I have semi apart Garrett turbos from a 603.96 (I don't remember if this is a 124 or 126 turbo) and the turbo that came with the 93 SD.

The intake sides are visually different because the .971 doesn't have ARV. The .971 doesn't have a fitting for a boost signal to the wastegate since the wastegate is actuated by a vacuum signal modulated by an ECU (I'd sooner keep the EGR hooked up). ARV makes the compressor housing ~95mm deep for the .96 vs ~64mm deep for the .971 compressor housing. The compressor wheels and contour of the business surface of the compressor housings are different but I can't quantify the difference. What I can measure is the base diameter of the wheels = both are ~60mm, and the tip diameter of the wheels = .96 is ~42mm, .971 is ~44mm. The pics show how the .971 wheel drops into the .971 housing but interferes with the .96 housing.

Maybe someone like Brian, Dave or ForcedInduction can explain the technical and flow differences in the compressor wheel and contour of the compressor housing. No hablo turbo tech.

The exhaust sides look pretty much identical. The key difference is the .96 wastegate housing is ~51.5mm thick while the .971 wastegate housing is ~23.5mm thick. The actuators are positioned differently. It's hard to say why but probably because the .96 has that funky trap oxidizer manifold while the .971 has the sleeker Euro manifold (it is a sight to behold). I'm hoping to fit the pressure actuator to the .971 turbo. That might require some modifications since the .96 wastegate flap arm is ~21mm long while the .971 wastegate flap arm is ~40mm long (see pics).

Finally, the .96 shaft has a LH thread at the compressor end while the .971 shaft has a more common RH thread. The nut at the turbine end of the .96 shaft is an assymetrical hex (if it's even a hex after I got done with it) while the nut at the turbine end of the .971 shaft is a more common hex that fits nicely in a 14mm socket.

Sixto
93 300SD