Between the years there are a few models of primary chain cases used. Using the parts manual as reference we can see 4 distinct versions, 2 aluminium and 2 steel.
Although there are differences in the model itself the part number used might not change, f.i. the 1937 inner chain case pictured below carries the part number of the later model T477. In this use case the parts are somewhat interchangeable, the inner and outer case will align however the later version had the addition of the rear chain oil adjuster.
1937 Aluminium chain case
This early model is only mentioned in the 1937 replacement parts book. This is the model that has the a knob to cover the shock absorber and is also missing the screw for the rear chain lubrication.
Inner chain case (T116)
This one is missing the breather tuber and had to plug the hole to keep the oil in.
Outer chain case (T117A)
1938 - 1940 Aluminium chain case
This is the most commonly known model. Next to the change in model there are a few other changes. This one has the addition of the lubrication screw for the rear chain. The inner chain case also had the breather tube repositioned, maybe due to it being to close to the chain.
Inner chain case (T477)
Note Post War T477: If you are looking for a T477 please be aware.
After the war they also used this part number for the inner chain case. At first glance it looks correct, however there are 2 minor differences. First is the that the hole for the engine is bigger, the other is that the chain case is thinner in the front. The engine seemed to be wider and therefore the spacer is missing.
Outer chain case (T478)
1937 - 1940 Steel chain case
The Triumph 3SE has a specific model of steel chain case. We have only seen this one time one a Triumph that was for sale.
(3SE, T365A and T365)
> 1942 Steel case
During the war aluminium became to expensive to be used for the chain case. So for the 3HW they switch to a complete steel chain case. However we have noticed overs the year that the early 3HW's still had the aluminum chain case fitted, along with a 3HW specific steering damper knob (information about this will be posted later).
Inner chain case
Note: this inner chain case is somewhat modified. The mounting bracket for the battery box holder is missing and the rear chain lubrication part has been modified.
Outer chain case