wouter,
The flybar shouldn't have any lateral play. It should be tight against the bearings in the head but not tight enough to cause any binding. There should be one of those tiny shim washers between the control arm on the flybar and the bearing on each side. Be sure to go over all the parts in the head to make sure they are tight. My flybar had one loose arm on it and it was off center about 1/4". The little block that keeps the upper swash oriented properly was also slightly loose and off time by about 45 degrees. I was fortunate as I had just built a fury where this timing was addressed. Had I not built the fury I wouldn't have noticed that it was wrong.