Suburban 3rd row seat/Blazer back seat floor mounts

[NCBurb]

Howdy. I am just beginning the process of adding a 3rd row seat to my 87 Suburban, which never had one. I picked up a floor section with mounts from a parts rig, and have been thinking the best/safest way to add the mounting "buckets" from the floor to mine.
At first I thought they were spot welded up underneath the floor. As I was cleaning them up though, there's seam sealer squished out all around the joint, and it looks like maybe some rivets or swages to crimp them together? Anybody ever removed those "buckets" clean? If I could do that, then I can just cut out the rectangular holes in my floor, which has the stamped bosses in place, and weld the buckets up from the bottom...
Basically I'm wondering if anyone knows how they were attached at the factory, and/or if you have ever taken some apart.
Also I want to confirm that they are the same "buckets" as the Blazer's 2 back seat mounts. One of mine is pretty rusty, so I'd rather expand my options to find 1 more clean one. No sense welding in rusty metal. thanks,

 

[bucket]

Iirc, they are spot welded to the underside of the floor pan.

I installed the 3rd seat in mine over a decade ago and I remember a few of the details. I got the floor mounts from a donor, I think they were cut out roughly 8"x10" in size. There was a substantial amount of donor floor around the mounts. I cut enough out of my rear floor so the mounts would sit down flush. I then drilled a series of 1/4" holes around each mount and used high grade 1/4" rivets to hold them down. Doing it this way, there's a lot of surface area and the load is spread evenly, making it a very stout install. I used seam sealer around the perimeter, on the top side. The theory is, any moisture that collects between the layers can escape out the bottom. It has held up very well over the years, despite the rest of the truck rotting out.

The Blazer mounts are the same design.

 

[NCBurb]

So how much overlap did you have around each mount? Also you put the mount plate on top of the floor, or underneath the floor to rivet it?
And I didn't realize you could spot weld through sealer? or maybe they just squished it everywhere at the factory...

 

[bucket]

I put them on top of the floor. Overlap is somewhere in the 2-3" range. I put the rivets more outboard of the panel, to more evenly distribute the load.

The factory can do all sorts of stuff with special sealers and welding equipment that us folks at home can't replicate. I'm not even positive that's what they did in this case, just going off my memory.

 

[NCBurb]

hmmm....I was thinking that in the case of a crash, the seat (and mounts) would tend to pull up, and so was leaning towards mounting the buckets under the floor, just to give it a little more resistance to an upward force. I wish I had a few extras to play with, I would like to try to drill out the spot welds and see if I could get them clean enough to weld up from below, but I don't want to mess anything up. Otherwise I hadn't thought of rivets, but that is intriguing as well.

 

[bucket]

hmmm....I was thinking that in the case of a crash, the seat (and mounts) would tend to pull up, and so was leaning towards mounting the buckets under the floor, just to give it a little more resistance to an upward force. I wish I had a few extras to play with, I would like to try to drill out the spot welds and see if I could get them clean enough to weld up from below, but I don't want to mess anything up. Otherwise I hadn't thought of rivets, but that is intriguing as well.

A frontal crash is something I took into consideration as well. But at the same time, if the truck were to get rear-ended hard by another large vehicle, the top mount would be preferred in that case. But also, the pockets and surrounding metal that I installed was large enough that the total secured area should be just as strong as a non-modified floor.

I also used rivets, not for the ease of install, but because of the close proximity to the fuel tank. I also forgot to mention, I used the stronger 'swell' type rivets and not the 'peel' type.

There is one more step that I wanted to do, but couldn't because I was working by myself. I would have liked to use a washer on each rivet, on the underside of the floor. But I didn't have anyone to hold up the washers from underneath while I installed the rivets.

 

[NCBurb]

you make a good point about the gas tank, although mine is diesel so it wouldn't be as big of a hazard...so 1/4 diameter rivets - by how long?

 

[bucket]

you make a good point about the gas tank, although mine is diesel so it wouldn't be as big of a hazard...so 1/4 diameter rivets - by how long?

I don't recall how long they were. Probably around 7/16". Also, a 1/4" rivet is going to require more than just the usual hand riveter. They take some force.