In a previous post James wrote...
Summary of Questions:

1. Am I planning the correct pilot hole sizes ?


The pilot hole should be no more than 60% of the lag shank.

For 3/8" (0.375") this works out to 0.225", or 7/32".

2. For drilling through 6 inch timbers, what type of drill bits are better,
the auger type, or spade ?

Either one works, but you will get a cleaner hole with an auger type bit.

3. Is the use of the spade bit the correct type of bit for the
countersinking ?

Again, you can use either one, but the hole will be cleaner with an auger
type bit. You might even consider using a 1-inch hole saw to cut a
groove and then use your spade bit to clean out the hole.

The countersink hole should be just slightly larger in diameter than the
washer under the head of the lag -- you are using washers aren't you?

--
Bob Morrison, PE, SE
R L Morrison Engineering Co
Structural & Civil Engineering
Poulsbo WA
bob at rlmorrisonengr dot com