Drill size for 5/16 lag screw


Old 03-02-05, 11:03 AM
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Drill size for 5/16 lag screw

I'm attaching a newel post for the first time with a 5/16" lag screws into doug fir. The screw is going into the doug fir carraige lengthwise. Although it looks like it's going into end grain, I can clearly see that the grain is perpendicular to the direction of the screw.

Usually I just use a drill about equal to the body size of the screw but since I have only one chance to get this right and I need very good holding strength... I also have to get a relatively expensive extra long bit so I can use the newel (with holes already drilled) as a pilot hole.

Also, when I demoed the old stairs, I noticed that the lag screw threads have squared edges, not a knife edge like the replacements. Is this significant?

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Old 03-02-05, 11:13 PM
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Moved to the "fasteners" forum.
Old 03-03-05, 11:47 AM
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Hi AlexH,
- In this case, I would use a pilot drill a bit smaller than the screw body. In the case of edges, i think the sharper edge offers greater holding power.

Do it Right - Do it once.
Old 03-03-05, 06:14 PM
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If you carefully compare the old screws with the 'squared edges' to the new ones with the 'knife edge' you will very probably see that they have different TPI or Threads Per Inch, and also have different thread pitch. In the most general terms, the more TPI of a screw, the 'sharper' the thread edge will be. As far as thread pitch - or angle - goes; a thread pitch of 55 degrees - for example - will produce a sharp edged thread, but 60 degrees - a simple 5 degree difference - will produce squared edged threads. As far as 'holding power' goes, well, there are - technically speaking - four (4) main 'strengths' to a threaded fastener; yield, tensile, shear, and pullout. There are several factors that are very carefully considered when establishing these ratings, and thread pitch - which in part affects the thread's embedment into the material (full or partial) - is a key to factoring pullout strength. Thread embedment is also affected by the diameter of your pilot hole. Too large a hole will offer only partial thread embedment and possibly poor performance. Too small a hole will very possibly cause unwanted fractures in the material, and a too small a pilot hole will also create dynamic friction when the screw is driven, and that heat will cause very minor but very relevent expansion in the screw resulting in poor thread embedment when it cools. In the most general terms there is only slight performance differences between the sharp and squared edged threads, the key is your pilot hole and the surrounding material it leaves for full thread embedment.

Also: screws are usually produced by one of three methods; machined, rolled or stamped, and at times two screws with the exact same TPI and the exact same pitch - and by the same manufacturer - will appear to have different edges - one sharp and one squared. This isn't an optical illusion, this is caused from the wear on the processing machinery and/or coatings.
Old 03-04-05, 01:12 AM
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Oh NO, not another "nut and bolt war" !
I take it back, honest, - I know I shouldn't have started mentioning things like "holding power" .

Do it Right - Do it once.

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