I need to bolt a bench to 3/4" plywood and I can’t get to the underside of the plywood. The two legs that attach are 3/4" square steel tubing and the pre-drilled holes look to be about 1/4" in size.
What would be stronger, threaded inserts and run two 1/4" bolts per leg or just use fairly heavy wood screws.
I want to avoid drilling through the plywood and through the bottom of my van floor. I’m sure that would be the strongest option, then I could use a big washer and a bolt to secure the bench. That’s plan B.
Before I do that, I think I’d just drill through the wood and the van floor. I’ve already put 4 holes in the van floor to keep the plywood from moving. I counter sunk carriage bolts with large flat washers. The plywood base seems secure between the bolts and the 1x3" firing strips I attached to the floor,. Just hate drilling more holes to attach the bench. The bench doesn’t have sea tbelts, more just to have a place to sit and it folds to a bed if needed.
Like you said, through bolts would be best but that would require you to drill through the floor of the van. Screws have good hold strength (especially in shear) but may loosen up with motion over time (a few years or strong jolts). If you can, I’d recommend oversize drilling the hole and filling it with epoxy. Then use either a threaded fastener or screw. The epoxy will soak into a larger patch of the wood creating a significantly larger holding surface.
If you think the screw will need to come out, then wait for the epoxy to soak all in and harden, then drill into that for the fastener. Good idea to coat the threads with soap / WD40 / Oil to keep any uncured epoxy from “sticking” to the screw. If the mount is permanent, then you can clean your fastener (remove oils) and either set it in the epoxy before hardening or wait until it hardens and add a little extra epoxy to the screw threads to make it extra special embedded.
That doesn’t take into account that this is a Bench in a van. Over time the screw holes will swallow out and then the pullout strength will be 5 pounds.
If the only reason for screws is to keep it from sliding around while driving, you may be fine with just some screws, especially if it’s up against a wall/side and can’t really lever itself in that direction. Worst case if the screws don’t hold you can either slide the bench a couple inches over or just drill all the way through and use the bolts. Probably no harm in trying the screws first.
If the only reason for screws is to keep it from sliding around while driving, you may be fine with just some screws, especially if it’s up against a wall/side and can’t really lever itself in that direction. Worst case if the screws don’t hold you can either slide the bench a couple inches over or just drill all the way through and use the bolts. Probably no harm in trying the screws first.
I’ll probably do the wood inserts, they are easy to install and less likely to wear out over time. The side of the jackknife bench will need to be bolted/secured to one wall of the van. It seems to rock side to side a little. It’s 68 long and weighs maybe 125 lbs. The other side is where you enter the sliding door and no place to attach.
Here is a link of what I’m trying to accomplish. Floor is in, my bulkhead is built, I have my fabric and coin flooring and have started with my panels. It’s really for my son but dad is the builder and we are learning as we go. lot’s of youtube videos on converting vans to campers but we are just doing a motovan.
Or what if you were to glue (or even screw) something ranging from a 1x4 to a 2x6 to the plywood first? With good adhesion that board isn’t going anywhere and would significantly increase both the holding power of screw both by going into “real wood” and by increasing the length. Downside is a slight increase in bench height.
Yeah that might not be a bad idea to use a flat 2x4 under the legs. If the bench is 2" higher that would give me more storage underneath. That’s the problem with these van builds, it’s like a chess match to constantly be thinking ahead and new ideas always pop up. below is a pic of the bench and its legs. It will be mounted on a 3/4" plywood base that is bolted to the van floor. It’s a jackknife and folds flat into a “bed”. Good enough for a 16 year old body.
