Does the motor cross mount have to flex?
 

I am looking to reinforce my engine cross mount due to more hp and the fact that I saw a crack developing near the place where the motor mount bolts in. I know others reinforce the corners, but because of torsional vibration on the mount induced by the idler for my supercharge set up I want to add torsional stiffness. This would make my mount stiffer in the front to rear direction. Which leads to my original question:

Does the mount need to flex in the fore-aft direction?

Is your engine mount early car style up thru 88 or 964/993 style? What is your engine size?

It's common for the mount bar to tear at the ends. Boxing that area of the mount at the threaded tubes is a good idea. Even plain unmodified cars have problems with the mount tearing. If the bolt gets stuck in the threads due to corrosion, loosening the bolt with a lot of force can tear the mount.

I should have said, flex front-to-rear of the mount is normal. The mount has a lot of vertical stiffness and little horizontal stiffness. The horizontal flex is actually helpful in getting the engine to line-up with the chassis mounts when trying to bolt the engine into place. I think if you beef up the horizontal stiffness you may have to shim the mount where it connects to the engine console. Otherwise you'll have difficulty lining up the motor mount bolts.