John -

I installed the new tensioner before commencing work on the driver's side; my concern was that all that loose chain flopping about might skip a cog and I would miss it. I suppose the fail safe is to mark the chain and cam gear on the passenger side with white paint to keep things properly referenced in the event this happens, should you prefer to leave the tensioner out. The 117 engine (yours is a 116 but pretty similar if my wife's 380SLC is anything to go by) factory manual is silent on this.

The increased chain tension from the new tensioner was not an impediment in practice as I found turning the driver's side cam a tad to secure alignment of the Woodruff key with the slot in the gear when replacing the latter needed little force using an 'English' wrench (the adjustable bodger's special).

One last thing - do check the part numbers before ordering as I am unsure whether the chain rails in the 380 are the same parts as those in the 560. My total parts cost was under $125. Labor was free but my Border Terrier charged me 2 cookies in exchange for moral support.

A beagle should work fine!