What screw should I use to bolt refer plate to floor?

[ramblinChet]

I am preparing to bolt a steel plate (32.0" x 18.5" x 1/16") down to the floor of my FWC and I would like to hear some suggestions on what would be the best wood screw to use. This steel plate will hold my 80 liter National Luna refrigerator and the plate has six 1/4" holes about the perimeter used to fasten it to the deck.

My first consideration was a #8 screw but since much of the loading would a shear type load with only some being a withdraw load I thought moving up to a #10 would be a good idea. So, six #10 with 1/8" pilot holes. Your thoughts?

Now onto the length...it appears that the plywood decking in my FWC is 5/8" thick and I know the steel plate I am bolting down in 1/16" thick. From what I can gather it would be best to sink the screw at least 1/2 to 2/3 of the thickness of the plywood.

So, shooting for 1/2 of the depth would require a #10 screw of 3/8" length while 2/3 depth would require a #10 of 1/2" length. Your thoughts?

For this application I am not sure of the screw head type matters that much although I am always open to suggestions. If I had the ability to remove my camper I believe it would be best to bolt through the floor but that is not an option right now.

 

[billiebob]

Either an elevator bolt or a carriage bolt and thru bolt it. Or a tee nut and bolt, and thru bolt it.



With a tee nut and bolt you might need to cut the bolt to the correct length.

 

[simple]

Looks like you have put a lot of thought into this already. If the plywood is solid, I would think 6 stainless steel 1/2" #10 pan heads should do the job. I might put some sealant / adhesive on the threads like 3m 5200.

 

[NatersXJ6]

I wouldn’t trust any screws that short not to rattle / back out of plywood. You can put some sort of glue on them to reduce the chances, but without through-bolting I’m guessing you don’t get 2 years before loosening is an issue. That being said, if you insist, I would use something like a #10x3/4 stainless Sheet metal screw, hex head for driving force, with a lock washer under it and drill the smallest possible pilot hole (1/16”) and apply something like bar soap to the threads. Use a sharpie to draw a reference line across the head so you can see if it is backing out over time.