Pretty much a drop in replacement. The hole for the stick to slide over the peg inside the controller is an exact fit as far as shape and width goes, but it is about 2 millimeters too shallow, so I had to take a hand file and go slowly until I had about 2mm shaved off the peg of the Dreamcast controller. It took a few test fits until it was freely able to move about. Now I have way more grip in games, and everything feels more accurate overall.
In the past I have done a similar mod with Xbox 360 sticks, but they never fit right for me without a lot of hacking needing to be done. These are the best fit by far, feels like a stock fit for sure. The 360 sticks used to get caught on the underside of the top part controller shell.
The exact ones I ordered were from here. https://www.ebay.com/itm/2pc-Replacemen ... 2749.l2649