One more thing to consider and it's a big one. The wobble box and ball joint need to be tight, with not a hint of slack anywhere. When all is working right, the knife slows and starts smoothly as it reverses direction. If there is just a little bit of slack anywhere, the knife will keep going as the linkage starts going the other way, That will result in a sudden stop for the knife and a sudden jerk on it as it reverses direction. That will break things and keep breaking them until everything is tightened up. I have welded several that have pulled the sickle apart near the head by cutting the knife so the head is put in a new spot, and taken the broken part and welding it at the end, where is has almost no stress. After one or two tries, I fixed the components in the knife drive and quit patching sickle whips.