I'm getting my rails done on Monday. I got the idler arm bushing replaced today and the shop owner showed me his 1987 420SEL. Beautiful car but it has a blown engine due to the plastic rail failing. The part of the rail that extends above the head broke off and jammed one of the cam gears. This caused the chain to skip or stop, I don't know which. A couple of valves broke off and severely gouged the cylinder walls and the pistons. He said that they can't be re-bored deep enough to repair and that putting in sleeves would be rather expensive. So, he is looking for a replacement engine for several thousand dollars. If my rails show that deep brown patina, I'll know it was time for the repair. If not, I will still rest easy knowing exactly what the inside of my engine looks like.