Now that I am into my 70s, my fingers are not as nimble as they were, and locating a replacement nipple into the hole in a small roarer burner is a real pain. There are various methods of doing this, for example using a guitar string pushed in the jet hole, or using a flexible plastic tube, but here is a method that I find easiest. Not only does it allow my arthritic fingers to locate the nipple in the hole, but it also helps (me) to start the threading. It may have been reported before, but if you want to try it all you need is a steel nut and a telescopic magnet. The nut should fit between the tubes in the burner and have a threaded hole slightly smaller in diameter than the long sides of the nipple head, (M10 works well for most nipples). File two flats at opposite sides of the hole until the nipple head is a push fit, and the nipple will not fall out or turn in the hole. Push the nipple into the prepared hole in the nut head first, and using the magnet locate it in the hole. Put a finger on top of the nut and pull off the magnet, and then thread as much as you can with your fingers. I find the six sided nut much easier to turn with fingers than the small nipple head. When it is finger tight, lift off the nut with the magnet (or flick off), and tighten down the nipple as normal. This is now my preferred method. I think it is easy to do and reduces the chance of a cross threaded result.