GM has had 3 different tensioner designs. The 1st design had a rubber O-­‐ring, had a larger tensioner piston, and was a 2 piece design. The 2nd remained the same except had no O-ring. The 3rd supersedes all previous tensioners, has no O-­‐ ring, has a smaller tensioner piston, and is one piece. This tensioner is shipped and installed deactivated and cannot be deactivated without disassembly. Tensioner is deactivated when the tensioner piston moves freely back and forth. Do not install activated.