Counter-sinking lag bolts

[James]

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 ?

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

2. 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--

[Bob Morrison]

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".

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

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?

[Brian V]

Make sure you do the counter-sunk holes first. Once you make the pilot holes it'll be too big for either a spade or auger tip and make a VERY sloppy hole, of centered as well. If it were me I'd use a forstner (some call em forsner's) bit for the counter-sunk followed by an auger for the pilot.

[Bobk207]

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

[Phil Scott]

works great if you like spending a lot money on bits... especially large diameter long ones.

[Bob Morrison]

Bob K is correct. Here's a quote from the National Design Specification for Wood Construction regarding the holes for Lag Screws:

"(a) The clearance hole for the shank shall have the same diameter as the shank and the depth of penetration as the length of unthreaded shank."

(b) The lead hole for the threaded portion shall have a diameter equal to

65% to 85% of the shank diameter in wood with G> 0.60, 60% to 75% in wood with 0.50

[James]

Thanks Bob for the good info !! You recommend 7/32 bit for the 3/8 lag screw. I have a 1/4 inch bit, which is only 1/32 of an inch larger. From your notes, I guess whether this would be too marginal depends on the species of wood. It will be pressured treated, and I "guess" it is Southern Yellow Pine. (My materials are in another location right now, 500 miles away) Either way, would the 1/32 likely make much of a difference ?

IF I am correct about the Yellow Pine species, what bit would you guess would be acceptable for the 1/2 inch lag screws ? Would the 3/8 inch bit be too big??

What is the difference between a forester (sp) bit and a "wood boring" bit ?

Thanks again guys !!!

--James--

[CC]

I agree with Brian. I used a 1 1/4 forstner bit to drill counter sink holes in 6X timbers that I used for my shed foundation. Make sure you drill the counter-sink holes first. If not it is really difficult to keep a large forstner bit centered with a hand drill. Use flat washers under the screw head. I used the same size washer as the hole drilled. Made a nice looking job and tightened down tight without any crushing of the wood fibers with the large washers CC

[Jordan]

But, not in that order! I used to have a job making timber children's playground equipment, and counterboring was standard procedure. The holes were further plugged with plastic caps. Drilling the hole before the counterbore I found is not good, pilot or no pilot. Unless of course you have some fancy guided counterboring drill (unecessary). Counterbore first, then drill for the bolt.

[Stormin Mormon]

Some grease injected into the bolt holes sure makes bolts go in easier.

[Jonny]

Do it backwards and save you some unneeded engineering aspects. Bore the larger hole 1/4" bigger than the intended washer first. Make yourself a countersink jig to maintain the same depth. Then drill the smaller hole to run the lag bolt in. I'd go with a 1/4" bit for the 3/8 bolt, and 5/16" for the 1/2" bolt. You don't need a particular bit type to make the hole, just be sure its long enough. You might clamp the timbers in place while drilling.

[mm]

If it is pressure treated, I think you have to use some sort of mask when drilling. I know you do when sawing. Arsenic, poison, lungs.

I may be missing something, but you know the coutersink hole is supposed to be in the top piece of wood, and the pilot hole is supposed to be in the other piece of wood.

You need another drill yet, bigger than the pilot drill, to drill through the top, the first piece of wood. There is no point in trying to force yourself all the way through two pieces of wood.

[James]

MM has made a good point. Yes, I do know that the countersunk hole is the top piece. And, I have also wondered if I should "force screw" the lag through the first piece, or just drill a hole big enough to allow it to "slip" through, and then allow the 2nd piece of wood to do the holding.

Maybe that is what one poster meant when he said I would need THREE bits. One pilot, one "shank" and one for the countersink. I didn't pick up on that when I first read it.

Do all of you concur, that the screw can go in rather loosely on the top piece, and the tight fit should only be on the second piece ??

You guys are very helpful , and full of experience. I appreciate the tips a lot, and I will use them.

--James--

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MM said:

I may be missing something, but you know the countersink hole is supposed to be in the top piece of wood, and the pilot hole is supposed to be in the other piece of wood.

You need another drill yet, bigger than the pilot drill, to drill through the top, the first piece of wood. There is no point in trying to force yourself all the way through two pieces of wood.

[Edwin Pawlowski]

Yes. Often, a washer under the head is used.

[J. Clarke]

You want the shank to be a tight fit, otherwise you don't get the full load bearing capacity.

A "Forstner" bit cuts a flat bottomed hole. A spade bit will also do that particular job, the Forstner is a bit neater, but more difficult to use hand-held. An auger bit will also do for the countersink but they're relatively hard to find and expensive.

[bitternut]

No one has mentioned one key item about your project. Since you said that the timbers are pressure treated I would assume they are the newer type that requires either hot dipped galvanized or stainless fasteners. I would also use the same material for the washers. The new pressure treated lumber eats regular metal fast. In my area most PT lumber is some type of pine from the south east part of the states. Running the screw threads on a bar of soap or wax ( beeswax the best) works very well for installing the screws also.

[James]

Yes, I am going to use hot dipped galvanized lagscrews and washers.

Thanks !!

--James--

[James]

Thanks for info on the Forstner bit. Looks like a real specialized bit, and it would do a neat, clean cut for the counterbore.

--James--

[Nick Hull]

Liquid Ivory soap works great

[nospambob]