There is always a first time. I have no other explaination for the chain to break. Either the chain just failed, or the rail broke causing the chain to fail. It may have slipped a couple of teeth until there was valve interference, causing the chain to break. The chain ends look as if it was pullled apart. The engine was not under a great deal of load, it was not accelerating but at highway speed. It was not the valve train or the tensioner, they were OK.
I did forget about the double row chain. I thought I had read posts here that seemed to indicate that the double row chain was no more reliable that the single. I would have thought that the double row is probably over-engineered. To me that would indicate that it is not the chain itself usually causing the failure but another source. i.e. tensioner, sprocket wear, rails, guides...
Thanks for setting me straight.
'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)