I have a 1984 380SL and when started cold at times the chain will clatter until oil pressure is up. I was wondering if the chain tensioner can be replaced with out opening the eng. If so are there any cautions to take

You can replace the tensioner without removing much more than the alternator. Maybe not even that. But I doubt that's the problem. Check the archives.

Sixto
95 S420
87 300SDL

These V8 engines are known for the chain guides getting brittle and breaking. When this happens, the engine is likely to be severely damaged. For this reason, it is highly recommended that you pull both valve covers and thoroughly inspect everything, better yet, replace all the chain guide and roll in a new chain. You should get a manual, study up and then inspect closely and thoroughly. Your engine is at stake.

Good luck,