I check the valve clearance on my 71 250C every two oil changes (6000 miles). I use the "go-nogo" method. Example, .003 fits, .004 will not fit. Since the gaps are "designed" to allow for thermal expansion to maintain the proper clearance, I don't believe any clearance mods are necessary. Perhaps some of the MB techs have a better handle on this one. I, too, get a light tapping when warm. My engine has 130K and the chain/tensioners are good so I have never felt there to be a problem for me.
BTW: I also run a bottle of Redline Total Fuel System stuff every two oil changes also.
Don't know for sure why I really do that - I think it just feels good (or something).
Anyhow, based on my experience (and my 250C is a "daily driver" - except in foul weather) I don't think you have a problem. However, as I stated earlier, perhaps some of the MB techs with all of the "good knowledge" have a better handle on this.
Enjoy the car
Dan Taylor/ Tulsa, OK MBCA '71 250C/'81 300TD-T
[This message has been edited by DANTRCAV (edited 06-01-2000).]