The "Killer Dowel Pin", or KDP, is a concern on 1989 to 1998.5 12 valve Cummins engines. A steel dowel pin is pressed into the cast iron block during assembly. The pin properly aligns the aluminum timing gear housing to the engine block during assembly. Over a period of time, vibration can cause this pin to back out entirely. When this occurs, there are basically 3 possibilities. One, the pin can fall out, bounce off the cam gears and end up in the bottom of the timing gear housing without causing any damage. Two, the pin can dislodge and be forced against the aluminum timing gear housing, cracking it before exiting or falling to the bottom of the case. And three, the worst case scenario, the pin vibrates loose, becomes lodged between the timing gears, and causes catastrophic failure. In this instance, the pistons slap into the valves and your engine literally demolishes itself.
The physical fix for KDP is simply a small tab that locks the pin in the block, therefore preventing it from backing out. The parts are inexpensive, but the procedure is quite labor intensive as many components must be removed to access the timing gear housing. Many argue that only a small percentage of pins will actually back out, but given the nature of the damage that it can cause, $100 and an afternoon of labor seems well worth the effort. The images below show the dowel pin location and what a typical KDP kit looks like installed. While there are many options available on the market, one popular example is BD Diesel's KDP Repair Kit.