OK, I realize that this suggestion is far-fetched, but Mikael's last report has bothered me and I am really stretching for a reasonable explanation.
Assuming that there are no offset keys in the cam(s).
All I can come up with is that the distributor drive gear may be off a tooth. This is the gear right below the left cylinder head. When it is installed, or the chain changed, it needs to be lined up to a mark in the crankcase. This is to allow the distributor to be set relative to #1 TDC. With the cams correctly set relative to TDC, which was a CHANGE from the way it was before, the ignition timing was changed relative to the valve opening and started firing with either the exhaust or intake valve open when the car was started.
The result is a lot of noise and a poorly running engine. Mikael sets the cam timing back and thus restores the ignition timing relative to the valves which makes the engine run better. By some miracle, in the current position, the valves are not hitting the pistons.
I am not suggesting that you pull the front cover off to check this, but take a look at where the firing point is with the cams and crank in closest agreement - the way you had it before.
All I can think of other than some wierdness like a wrong part somewhere in the valve train.
Offered almost apologetically.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe