1). The Engine.


The engine utilises five rolling-element (ball) bearings, all of the single-row deep-groove type:-

* Two rear-main bearings

* One front-main bearing

* One crankshaft ‘nose’ bearing

* One (front) camshaft bearing.

It seems likely that originally, these were all of the “full complement” type (crowded balls with no cage), to provide greater load-capacity, but probably with shorter life. They can all be satisfactorily replaced with modern caged bearings.

All bearing sizes are still current, (a century after the last Sopwith was assembled!) except one. This is the front main bearing, originally of (¾” x 1 7/8” x 3/8”) size.

The best recourse here is to fit a short sleeve (¾” x 7/8” x ½”) to this part of the crankshaft, and use a bearing of the rear-main size.

The front main bearing is the one that resists the forward thrusts of the clutch release-mechanism, during gear changes. This situation is not ideal, but such thrusts are not high, nor continuous, so that a deep-groove bearing manages well enough.

                                                                                                                                                            Inner                                                    Outer                                             Width


Crankshaft Main Bearings                 (3-off: see above)                                                                       7/8”                                                     1 7/8”                                              3/8”

Crankshaft Nose                                (1-off)                                                                                         5/8”                                                     1 3/8”                                             9/32”

Camshaft Front                                  (1-off)                                                                                          ½”                                                      1 1/8”                                              ¼”


2). The Gearbox.


This unit has four rolling-element bearings, of which three were originally of the double-row, self-aligning type, but which can safely be replaced with bearings of the single-row deep-groove type.

The fourth is a compound, purpose-made device that runs with cylindrical rollers, and is definitely not commercially obtainable. However, they all lead cushy, well-lubricated lives, and are unlikely to need replacement. Just for the record, then:-


Mainshaft Rear                                  (1-off)                                                                                         7/8”                                                        2”                                                 9/16”

Layshaft                                             (2-off)                                                                                         5/8”                                                     1 9/16”                                           7/16”


- These are both currently-obtainable sizes, in both single- and double-row types.


3). The Bevel-Box.


This contains two rolling-element bearings, originally of the double-row, self-aligning type.

The smaller of the two can be safely replaced with a single-row deep-groove bearing. The larger one, adjacent to the large bevel gear, absorbs a degree of thrust, to say nothing of the full chain-loads, and should preferably be replaced by a bearing of the original type.


Small Bearing (RHS)                           (1-off)                                                                                           ½”                                                  1 5/16”                                               3/8”

Large Bearing (LHS)                           (1-off)                                                                                          7/8”                                                    2”                                                    9/16”


These are both currently-obtainable sizes, in both single- and double-row types.