On the back wall of the niche I mounted the base plate for the mount off center so I could attach it to two studs (2x6). Didn't like the lag bolts included with the mount so I bought four 5/16" x 3" to use instead.
Wanted to seek some input if this should be sufficient to securely hold the TV? Originally had thought about adding a pair of toggle bolts as well to anchor the center of the base plate between the studs, but not sure they're needed.
I used the ones that were included in my Sanus mount to hold my 52" Samsung, though a lot of people bought hardware store bolts as you did. I also modified the mount to reposition the TV in relation to the wall mount. My wife had doubts it would hold our new TV. Funny, as many times as I have been correct, she still has doubts - WTH???
So, once I mounted it, I had my son hang from it for awhile to demonstrate to her it would hold. This was with the mount at full extension (29"). Those lags were probably 5/16 x 2 1/2 so I would say I would worry more about the nails securing those studs to the sole and top plates ripping out before I'd ever worry about those lag bolts failing!!! Some of those wall mounts, like the Sanus, are built like boiler plates!!
I have pics in my thread of him hanging from the mount!!
Frankly, there is no telling whether ungraded lag screws will be up to the job without testing them somehow. But I wouldn't do it by hanging a kid from the mount, just on the off chance that something goes wrong. If you want some assurance, at least use grade 5 lag screws, or better yet, construction screws.
An unmarked bolt from a hardware store would be grade 2, that bolt with 3/16" diameter shank would have 2043lb breaking strength. That would not be my concern. It's far more likely the bolt would pull out for some reason than break.
Drywall anchors? No way. That's peanuts compared to what you already have.
An unmarked bolt from a hardware store would be grade 2...
True for most bolts, but I don't think so for lag screws. I read a post somewhere from a distributor about it some time ago, but don''t recall just where. There is certainly an issue with lag screws. Monoprice had a problem with the ones that were included with some of their mounts. I suspect besides inferior screws part of the problem was that people were not properly drilling before installing the screws and they sheared off before making it all the way into the wood. Makes me wonder how many installations have screws that didn't shear off but were seriously compromised during installation and on the verge of failing.
Don't know what breaking strength is. I think you are referring to ultimate tensile strength. A grade 2 bolt is about 74 ksi IIRC. An ungraded bolt could be much lower. And tensile strength doesn't tell the whole story.