Contrary to what I have read to this point, the 400E - 4-POT calipers are a
DIRECT SWAP onto the 300E (W124) 124.030 chassis ranges from 1986 - 1992.
They are also a direct swap onto the 190E 2.3-16, as I have done the swap to that chassis 3 times thus far. It will work on ANY (W201) that utilizes ABS, which is mid year 1985. By direct swap, meaning they will bolt right on to the carrier or knuckle if you will. The rotor & pads need to be upgraded & that is pretty obvious, wouldn't you all agree...?
Once you pull the 19mm bolts from the back of the caliper & slam the new calipers on you will see exactly what I mean. Yes, you can use your original caliper bolts from either the (W201) or (W124), just remember to use a bit of
loc tite
Clearance of the dust shield on the lower portion where the ball-joint is on the control arm may be your only issue. You can upgrade to the 400E/E320/E420 dust shield or just cut away a small un-noticeable portion. This same caliper is used on the later 1994 - 1995 E320 (W124.052) & fits with no problem.
But to say or even
assume that the calipers are not a direct fit for ANY (W124) chassis is ridiculous. The only brake modification that requires any sort of knuckle/hub/carrier modifications are the installation of the 500SL/600SL (R129) chassis brake. Which you need to swap the knuckle/spindle & the hub assembly.