I have a 1977 Maxi with the bicycle style handlebar set up. That is it has a handlebar welded to a stem and quill. A bolt goes down through the quill and threads to a wedge nut. When tightened the wedge nut jams against the quill and the downtube holding the handlebar in place, like a bicycle.
The set up on newer maxis has the handlebar clamped to the top plate of the triple tree.
In the bicycle set up, The quill is 21.1mm. This size was used on old bmx bikes and old schwinns. Stems and quills (called goosenecks by some) can be found on ebay and at some bicycle shops.
The bolt and wedge nut on the Puch has M8×1 threads. The bolt is made of steel, the wedge nut is a non magnetic alloy.
To free the handlebars you loosen the bolt and the strike the bolt with a rubber mallet. This causes the wedge nut to release and allows you adjust the handlebar height and angle.
Once adjuseted you tighten the bolt a reasonable amount and the whole thing locks up.
DO NOT torque the snot out of the bolt. Remember the wedge nut is made of an alloy. If you torque the snot out of it it will get tighter and tighter until the threads in the wedge nut fail and it gets looser. Then it sucks to be you and you will not be riding for a while.
So what then?
One solution would be to get a replacement wedge nut. This should be possible, but you need to find one with a similar angle and threads. Replacement Puch wedge nuts do not seem to be available.
Another solution would be to get a handlebar, clamps and the triple tree top plate from a newer Puch and change over to this system. Sometimes Treats has all of the required bits, but not this past week.
A third solution is to repair the wedge nut threads. I did this. I found an M8×1 thread repair kit on ebay and bought it. The one I found was a Kato Kit. (There are other kit manufacturers. A popular one is Heli Coil.) M8×1 is less common that M8×1.25 and is a bit harder to find.
I drilled the wedge nut with 5/16 drill. (The kit called for a 9/64 drill. I had a 5/16 bit and it’s just a 64th smaller.) The kit comes with a special tap. The wedge nut was easy to tap even with the smaller hole than recommended. Since these are fine threads, the installer prewinds the repair coil. The prewinder did not fit the top of the wedge nut. The top is cut at an angle with a counter sunk hole too small for the prewinder to fit in. I installed the coil through the bottom of the wedge nut. The coil has a tang to hold it to the installation tool. After installation the tang breaks off easily with a punch. The long bolt threads through the top just fine. The Kato Kit coil has one coil that is a bit out of round to act as a locking device.
Repairing the wedge nut threads with a coil insert seems to be a good repair.