Brake Booster Modification See Brake Booster section. There are several brake boosters that are commonly used with the M/S5x swap. E21 320i E10 2002 E30 325ix Porsche 924 Porsche 944 The 2002 and E21 320i booster both need to be modified before they can be used with the E30 chassis. The booster rods are too long. They must be shortened, and the E30 rod must be either welded on, or you must thread the rod. The thread size is M10x1.5. The 320i booster has a 12mm shaft, so it must be ground down in order to be threaded properly. I threaded my 320i booster with a 12x1.5 die first, then filed off the threads evenly. I then threaded it further with an M10x1.5 die. The Porsche 924 and 944 boosters have a 10 mm shaft, so they merely need to be shortened and threaded around an additional 1/2".