Across the production years, several distinct versions of the primary chaincase were used. Working from the parts manuals, we can identify several distinct versions.

Although the case itself changed over time, the part number didn’t always change with it. For example, the 1938 inner chaincase pictured below carries the same part number as the later T477. In this case the parts are largely interchangeable — the inner and outer cases still align — but the later version added the rear chain oil adjuster.

1937 Aluminium chaincase

This early model appears only in the 1937 replacement parts book. It’s identifiable by the knob covering the shock absorber, and by the absence of a screw for rear chain lubrication.

Inner chaincase (T116)

This version lacks a breather tube; the hole had to be plugged to keep the oil in.

Outer chaincase (T117A)

1938 Aluminium chaincase

This is the change to the droplet model. The inner chaincase also had the breather tube repositioned, maybe due to it being to close to the chain. This model still does not have the rear chain lubriacation screw.

Outer chaincase, coil ignition (T478)

1939 – 1940 Aluminium chaincase

From this year on, the parts manual specifies the addition of a lubrication screw for the rear chain. Note that the part number didn’t change, even though the screw was added.

Inner chaincase (T477)

A note on post-war T477 cases: if you’re sourcing a T477, be aware that this same part number was reused after the war for the inner chaincase. At first glance it looks correct, but there are two small differences: the engine hole is larger, and the case is thinner at the front. The engine itself appears to have been wider, so the spacer is absent on these later cases.

1939 – 1942 Outer chaincase (T478)

Includes the rear chain oil adjuster screw.

1937 – 1940 Steel chaincase

The Triumph 3SW had its own dedicated steel chaincase. We’ve only encountered this once, on a Triumph that was for sale.

Chaincase T365A and T365

Post-1942 Steel case

During the war, aluminium became too expensive to use for the chaincase, so from this point the 3HW switched to an all-steel case. That said, we’ve noticed that some early 3HW models still carry the aluminium chaincase, paired with a 3HW-specific steering damper knob.

Inner chaincase (T638)

Note: this particular inner chaincase has been modified — the mounting bracket for the battery box holder is missing, and the rear chain lubrication fitting has been altered.

Outer chaincase (T639)