I recently had a contractor to help me install a TV mount. The mount is fixed to the wall and studs behind it using four 5/16x2-3/4 lag bolts. Later I found out you are supposed to drill pilot holes before putting in lag bolts, but the contractor did not do this. How likely are the 2x4 studs behind the wall damaged?
Pilot holes are done for two reasons. One is to help prevent the stud from splitting and the second is to make the lag bolt or screw easier to install. The ease of installation is important because the head of a lag bolt can shear off when excessive torque is applied.
As long as your lag bolts are firmly holding the TV in place with no wobble in the mounting bracket you should be fine.
There's more to this than splits and installation torque. Final holding power is at stake.
If a lag screw (not "bolt") is run in without a pilot hole, the wood (unless it's very soft) tends to split locally around the screw shank. This may not result in a full split of the lumber, but it effectively reduces the grab of the threads on two opposing sides of the screw.
Here's a simplified diagram of what happens:
|||| <-- wood grain /\ <-- gapped area /--\ ( ) <-- screw shaft \--/ \/ <-- gapped area ||||
The wedge-shaped gaps above and below the screw shaft in this diagram provide little or no hold--the threads are barely or not at all in connection with the wood. The screw will pull out with less force than it would from a proper pilot hole.
It's worth mentioning that lag screws rarely pull out of modern SPF lumber, but if you're trying to snug hardware against a wall with force, for example, this can be an issue. Your gizmo probably won't fall off the wall, but the screw may slip more than you'd like during final tightening. In extreme load scenarios like extendable TV mounts and floating desktops, pullout strength is critical, however, but screws are usually sized with enough overhead capacity that it's not a concern.
Even if the studs are damaged due to shoddy work, it doesn't necessarily mean there's a problem with those studs or their ability to carry a load. Studs can take a lot of drilling & notching (or chipping) before they fail their structural purpose.
The bigger problem could be that the stud is not adequately able to hold the screw: little can be assured about the withdrawal force of the screw if the wood is split.
Also, we don't know if these large diameter screws are perfectly centred into the stud, or whether they partially missed the stud. Stud finders can be off. This contributes to the uncertainty of the holding power.
So, the stud may be able to carry the load but not hold the screw.
Vibrations from movement or seismic activity matter too. If the mount has a retractable arm, and the TV is relatively large, it may be best to take your losses and have the work redone. The mounting plate will likely cover the old screw holes.
Ideally you'd mount to different studs, or mount a few inches below (not above) the existing location. This allows the bottom screws to transfer their load into pristine studs, downward.
Thanks for your question. Yes, it is generally better to pre-drill holes big enough so that only the threads of a lag screw grab to help prevent splitting the studs.
However, I've seen people install punching bags with screws that had larger diameter than the ones in your kit, and just as long, without pre-drilling, and the bags are holding fine with 120+ pound kids swinging from them daily for over 10 years. Some of the screws didn't go all of the way in. The studs were split by the screws and it was no problem.
Moral of the story, while pre-drilling is good practice, even if the wood split the TV will probably hold just fine. I wouldn't worry about it.