I use 316 stainless....but it's just coroplast, grade really doesn't matter.

316 - mind sharing what you used? I did not see that grade in my searches. I want something stronger than grade 2 so I don't twist the head off when snugging them up. I have had the self drilling break just half way installed to.
Thanks

Combining the drilling, threading and fastening into one component is a “compromise” for all three. Drill the hole with a quality drill bit. Tap the threads with a quality tool. Use a 316 SS bolt. (In my opinion).

Rob

Keith--boltdepot.com has everything, including fantastic reference information. The snippet below was taken from one of their downloadable information booklets.

McMaster Carr will have lots of information on the materials and there strength. I have added a couple screws to the coroplast area into the frame and drilled a hole first before using a self drilling/tapping screw this puts less stress on the screw, also back down the clutch setting on your driver. I have also tried to use stainless self tapping in steel with a predrilled hole and broke the stainless screws so I would not recommend stainless unless you are drilling and tapping.

Brian

316 is the alloy, not the grade.... best I can tell it is close to grade 5 (What a millwright told me)

Pre-drilling a pilot hole will help to avoid breaking off screws in thick material. The self drilling screw was not designed to be used in material as thick as the flange on our frames since the metal can be thicker than the drilling pilot/point toward the root. This sets up a condition where the drilling point is penetrating at a different rate than the threads in overly thick material which results in screw breakage. The self drilling screw is designed for a specific material thickness, installation RPM and torque. We do not need a torque wrench but rather a reasonable setting on the overriding clutch of the drill and a lower speed setting.

Jim

The plated steel drilling/self tapping screws are not a "graded" product. Their hardness, durability and strength are mostly dictated by the quality of the material used which varies wildly and is mostly dictated by pricing. Cheap screws arent good and good screws arent cheap.
Most 18-8 stainless screws are harder than a grade 2 and roughly equivalent to a grade 5.
If you purchase a stainless replacement for the hex washer head self drilling/self tapping screws that GD uses they will likely be more durable than the steel ones that were on it when new.

The OP is mixing terms for what GD uses on the corplast. A 1/4-20 screw is a machine thread typically but can be specified for a "thread cutting screw" like a type F or a type 23 but thats not what GD uses.
In most cases (but certainly not all) a sheet metal screw will not have a noted thread count but would be expressed as a 1/4x1 hex washer head self drilling/self tapping screw. If youre looking for a direct replacement for what came on the trailer then that is the nomenclature, if you want stainless then add that to the description also and see what pops up. No telling what types of stainless are available. Also note that 18-8 stainless is a very broad classification but anything shown as that would be more than suitable for your application.

Although material thickness is a factor, I believe material quality is the biggest factor in breakage on the screws used in the RV industry.

Wow
Thanks folks for the wealth of information. Sorry about mixing terms.

My initial plan is to pre-drill the holes so a self tapping screw/hex bolt could be used. I was not planning on tapping that many, but that is an option I guess. I found a tap adapter to fit my drill driver, so I can use it to go very slow and not hurt my hands as much. It has a fully adjustable clutch system (12-14 settings) My other option is the self drilling and still doing a hole pre-drill the diameter of the screw drilling portion. However when I did this in the past with my 1/4" impact driver I broke a few, but that could have been due to my error. Attachment points will be in 1/8" mild steel, either the tubing , the flange, or the cross braces. All appear to be about 1/8" thick.

I really appreciate the links and had saved a few options. Many of the self tapping/drilling are labeled 410 SS Boltdepot and McMaster Carr Some 8-18. If I tap the 316 is common for bolts.

I ran across this option which is a hair smaller than 1/4" https://www.boltdepot.com/Product-De...x?product=9176 It appears to be a match (style wise) to what GD used. I already have some self sealing washers in my stock from Fastenall I bought a few years back. This appears to be the same screw without the washer https://www.boltdepot.com/Product-De...x?product=7174

I also ran across this self tapping in 8-18 https://www.kljack.com/products/25c1...SABEgJydvD_BwE McMaster Carr has the same in 410 SS https://www.mcmaster.com/type-f-thre...ews-for-metal/ Screw detail here https://www.mcmaster.com/96024a430/

Thanks again folks - lots to think about and research. Ill probably send a few emails to the suppliers too.

Keith

A little lube on the screws will help prevent galling with ss screws whether self-tapping or pre-threaded installation.

Agree screw material properties are important, but if the pilot portion of the screw does not drill completely through the material, the thread will catch and try to control the rate of the pilot drill. Most of the time this will result in the screw breaking. The other issue if the pilot portion does not extend through the material before the threaded section is binding due to the cuttings not having a path to escape which will also result in breakage.

Jim

OK, slight tangent here: IMO I don't like the whole system. To me there are 2 major issues with the factory standard way of coroplast as the underlayment.

First, it sucks. You need to access something in the middle of the trailer, you gotta drop the majority of the underlayment. That's lame. I've seen some older trailers with plastic sectional panels. You just remove the specific panel in the area you want to access. Easy. Why can't RV industry innovation lean toward easier maintenance/repairs?

Second, I do not like drilling holes in the frames of the trailer. It compromises the structural integrity of the beam. Ther are very specific areas and hole sizes that are allowed in a beam, and the holes for the coroplast are not in those acceptable locations. This is why I welded the shock brackets to the frame of our trailer rather than putting more holes in the bottom of the beam.

Yoda Keith, I will just drop this here and let you decide if you have not already: https://www.homedepot.com/p/DEWALT-M...SETS/300703696

This may help others. And yes there are now several brands out there. You usually get what you pay for.