The guibo, or flex disc, is a flexible rubber coupler connecting the transmission output shaft with the driveshaft. It acts to reduce vibration and transfer power from the drive train to the rear differential and wheels. If your guibo cracks or begins to disintegrate, you need to replace it or you might be stranded on the road. Inspect every year or so. Replacing the flex disc is a medium difficult job. Manual transmission cars will need a different replacement part.
Deep socket wrench set
1) Jack up the car and place it on stands.
2) Remove the muffler and catalytic converter. Do this by disconnecting the whole rear exhaust system from the forward flange and removing:
UNFASTEN SMALL EXHAUST RETAINER CLAMP
SPRAY SOME WD-40 ON THE SIX BOLTS FROM THE EXHAUST FLANGES AT THE FRONT OF THE TRANSMISSION. ON LATER CARS, MAKE SURE YOU REMOVE ANY OXYGEN SENSING WIRES, AND THEN REMOVE THE BOLTS.
UNDO REAR MUFFLER CLAMPS AND DROP THE ENTIRE ASSEMBLY. ITS HEAVY SO BE CAREFUL.
1) Remove the heat shield and the lower transmission support bar. If your transmission mount look worn, take the opportunity to replace them.
2) Mark the output flange and driveshaft with white marker so you know which flanges connect with each other.
3) Remove the six bolts from the guibo. Use your deep socket and open-ended wrenches. You’ll need to release the parking brake, take the transmission out of gear, then hand-rotate the driveshaft until you reach the bolts and nuts and re-engage the parking brake. You’ll have to do this a few times to get at all the bolts.
4) Remove driveshaft tunnel cross-brace at the rear. Release the rear driveshaft bearing by removing the two nuts as you support the driveshaft.
5)Remove driveshaft from the transmission output flange. You should support the driveshaft with a jack stand. On older cars, remove the center driveshaft at this point.
6) Remove the flex disc. Inspect and if necessary replenish you shift bushings.
7) Attach replacement guibo to driveshaft. Align arrow with mounting flanges.
8) Tighten the three driveshaft bolts, then move the rubber disc to the transmission flange and insert the three remaining bolts, aligning your white marks as you go. Slightly tighten the three bolts.
9) Reattach the rear driveshaft support bearing, pushing it forward as you tighten.
10) Tighten the remaining bolts.
11) Reattach everything: heat shield, exhaust system, and, if necessary, oxygen sensors.
You can also find this DIY along with many others and more to come here: https://www.bimmerhq.com/blog/bmw-3s...o-replacement/