The newer Zhumell Dobsonian models come with a roller bearing setup for the azimuth axis. This roller bearing setup is comprised of the top and bottom sections of a lazy susan, with two bearing plates and the bearing retainer between them. Although this gives an incredibly smooth azimuth motion, some people, including myself, find this too smooth. Sometimes just a slight breeze can blow the telescope off target; a real pain at high magnification. This mod is a two-fold mod, however. While I had the base apart, I also replaced the problematic center bolt. The center bolt is held on by a nylon lock-nut, but it doesn't seem to lock very well. This is how I fixed both.
This picture shows the disassembled turntable base. To add a little friction, and stiffen up the azimuth axis, I added four 2" long pieces of Velcro to the bottom half of the turntable. The Velcro was not tall enough to touch the top half of the turntable when assembled, so I simply stuck two pieces on top of each other, a sort of Velcro sandwich.
This picture shows both the old center pin setup and the new parts, which I bought at Lowe's. The bolt in the old setup would frequently come loose after a night of observing, and when I went to pick up the base, the two halves would come apart, sending the bearing plates flying. Make sure to take the old setup with you so you can get the right size pins!
Insert the new pin just like the old bolt, with the stock washers installed in the same manner. Then simply put the retainer pin through and you're done! No more loose nuts!
Here you can see, from the bottom, the new retaining pin. Putting the pin on the bottom is nice, because from the top, all you can see is a round silver button where the bolt used to be.