James wrote:
I am building an outside shed, using large timbers and lag screws of two
sizes, 3/8 inch and some 1/2 inch. In some places, I want to
counter-sink the lag screws. I want to see if I am on the right track.

For the 3/8 inch lag screws, I plan to use a 1/4 auger drill bit for a
pilot hole. Would that be the right size ? To countersink the head of
the screw, I plan to use a 1 inch spade drill bit.


For the 1/2 inch screws, I plan to use a 3/8 auger drill bit for the pilot
hole. Again, is this the ideal size ? To countersink the screw head, I
would use a 1 1/2 inch spade drill bit.

I will be using washers, so the countersunk holes must accomodate the size
of the washers.



Summary of Questions:

1. Am I planning the correct pilot hole sizes ?

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

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



I know these are rather "green" questions, but I learn from this group.

Thanks !!

--James--

The holes you are suggesting are just a little too big. The pilot hole
size should be a function of the wood species as well. That said,
since you're most likely building with a softwood I would cheat
sligthly smaller.

I usually measure the root diameter of lag & go just slightly smaller
(in softwoods) . This way, I get a lttle bit of compression of the
wood & it makes for a nice tght assembly. Hardwoods I go the other
way; esp oak!

I want to have as much thread as possiblw w/o danger of splitting the
wood or breaking the lag.


A slight 'nit' about the OP's teminology........

a countersink is a tapered feature desinged to accept the tapered head
of a flat head bolt or screw.......... usually 82 deg or 100 deg or
other

the feature the OP is creating; a flat bottomed hole is actually
refered to as a counterbore

cheers
Bob