I went with the Moroso 7/16"-20x2-7/8 on the front of my 521. Go with the .480 dia knurl. Part # 46150. I had to grind down the raised ridges on the knurl part flush to get them to seat all the way down. You can cut them down to whatever length suits you best. I went with these to hold my Titan 18's better since I used a spacer. I also have the ARP part # but you'll have to wait till tomorrow , I need to find the package. I'll try and add some pics also.