Roymech engineering encyclopedia

BS 46 Parallel Rectangular Key and Keyway Dimensions


This page provides imperial dimensions for parallel rectangular keys and keyways to the legacy British Standard BS 46 Part 1:1958. It includes shaft diameter ranges, key sizes, shaft and hub keyway dimensions, tolerances, corner radii and preferred key lengths.

Need help understanding the different types of keys?
See the Key and Spline Selection Guide for an explanation of parallel, square, taper and gib-head keys, where they are generally used, and the main considerations when selecting a keyed shaft-to-hub connection.
Legacy standard: BS 46 Part 1:1958 has been withdrawn. The dimensional information below is retained by RoyMech as a reference for existing machinery, legacy drawings, replacement components and older imperial designs.

For new metric designs, see BS 4235-1 Parallel Keys and Keyways and confirm requirements against the applicable current standard.

BS 46 Part 1:1958 Parallel Rectangular Keys

Reference standard: BS 46 Part 1:1958 – Keys and keyways and taper pins. Specification. Keys and keyways.

All dimensions in the following tables are in inches.

BS 46 Part 1 parallel rectangular key and keyway dimensions

Parallel Key and Keyway Dimensions

Use the shaft diameter range to identify the corresponding nominal key width and height. The table then gives the dimensional limits for the shaft keyway and hub keyway together with the specified corner radius.

The numerical values below reproduce the legacy BS 46 dimensional data previously published by RoyMech. Where components are being manufactured, inspected or replaced against an existing drawing, the original drawing and applicable standard should take precedence.

Shaft Diameter Key Width Key Height   Keyway – Shaft   Keyway – Hub Radius r
D W T   W H   W h
Over To / Inc. Nominal Nominal   Min Max Max Min   Min Max Min Max
1 1 1/4 5/16 1/4   0.311 0.312 0.146 0.152   0.312 0.313 0.112 0.118 0.010
1 1/4 1 1/2 3/8 1/4   0.374 0.375 0.150 0.156   0.375 0.376 0.108 0.114 0.010
1 1/2 1 3/4 7/16 5/16   0.437 0.438 0.186 0.192   0.438 0.439 0.135 0.141 0.020
1 3/4 2 1/2 5/16   0.499 0.500 0.190 0.196   0.500 0.501 0.131 0.137 0.020
2 2 1/2 5/8 7/16   0.624 0.625 0.260 0.266   0.625 0.626 0.185 0.191 0.020
2 1/2 3 3/4 1/2   0.749 0.750 0.299 0.305   0.750 0.751 0.209 0.215 0.020
3 3 1/2 7/8 5/8   0.874 0.875 0.370 0.376   0.875 0.876 0.264 0.270 0.062
3 1/2 4 1 3/4   0.999 1.000 0.441 0.447   1.000 1.001 0.318 0.324 0.062
4 5 1 1/4 7/8   1.248 1.250 0.518 0.524   1.250 1.252 0.366 0.372 0.062
5 6 1 1/2 1   1.498 1.500 0.599 0.605   1.500 1.502 0.412 0.418 0.062
6 7 1 3/4 1 1/4   1.748 1.750 0.740 0.746   1.750 1.752 0.526 0.532 0.125
7 8 2 1 3/8   1.998 2.000 0.818 0.824   2.000 2.002 0.573 0.579 0.125
8 9 2 1/4 1 1/2   2.248 2.250 0.897 0.905   2.250 2.252 0.619 0.627 0.125
9 10 2 1/2 1 5/8   2.498 2.500 0.975 0.983   2.500 2.502 0.666 0.674 0.187
10 11 2 3/4 1 7/8   2.748 2.750 1.114 1.122   2.750 2.752 0.777 0.785 0.187
11 12 3 2   2.998 3.000 1.195 1.203   3.000 3.002 0.823 0.831 0.187
12 13 3 1/4 2 1/8   3.248 3.250 1.273 1.281   3.250 3.252 0.870 0.878 0.187
13 14 3 1/2 2 3/8   3.498 3.500 1.413 1.421   3.500 3.502 0.980 0.988 0.250
14 15 3 3/4 2 1/2   3.748 3.750 1.492 1.502   3.750 3.752 1.026 1.036 0.250
15 16 4 2 5/8   3.998 4.000 1.571 1.581   4.000 4.002 1.072 1.082 0.250
16 17 4 1/4 2 7/8   4.248 4.250 1.711 1.721   4.250 4.252 1.182 1.192 0.312
17 18 4 1/2 3   4.498 4.500 1.791 1.801   4.500 4.502 1.229 1.239 0.312
18 19 4 3/4 3 1/8   4.748 4.750 1.868 1.878   4.750 4.752 1.277 1.287 0.312
19 20 5 3 3/8   4.998 5.000 2.010 2.020   5.000 5.002 1.385 1.395 0.312


Preferred Key Dimensions and Lengths

The following table gives the nominal key dimensions, dimensional limits and preferred lengths listed on the original RoyMech BS 46 Part 1 reference.

All dimensions are in inches.

BS 46 Part 1 parallel rectangular key dimensions

Width Height Width W Height T Preferred Length
W T Max Min Max Min From To
5/16 1/4 0.314 0.312 0.253 0.250 3/4 3
3/8 1/4 0.377 0.375 0.253 0.250 1 4
7/16 5/16 0.440 0.438 0.315 0.312 1 1/2 4
1/2 5/16 0.502 0.500 0.315 0.312 1 3/4 5
5/8 7/16 0.627 0.625 0.441 0.438 2 1/2 5
3/4 1/2 0.752 0.750 0.503 0.500 3 6
7/8 5/8 0.877 0.875 0.629 0.625 3 1/2 6
1 3/4 1.003 1.000 0.754 0.750    
1 1/4 7/8 1.253 1.250 0.879 0.875    
1 1/2 1 1.504 1.500 1.006 1.000    
1 3/4 1 1/4 1.754 1.750 1.256 1.250    
2 1 3/8 2.005 2.000 1.381 1.375    
2 1/4 1 1/2 2.255 2.250 1.506 1.500    
2 1/2 1 5/8 2.505 2.500 1.631 1.625    
2 3/4 1 7/8 2.755 2.750 1.881 1.875    
3 2 3.006 3.000 2.008 2.000    
3 1/4 2 1/8 3.256 3.250 2.133 2.125    
3 1/2 2 3/8 3.506 3.500 2.383 2.375    
3 3/4 2 1/2 3.756 3.750 2.508 2.500    
4 2 5/8 4.008 4.000 2.633 2.625    
4 1/4 2 7/8 4.258 4.250 2.883 2.875    
4 1/2 3 4.508 4.500 3.010 3.000    
4 3/4 3 1/8 4.758 4.750 3.135 3.125    
5 3 3/8 5.008 5.000 3.385 3.375    

Using Legacy BS 46 Key Dimensions

These imperial dimensions remain useful when maintaining older machinery, interpreting historic engineering drawings or manufacturing replacement components for equipment originally designed to BS 46.

Do not assume that an existing imperial key should be replaced by the nearest metric key solely on nominal size. The shaft and hub keyway dimensions, tolerances, key height, engagement and required torque capacity should all be checked.

Where a new shaft-to-hub connection is being designed, current standards and current engineering requirements should normally be considered rather than specifying the withdrawn BS 46 standard simply because the dimensions are available here.

Related Key and Spline Information