How many rivets or pan head screws to use?

[peterh226]

There are a couple of areas where there seems to be a wide range of fastener density being used.
On several of the latest builds on the FFR site, the firewall is held on with a very small number of fasteners - certainly not even close to every 3 or 4 inches. Does using a good silicone mean we don't need all of those attachment points?

On the Blue Hot Rod with the 32 grill, they have 3 fasteners at the door jamb into the frame - a lot less than the instructions would lead you to use.

Any of the wizards that have built a bunch of these have any comments on the changes especially in body fitment? The Gen 2 seems to be a bit of a different beast - in a good way.

[TxMike64]

I think generally from the builds I've seen the number of fasteners used on the firewall - especially those using screws - is way more than needed. This is my opinion as a mechanical designer, and based off the strength of a 10-32 screw and the strength of the aluminum plate used for the firewall. That spacing (2-3") may be correct for the smaller rivets, and factoring failure rate over age of rivets, I don't design using rivets, so I have very little experience/knowledge of their characteristics. But I do use a lot of machine screws in my design of medical device and manufacturing fixtures, and in my opinion the size and number of screws used is generally too big and too high on the firewall.

The tensile strength of a mild steel 10-32 screw is 120,000 PSI, so the shear strength is about 2,000 lbf. (Much higher strength than the thin wall square tube it's threaded into.) The aluminum firewall panels, if I understand correctly, are 6061-T6 which is fairly rigid and resistant to stress fracturing from vibration (work hardening), so 4-6" might be more practical. If using a #10 screw, use 10-32 instead of 10-24 to get more thread engagement with the thin wall square tube - not necessarily for strength, but will tap easier and more threads will be less prone to backing out. I'm planning on using 8-32 BHCS at 4" spacing with blue loctite.

Where screws are used for holding the body on you need to worry more about the size of the clamping area (underside of screw/rivet head), strength of the body material, if the body is supported fully underneath, and the strain in that area. From what I've read about the Gen2 body it fits much better than the previous body and in the door sill area will probably have lower stresses - hence the lower fastener count seen on the blue car. Or this was just the choice of the builder, not F5.

This is all conjecture and keyboard engineering on my part. The best would be to get in touch with Tony and the other engineers at F5 for their opinion.

Another option to consider for the firewall is using panel bond and very few fasteners. This eliminates vibration issues and the ordeal of drilling and tapping all those holes, not to mention having a smooth firewall!

[SSNK4US]

Ummm ok. Lol

[Svtfreak]

Mike gives the perfect engineering explanation. He's right.

The gut feeling though is.....

If it's high stress area, lots of pull or push, use 1 every 1.5" or so. Just holding a panel on (like firewall?) use every 3" or so. If you like the look of rivets/screws, use more. I used one every 2" on my firewall and, while it looked great empty, once everything was in, it's too crowded looking. Shoulda used 3".

Places like the side behind door holding the body on, those are mostly for location, so I used 4 across there.

Just make sure you use enouhh seam sealer for the amount of fasteners. You don't want any weird vibration rattles.

[peterh226]

My thinking is to use 1/4-20 button heads for location as much as anything on the firewall then silicone adhesive/sealer to lock it to the frame. There really is not much in the way of forces on the panels. Reservoirs, A/C ports, and now the hood latch are the main items. I’m not sure how much this panel contributes to the shear resistance to twisting forces. Probably would take a long time of drag racing or other chassis abuse to see any affect on the firewall.

Thanks folks...