Both of those auctions show complete third members, so you would unbolt and remove your old one, and then replace it with these complete units. However, the one with the 3:91 is a sure grip, the other with the 3:23 is not. So, if your swapping the entire case assembly, it doesn't matter 489, 741 or 742. The 741 is the weakest of the three, but if your just a street cruiser and never go to the drag strip and use slicks, you probably would never have problems with it, even witha 440. If your changing just the ring and pinion (gears), then the case number definetly matters. If your just adding a sure grip to you existing carrier, it doesn't matter what case you have.
So we know for sure you have a 489 case. Have you figured out what gears you have? If you have a nice ratio in there now, all you would need to do is add the sure grip device.
For some clarification;
Axle housing- this is the large steel case that houses the axles and third member and sits on the leaf springs.
Gear case- the large iron piece that everything fits into that bolts to the axle housing. These are the 489. 741, 742 designations.
Third member-the complete assembly of case, gears, carrier and bearings.
Gears/ring and pinion- the actual gears that convert the longitudinal twist of the driveshaft to forward rotation that goes to the axles. These are the helical cut pieces that are case specific and do not interchange. Gears MUST designate what case they match, 741, 742 or 489 as the cases are all physically different in relation to the pinion shaft size.
Carrier-this is unit that the ring gear bolts to. It is either open, limited slip or spool in design. The open one is a large open case with some gears and pins, like the 3:23 auction posted above. Limited slip is more closed and contains the clutch and springs or cone assy, like the 3:91 auction above. Spool is a single solid piece that locks both axles together so they have NO differentiating action for wheel speed. These are a race only piece.
The pinion gears goes into the nose of the the gear case. The ring gear bolts to the carrier. The carrier mounts in the gear case. The whole assembly goes into the axle housing.
So, you have a few of choices of how to go form where your at. Buy a compelte third member, like those above, in a gear ratio of your choice and just swap them with your existing unit. You then have a spare third member. You can pull your existing unit and swap the gears and carrier to a ratio you like with the sure-grip. Or if you have a ratio you like now, just add the sure-grip carrier to it.
