At unknown mileage I rebuilt the engine of my '76 450SL. It was running fine before but smoking a little. Rebuild included new timing chain, rails, timing gears, hydraulic compensators & one new cam. After rebuild, with about 5000 miles the valves on the left head bentand woodruff key sheared off on cam. I removed head and corrected the problem. After another 1000 miles the same thing happened, more bent valves on left head only with woodruff key sheared off again. Timing chain looks a little loose only at top idler gear between heads. Any ideas?? I'm getting more experienced at head removal than I ever wanted to be.