Replacing the upper timing chain rails will require that you have the proper pin puller. A source for this tool is Performance Products4benz.com.
The next step is to place the engine on top dead center and aligh the timing marks on the cams and crankshaft pulley. Disconnect the battery . Next remove the power steering pump complete with mounting bracket and the alternator complete with mounting bracket. Using plastic ties affix the timing chain to the timing gears. Back off on the big cap nut on the timing chain tensioner but do not screw it all the way off.Stuff a rag into the cavities of each cylinder head to make sure that you do not drop anything into the engine. Remove the two timing gears off of the cams being careful of the washer on each cam. Next thread the pin puller into the chain guide pin and pull the pins out freeing the old rails. Change one rail at a time to make sure that you don't mix them.The pin puller threads are locater under the alternator and p/s mounts on the front faces of the cylinder heads. After the new rails are installed replace the timing gears back onto the cams starting on the drivers side so that all slack is pulled out of the chain on the driver's cam from the crank gear.Ditto from the left gear to the right gear.The gears should slide onto the camshaft keys without moving the cams. Check timing marks and rotate engine with socket wrench and recheck timing marks. If the chain is too tight to get on compress the right side tensioner rail with a large screwdriver.