Cam Timing Question

Is it possible to change the timing by rotating the engine using a wrench on the nut that holds the cam sprocket on? Yesterday I did a leak down test on the cylinders to see if any of the valves were leaking. They all tested around 95%. Today I put the covers back on and when I started the car the valves were really clacking and the car will only go about 20mph. :(
I brought the sprockets around and the timing marks lined up, but I'm wondering if I could have turned the camshaft inside of the sprocket. Do the sprockets have a key way or a flat or something so they can only be put on the cam in one position?

If I did change the timing how do I go about getting the timing mark back to the right position on the camshaft?
I wanted to check the valves before going to the dealer for a crank sensor relearn to try an solve my PO300 code.
The car was running great except I can't pass inspection with that code.
Any help would be greatly appreciated.:D