You have to be careful when you put the torque converter into the transmission. Guide the converter over the input shaft, but don’t let its full weight lay on the shaft. The reason for this is that the O-ring (see picture below) seals the input shaft to the converter, and could be damaged during installation. You must always replace the O-ring when the converter is removed. OE manufacturers describe in the repair manual that when handling the converter, you must use a handle that can be screwed into the converter. This helps to guide and control the converter over the splines and O-ring.
The next point refers to the driveshaft. The driveshaft balance can influence the driving behavior of the car. For this reason, mark the position of the shaft to the differential and bring it back to the same position when reconnecting them. This is important for the balance of the shaft. Some vehicle manufacturers provide markings you can use as a guide. 
Check the guibo (flex disc) for wear. ZF recommends changing the guibo when you replace the transmission. In the picture below, you can see wear between the rubber and the metal part that guides the bolts.
This wear can cause a second to first shift bump, which is often associated with a transmission error.
The last point to consider is flashing. In order for the transmission to operate efficiently, it is necessary that the TCU has the latest available software from the vehicle manufacturer.
Courtesy of ZF Services LLC
