Theres a small gasket. I'd check to make sure the tensioner is torqued down to spec before replacing it though. No point in fixing something that isn't broken.
If it needs replacing;
1. Remove the plug (8mm or 10mm i think....cant remember)
2. using a small screwdriver, back the tensioner into itself, it must be full retracted or it will just release. Don't remove the tensioner until it is full retracted and stays that way, its far easier than doing this with it off the bike.
3. remove the tensioner.
4. replace the gasket.
5. reinstall (torque to spec)
6. tap the tensioner with the butt of a screwdriver till it releases.
7. reinstall plug (torque to spec)
V-Star 1100 Silverado