That serrated wiper shaft and nut holding the arm to it is part of the
Teutonic pledge of difficulty.
It is so the arm is adjustable.
A hex shaft or square shaft to lock the arm down would have been too
easy.
Most all vehicles use this type of serrated system with a more
efficient, simple locking devise.
Take a 10mm flat washer's and put one under each nut.
Tighten them up.
The arm's will stay put for some time.

RC

Subject: Re: windshield wiper help
How come they don't have a normal nut and bolt on the windshield wiper,
instead of that very finely serrated post?
Joy