Genuine Beru glow plugs are the only glow plugs you should ever use in your 6.9 or 7.3 IDI engine. Other brands are known to fail early, and when they do, their tips swell and if that happens, they can break off in the head! Don't make the odds of this high with an inferior glow plug. Motorcraft glow plugs are always Beru for this reason. Use only quality genuine OEM parts in this application. Sold individually, be sure to replace all 8 for optimum results. As one glow plug fails, the available current to the other glow plugs increases, potentially shortening glow time and/or glow plug life. Coat the threads with a little bit of nickel anit-sieze compound when installing. Don't over-torque. ZD1A glow plugs have the flat tab, for 6.9L wiring harnesses. ZD9 glow plugs use a bullet connector on the 7.3L IDI wiring harness. Buy these glow plugs from Moose Diesel, and receive a free troubleshooting guide with your purchase.