Quick question re: replacing 420SEL cam chain
 

I'm going to be pulling the engine and transmission from my 420SEL in order to replace the transmission's front seal. Should I replace the engine's cam chain and guides while the engine is out of the car, or is there any reason why the job is easier to do with the engine in the car?

it really depends on your method of replacing the chain, most technicians including myself replace the upper guide rails and then roll in the new chain by connecting the new one to it and turning the engine by hand. i would say this will be easier done in the car, but would also be easy with the engine out and attached to am engine stand.

if you are going to remove the front engine cover and replace it like that with all the rails, it would be easier with the engine out and also mounted to an engine stand. this method is the most invasive and is quite involved

but to replace the front seal on a transmission the engine does not have to come out, i have done many by simply pulling the transmission, if you go to the extent of pulling the trans you should also at minimum also replace the o-ring that goes around the front pump, this has been known to leak more often than the actual radial seal.