fad64b said:
I believe the upper arms and ball joints were the only change TR3 to TR4, the lower trunnions change in the TR4A.
I disagree. The trunnion change only altered the angle between the pin & the vertical link. But when the vertical link was tilted, the upper pivot point had to move as well. The change in the arms moved the pivot point (as well as mating to the new ball joint).
Hence both the arms and the trunnions changed at the same time, which was NOT at the TR3-TR4 transition, but early in TR4 production (CT6343/CT6389, depending on whether wire or disc wheels were fitted).
Just had a quick peek in my TR4 SPC, which confirmed the above and also reminded me that the steering lever also changed. If memory serves, the upper ball joint moved to the rear (creating the 3 degrees positive caster), and the steering lever got longer to retain the geometry. But I could have that backwards, and I'm too lazy to go check my own parts. The change points are also listed in the Moss catalog.
The TR4A did also get a redesigned trunnion (similar if not identical to the TR6 unit), but the later TR4 got the 'handed' trunnions at the same time as the upper A-arms changed. The later TR4 trunnions look almost identical to the earlier ones, except for the change in angle (which is visible if you look closely or compare sides).