The contra piece is very tight on my engine as well. At first I thought it was the threads, but it is the cylindrical section below the threads that fits into the head. The fit there is extremely tight, gets tighter the farther you screw the contra in. Make sure to find the very best fitting allen key when adjusting the contra. If need be, you can unscrew the thing and gently relieve the fit by various methods. Perform at your own risk, of course.

Once set in a good position, the contra should need little or no adjustment in the future.