As Donnie pointed out the lower cover must be removed to replace the rail in question. He pointed out that in his world that would add up to $300. What he failed to address was that he probably is charging 12 to 15 hours of that $75 hour to do the rest of the work. Anyone who is only charging $500 labor to do the original valve job doesn't live on the same block as the $300 timing cover replacement.
I think he is trying to get paid for the original job that he misquoted.
The rail in the 300 is not the same situation as the 500SEL. The chain rides on the 300E rail. It DOES NOT ride on the rails that cause problems in the V8. The rails in the V8 are there to guide the chain when it is untentioned. The problem is that they become brittle in 80-100K and during the occasional, unpreventable, untensioned slap, they fracture with pieces running through the works. It is the pieces rolling between the gears and the chain that jumps the timing in 90% of all of these failures.
Bosch Master, ASE Master, L1
33 years MB technician