W116

I recently replaced my exhaust manifold on my 617.951 diesel and couldn't find new copper locking nuts anywhere so I reused the old ones with red thread lock. It's been about five months since then and I noticed a little oil around the front of the gasket, I checked the front nut that I can get to and it was loose enough that I could turn it with my fingers.
My question is, where can I find new locking nuts and what is correct torque for these nuts? My Chilton manual said 18-21 ft lbs but that doesn't seem very tight. Thanks for your help