 0.376 | 0.148 | 0.154 | 0.010 | ||
| 1 1/2 | 1 3/4 | 7/16 | 0.437 | 0.438 | 0.248 | 0.254 | 0.438 | 0.439 | 0.175 | 0.181 | 0.020 | ||
| 1 3/4 | 2 | 1/2 | 0.499 | 0.500 | 0.283 | 0.289 | 0.500 | 0.501 | 0.202 | 0.208 | 0.020 | ||
| 2 | 2 1/2 | 5/8 | 0.624 | 0.625 | 0.354 | 0.360 | 0.625 | 0.626 | 0.256 | 0.262 | 0.020 | ||
| 2 1/2 | 3 | 3/4 | 0.749 | 0.750 | 0.424 | 0.430 | 0.750 | 0.751 | 0.310 | 0.316 | 0.020 | ||
| 3 | 3 1/2 | 7/8 | 0.874 | 0.875 | 0.495 | 0.501 | 0.875 | 0.876 | 0.364 | 0.370 | 0.062 | ||
| 3 1/2 | 4 | 1 | 0.999 | 1.000 | 0.566 | 0.572 | 1.000 | 1.001 | 0.418 | 0.424 | 0.062 | ||
| 4 | 5 | 1 1/4 | 1.248 | 1.250 | 0.707 | 0.713 | 1.250 | 1.252 | 0.526 | 0.532 | 0.062 | ||
| 5 | 6 | 1 1/2 | 1.498 | 1.500 | 0.848 | 0.854 | 1.500 | 1.502 | 0.635 | 0.641 | 0.062 | ||
The following table gives the square-key dimensional limits, gib-head dimensions and preferred lengths contained in the RoyMech BS 46 reference.
The dimensions A, B, C, D and R refer to the gib-head geometry shown in the diagram. Blank preferred-length entries have been retained where no value was given in the source data.
All dimensions are in inches.
| Key | Gib Head | Preferred Length L |
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| Width / Height | W | T | A | B | C | D | R | ||||||
| W & T | Max | Min | Max | Min | Taper | Gib Head | |||||||
| 1/8 | 0.127 | 0.125 | 0.129 | 0.124 | 3/16 | 1/4 | 5/32 | 0.1 | 1/32 | 3/4 to 1 | |||
| 3/16 | 0.190 | 0.188 | 0.192 | 0.187 | 1/4 | 3/8 | 7/32 | 0.2 | 1/32 | 3/4 to 2 | 1 1/2 to 2 | ||
| 1/4 | 0.252 | 0.250 | 0.254 | 0.249 | 5/16 | 7/16 | 9/32 | 0.2 | 1/16 | 3/4 to 3 | 1 1/2 to 4 | ||
| 5/16 | 0.314 | 0.312 | 0.316 | 0.311 | 3/8 | 9/16 | 11/32 | 0.3 | 1/16 | 3/4 to 3 | 2 to 4 1/2 | ||
| 3/8 | 0.377 | 0.375 | 0.379 | 0.374 | 7/16 | 5/8 | 13/32 | 0.3 | 1/16 | 1 to 4 | 2 to 6 | ||
| 7/16 | 0.440 | 0.438 | 0.442 | 0.437 | 1/2 | 3/4 | 15/32 | 0.4 | 1/16 | 1 3/4 to 4 | 2 1/2 & 3 to 6 | ||
| 1/2 | 0.502 | 0.500 | 0.504 | 0.499 | 9/16 | 7/8 | 17/32 | 0.4 | 1/16 | 2 to 6 | 2 1/2 & 3 to 6 1/2 | ||
| 5/8 | 0.627 | 0.625 | 0.630 | 0.624 | 11/16 | 1 | 21/32 | 0.5 | 1/8 | 2 1/2 & 3 to 6 | 4 1/2 to 7 & 8 | ||
| 3/4 | 0.752 | 0.750 | 0.755 | 0.749 | 13/16 | 1 1/4 | 23/32 | 0.5 | 1/8 | 3 1/2 to 6 | 4 & 5 & 6 & 8 | ||
| 7/8 | 0.877 | 0.875 | 0.880 | 0.874 | 15/16 | 1 3/8 | 27/32 | 0.6 | 1/8 | 3 1/2 to 6 | 6 & 7 to 8 | ||
| 1 | 1.003 | 1.000 | 1.007 | 0.999 | 1 1/16 | 1 5/8 | 1 1/32 | 0.6 | 1/8 | ||||
| 1 1/4 | 1.253 | 1.250 | 1.257 | 1.249 | 1 5/16 | 2 | 1 9/32 | 0.7 | 1/4 | ||||
| 1 1/2 | 1.504 | 1.500 | 1.509 | 1.499 | 1 9/16 | 2 1/2 | 1 17/32 | 0.7 | 1/4 | ||||
These imperial dimensions remain useful when maintaining older machinery, interpreting historic drawings or manufacturing replacement components for equipment originally designed using BS 46.
A square taper key has equal nominal width and height, but the shaft and hub keyway depths are not simply equal divisions of the nominal key height. The specified keyway dimensions and tolerances should therefore be used when reproducing an existing BS 46 connection.
The taper key develops its fit by being driven axially into the corresponding shaft-to-hub connection. Where a gib head is provided, the head supplies a feature to assist removal. The taper and keyway geometry are part of the connection and should not be varied arbitrarily to obtain a desired fit.
When replacing an existing key, check the actual shaft and hub keyways, key dimensions and taper against the drawing or original component rather than selecting a replacement from nominal key size alone.
Do not substitute the nearest metric key solely because its nominal dimensions appear similar. For a new design, current standards and current engineering requirements should normally be considered rather than specifying the withdrawn BS 46 standard simply because the imperial dimensions remain available.