I measured a 1/4", just one, and its root measured .186-.187 top to bottom.
I don't imagine lags are a precision item, tho.
And one of the links someone posted gives the pilot as 3/16 in softwood,
7/32 in hard!
It also mentioned grease or vegeteable oil as a lube, but cautioned against
1/4 lags are fragile, tho. I'd use 5/16 on anything semi-substantial.
> > The next natural drill size is .187, or 3/16".
For optimum strength in solid wood you actually want to drill the pilot
hole smaller than the root diameter. Specifically between 0.7 and 0.9
times the root diameter, depending on the density of the wood in
question--softwood gets a smaller hole.
Long lore, at least...
I don't have a direct URL; I'd expect you'd find the information in some
of the US Forest Products Laboratory technical publications. It was
something I was taught way back in one of first HS ag-ed classes is
first I recall it personally, anyway...don't recall if it was taught as
a specific ratio, only "tubafores get smaller, rr-ties get bigger" was
the gist of it. :)
I don't see the point. The pilot hole sizes quoted work for nails
where compression determines strength but I don't see any benefit in
having a pilot hole of less than the root diameter of a threaded
The point is to have a larger pilot-hole in softer woods than hardwood
to minimize the effort of installation but to ensure a full bite which
can be marginal if use a full root diameter for pilot, particularly in
softwoods that tend often to "crumble".
No claim made (at least by me) that there's any _precise_ ratio other
than the aforementioned bigger/smaller based on the material.
I'd still wager there is some information at US FPL but I've not taken
time to search for it.
It's in "Wood As An Engineering Material", Page 7-11. What they say is:
"For low-density softwoods, such as the cedars and white pines, 40% to 70%
of the shank diameter; for Douglas-fir and Southern Pine, 60% to 75%; and
for dense hardwoods, such
as oaks, 65% to 85%. The smaller percentage in each range applies to lag
screws of the smaller diameters and the larger percentage to lag screws of
Excellent reference, J. Thank you for posting that, it is very much
The lag bolt which snapped off had an average shank diameter of 0.182".
Sixty percent of this value is 0.1092, while seventy five percent of this
value is 0.1365, which puts a pilot bit of 1/8" right in the middle.
On Thu, 17 Dec 2009 11:19:09 -0800, "Jon Danniken"
What do you mean by "shank diameter"?
Machinery's Handbook lists the "body" or "shoulder" diameter of a 1/4"
lag bolt as between .237" and .260". This is the area that is not
The "root diameter" is listed as .173". This is the diameter of the
remaining cylinder after the threads are formed.
(American National Standard Square Lag Screws - ANSI/ASME
BUT Jon,,,,,, While it is a kewl reference that agrees with what you were
using as a pilot hole, how did that work out for you?
The information could be out dated for readily available fasteners today.
If might be a new publication using old data.
It works out great, Leon, once I purchased lag bolts that weren't from the
bulk bin at Home Depot. The bolts are currently holding up a home-made
welded bracket, which in turn is holding up my bathroom sink.
The sink itself weighs maybe 20 or 30 pounds, so I tested the bracket by
holding myself up with it, and it didn't budge.
The numbers are based on maximizing holding power, not making a crap
fastener survive being driven. They state the assumptions, which are 35,000
PSI yield and 77,000 PSI UTS, slightly higher than required for a Grade 1
Are you saying that the properties of wood have changed so radically in the
last 30 years that lag screws hold differently in them now? Because the
recommendations are not about allowing you to use crap screws without
breaking them while driving, they are about sticking wood together so it
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