This jig is designed to be used with a router equipped with a 5/8" O. D. Porter Cable style guide bushing and a 1/2" Diameter mortising bit.
 

The lock bolt mortise jig is used to locate the mortise for the lock bolt cover plate on the edge of a heavy wooden door.  Begin by cutting a 10-1/2" by 10-1/2" piece of 3/4" maple or birch plywood as a base for the jig.

To determine the size of the hole in the jig, carefully measure the height and width of the plate that covers the lock bolt.  (Mine measured 1" x 2-1/4")  Then add 1/8" to each dimension to obtain the dimensions of the hole.  e.g.  1" + 1/8" = 1-1/8" width and 2-1/4" + 1/8" = 2-3/8" height.  Make two router guide blocks (A) about 8" long and about half the width of your router base plate from scrap 3/4" thick lumber.  Make two more (B) about 3" long and exactly the width of your cover plate +1/4"  e.g.  1" + 1/4" = 1-1/4" (Because we are making a jig to make a jig, we need to allow another 1/8").  We will use these four blocks to precisely locate the position of the hole on the jigs base plate.
 

Place the router guide blocks on the lower face of the jig base, as shown in figure 1. Then fasten them temporarily using double face carpet tape.  The tall block is used as a temporary spacer to position the two narrow blocks at a distance of the length of the lock bolt cover plate plus 1/4".  e.g. 2-1/4" + 1/4" = 2-1/2" (Because we are making a jig to make a jig, we need to allow another 1/8")

Remove the temporary spacer. Then fit a 5/8" guide bushing to your router. Using a 1/2" mortising bit route the hole completely through the jig base.  DO NOT REMOVE THE GUIDE BLOCKS YET!
 

Fig. 1