More than once in various topics I have admonished builders to use the correct size pilot hole when fastening with lag screws. Nobody's ever asked what the proper size is; I assume you've all looked it up. Anyhow here's a little list of proper size pilot holes for both hard wood and soft wood. Along with this is a notation to also be sure to drill a larger hole for the shank of the lag screw. (some flks call lag screws lag bolts; same thing, different name)
The shank size hole should be drilled completely through the piece of wood being attached to the larger piece of wood. The reason is if the screw threads are biting into the board to be attached, the screw threads can not pull the two pieces together. The total depth of the shank hole should be equivalent to the length of the shank portion of the lag screw.
The size of the pilot holes is somewhat dependent on the species of lumber and its age and because of that, some experimentation with the correct pilot holes for your project might be undertaken.
However, there is a rule of thumb that can be used. The pilot hole for the shank should be the same as the diameter of the bolt and the pilot hole for the threaded portion should be 3/4 of the diameter for softwoods and slightly larger for larger diameter bolts used in hardwoods as shown in Table 1.
Some suggest the use of soap on the end of the threaded portion of the lag bolt in order to provide some lubrication to the threads. This is definitely not
a good idea, as soap will cause a steel lag bolt to rust prematurely, although it will have no effect on stainless steel or galvanized lag bolts.
If you would like to use a lubricant, bees wax is the best choice. If you do not have bees wax readily available, use some vegetable oil.
FWIW, I prefer to drill the pilot hole full depth required first using a auger type wood bit.
Then I drill the pilot hole out to the shank size using a regular twist bit. I have a set of stop collars that can be fitted to the bit to easily set the drill depth. I also use masking tape wrapped around the bit as a visual assist when using the auger bits.
Here's a chart for regular wood screws
, sizes 0 to 20, with common sizes in color...
To prevent brass screw heads from twisting off in hardwood use a steel screw of the same gauge to thread the wood, remove and then insert the brass screw. As noted above use wax rather than soap for a lubricant if you desire.