It's pretty easy to make a replacement nipple. I've made them out of old bolts with the correct threads. Cut a piece of all thread from the bolt. put the all thread in your electric drill. Put drill in your vice and run on high speed, while grinding with your angle grinder to three diameters at the proper lengths. 1- threads, 2- flats for wrench, 3- nipple diameter. (You can figure the lengths). Fine tune with a file. drill the holes to your preferences. Most of us have a 1/16" bit. If you want a spark channel smaller than that, carefully, stop drilling just before breaking through to the larger interior hole and, with a sharpened finishing nail, tap it until it breaks through. I've had mild steel nipples last a long time, but you can use high strength bolts that will last much longer.