how can i change the split drive shaft gaiter VW Mk2 1991 golf?

You've got two options: First (easy) is to purchase a split drive shaft gaiter from Car Parts Direct, cut the old one off, CLEAN, CLEAN, CLEAN, install gaiter, clamp and drive. I've used these before and they work well as long as you clean and keep the seam very clean. If not they leak quickly...

The other option is the better long term method that requires disassembly of drive shaft from the suspension/hub. This method is preferred if you've got the time, tools, and patience as is it far more reliable. Add to that there is no seam to fail causing a leak and subsequent cv failure...

Kiro
November 2006