Some Interesting info found on Thumper Talk
1. 80 and 81 XT's had electronic ignition cdi (US Models)
2. All SR's were 12volt and had electronic ignition
3. All TT's were point motors. Pointless ignitions fit into points cases
4. Bigger intake valves started from 79 TT500 onwards and 80 XT500 onwards... I think the SR's may all have the bigger valves (2J2)
5. 1976 motors have no kick indicator window on the cam all others do
6. The head fins on the earlier TT/XT500's were not as wide as the later models
7. No tach drive on the TT500 Head
8. XT500 carb was 32mm, TT500 34mm
9. The aforementioned big intake valves (2J2) were in the big fin heads. That means they came on all SRs, late TTs and XTs. 2J2 intake valves can be installed in early TT/XT heads by recutting and reaming valve seat.
10. The SRs have a heavier crank which is larger but will fit in the TT/XT cases with some modification to the cases. TT/XT cranks fit into SR cases without mods. The heavy SR crank is great for torque motors and the lighter TT/XT crank is great for quick revving motors.
11. SRs have needle roller shift drum and different profile shift drum detent wheel. This means the SRs shift sweeter and don't miss shifts as easily.
12. SRs have a heavier duty clutch basket. Not sure but I think some of the late XTs also have this.
13. SRs use a 530 chain. TT/XTs use 520. You can swap one to the other if you use the appropriate sprocket spacer behind the sprocket
14. Early 500s have 3 bolt oil pump. Lates have 5 bolt oil pump
15. Actually the SR engine is my first choice because it has the better shift system and tougher clutch basket. I also like how the heavier crank works with a stroker kit and big bore piston if cammed appropriately. When a good port job and a megacycle 251-80 cam are mated with this combination you'll have a 605cc rocket without the stresses involved with high RPM builds. The heavy crank does an awesome job of controlling wheelspin. It acts similar to having a moose flywheel weight added to a WR... better traction means better lap times and more control.