Are you referring to the engine crossbar which hangs the engine in the chassis, or the engine console that mounts on the engine itself? Whatever the case, Walt's suggestion is spot-on.
If you really want a number, I found a tech tip in the 74-89 911 Parts Reference years ago that says slight misalignment of the engine and transmission can transfer more noise into the car. They suggest the crossbar should loosened at the console, then tightened to 40 Nm/29 ft-lbs. The console itself can be attached to the engine at that same torque setting for a typical mild strength M10 fastener which is 40 Nm/29 ft-lb. Here's a nice chart to use when you're not sure
http://www.metricmcc.com/catalog/Ch10/10-1027.pdf