First remove the belt pulley from the inner crankshaft pulley. You will see 2 cutouts in the inner pulley at 180 degrees. With the engine turned to the correct position, a 13MM deep socket can be wedged between one of the cutouts and one of the 6MM allen head bolt heads which hold the oil pan on at the front of the block, It will be necessary to turn the engine backwards only slightly to lock the socket in place after which you can really pull on the center crank bolt. Works in reverse for retightening also.
JK - ASE Certified Master
Technician, Mercedes-Benz specialist