BMW used two different fuel pumps in the K75/K100/K1100, the 52mm diameter pump prior to 1/93, and the 43mm diameter pump after 1/93. The third common pump used in a K100/75 is a 37mm aftermarket pump that is common in some autos.

I prefer to change over any 52mm K that I work on, that needs a pump, to the 37mm aftermarket pumps for 3 reasons:

1. The 52mm pump uses a VERY fragile cup style inlet filter. They tear very easily, especially when removing them from the bracket in the tank. Any tear will allow grit into the pump which will seize it. The cup, which is snapped over the bottom of the vibration damper will sometimes deform when the vibration damper swells, making it almost impossible to get out of the pump bracket without destroying the filter. This is probably one reason BMW replaced these pump with the 43mm ones that have a robust sock filter on the inlet.

2. The 37mm pumps are much less expensive. So much so that I carry a spare (for me or people I ride with. Haven't needed it yet.) They take up about half the room that a 52mm pump does.

3. If you have problems on the road, most car parts places will stock a suitable 37mm pump, and if you are already adapted for the 37mm pump, replacement will be minutes once you get a new pump.

I use Euromotoelectrics FP-231KitPlus:
It includes a fuel resistant rubber sleeve to adapt from the 52mm damper to the 37mm pump.

In 10 years and probably a dozen pumps installed, I have not heard of any problems. On the other hand, the two used 52mm pumps that I installed on my bike each failed within a year. YMMV

The aftermarket pumps put out less ultimate pressure than the 52mm pumps (mid 40s vs almost 100psi) but as long as its over the regulator setting (about 38psi) it works fine.

To remove the old pump (any size):

Cover the tanks paint! (I had my wife take an old bath towel and cut a hole in the middle of it the size of the filler cap and then hem the edge of the hole).

Remove filler cap assembly from tank (4 screws under cap).

If you have a 52mm pump and it is not an '85 K100, there will be a 5/16" O.D. thin wall rubber hose that runs from the pump to an aluminum tube at the top of the tank just to the rear of the filler opening. Remove the hose from the tube.

Loosen hose clamp on short (2") hose, that is connected to fuel filter, on the end of the hose AWAY from the filter.

Remove that hose from tube in tank.

Pull that hose and the fuel filter straight up out of the tank. The filter will still be connected to the fuel pump by a hose about a foot long.

Plug the end of the hose with a 5/16" bolt and tighten the clamp.

While putting some upward tension on the fuel filter hose, reach into the tank with your other hand and pinch the two clips on the plastic pump mount ring towards the pump. There is one clip to the front and one clip to the rear of the pump and the are about 3/4" wide. Do one at a time with a slight rocking up on the tab you are pinching until they are both released. Again use some tension on the filter hose to help.

Once the clips are both released try lifting the pump, plastic ring, and vibration damper up out of the hole in the shelf of the tank as straight up as possible. Use the hose to pull (CAREFULLY) and your other hand inside the tank to guide. If it is a 43 or 37mm pump with the sock filter, the sock runs fore and aft with the longer end to the rear, so once the sock hits the underside of the shelf, rock it slightly to the rear and lift out.

Once the pump is clear of the shelf, lift the entire assembly straight up and out of the tank using the filter hose.

The wires are plenty long enough that you can remove the wires from the pump with the pump outside of the tank. On pumps with studs and nuts for the wire ends; large stud is ground (brown wire) small stud is +.

Check vibration damper (rubber surround that holds the pump) for deterioration. It should have the consistancy of a tire. If it is very soft or gooey, replace it with the newer alcohol resistant ones from Euromotoelectrics or Beemerboneyard.

Check for debris under the pump mounting shelf (where the pump would suck it in).