Re-using lag bolt holes?

Question

Can holes in wood studs that previously held lag bolts be re-used with lag bolts of the same diameter or will this not hold safely?

More detail: I have a piece of fitness equipment mounted to wood studs in my garage. It needs to support ~350lbs of weight. The equipment needs to be removed and replaced. Can the replacement go back into the same holes or will new ones need to be drilled? I cannot use larger diameter lag bolts since they will not fit through the mounting holes on the fitness equipment.

Possibly irrelevant: It is a Rogue 3x3 Strip used to hold a Matador lite dip station. Since the load is very heavy, I used poplar boards as stringers to distribute the load across 3 studs. The strip has 4 holes for lag bolts so a total of 12 lag bolts were used. Unfortunately, one of the lag bolts broke off in the wall as it was being tightened. It was in the middle, directly anchoring the strip. In order to get it out, I'm pretty sure I need to remove everything to gain access to the hole and extract it.

Answer 1

The important thing when reusing holes for lag bolts (or screws in general) in a material that the screw will cut threads in (i.e. wood, but also applies more generally to any material much softer than the screw being driven) is to turn the bolt backwards (counterclockwise for a normal right hand thread) until you feel it "drop in" to the existing thread - then start turning forwards. That way you are following the existing thread in the wood to reinstall the screw, not starting to cut a new thread just a bit off from that one and reducing the strength & holding power of the wood.

Answer 2

Yes, you can reuse screw holes in wood or plastic. Just make sure the new screw has the same diameter, thread pitch, and thread shape.

When reattaching, roll the screw backwards a couple of revolutions and feel for the "hop" of it dropping into the existing thread. Sometimes there's a little hop and a big hop; you want the big hop. If you do much mechanically, this should be a daily habit.

Expect insertion torque to be much lower. This isn't a metal screw; it doesn't create clamping strength by stretching the fastener shank. The torque cuts the thread initially, since screws are by nature self-tapping. There will be much less torque when the thread is already cut.

If you are snapping off threads, it's time to remember "monkey tight, not gorilla tight". If monkey-tight is insufficient to drive the fastener, then you need to pre-drill more. If you want to get scientific, consult the manufacturer as to maximum elastic torque before introducing metal fatigue, and don't exceed it. The material you're going into is irrelevant to the torque capacity of the fastener.

Answer 3

Using the existing holes should be fine, so don't worry about that. There's a few other things I'll mention tho.

The lag bolt shouldn't have broken unless it had some severe manufacturing defect. When inserting large lag bolts, you should always pre-drill with the correct sized bit. You can also lubricate the bolt with a bit of wax or soap when screwing it in to make it a little easier. Lubrication will help it twist into the hole but will not reduce the strength needed to pull it out.

Also, poplar is a very soft wood. A standard southern yellow pine stud would probably be a better choice for the load sharing.

Getting the broken lag out will be very difficult, so consider repositioning the mounting bracket slightly and starting over if possible.

Answer 4

Why reuse the existing holes? Just offset it especially given one of the lags snapped and you wouldn't be able to reuse that hole anyway.

I've snapped lag screws before probably because I didn't pilot with the correct size drill bit or pilot at all.

If you did want to go up a size in lag, you could probably just drill the holes out the tiny bit extra that you need without much work and with compromising the strength of the bracket but it sounds like the easiest thing is just to offset and not worry about any of those details.

In terms of strength it doesn't sound like you are taxing the pull out strength and are really taxing just the shear and the holes being a little less tight should not affect the shear. If you were mounting something to the ceiling and the lags were being taxed in pull out then I'd be a little more worried about re-using the holes.