Apr 27, 2024     Firewall grommet shields - (6.5 hours)       Category: Firewall

While thinking about what to do next and where to mount stuff on the firewall, I realized not everything going through the firewall will have or need some trick fitting. Some stuff can get by with a simple grommet and a stainless steel protective shield over it. (typically wires). I used a few of these shields on my RV build. They're simple and light and do the job on Cessnas and Pipers for decades. Probably good enough for me too.

They're about $12.50 a throw from Acft Spruce (plus shipping), but anybody can do that. I recalled reading an article by Tony Bingelis in an EAA publication about how to make your own shields (for less).

Tony used a socket and a larger piece of steel tubing to telescope the socket into with the stainless sheet pinched between, squashed in a giant bench vise. I did about the same, except I used a pair of big and little sockets squashed in an arbor press. The bigger socket is twelve-point, allowing a slightly larger small socket to fit into it.
I had a bunch of leftover SS scrap from the firewall fabrication, so I cut some generously sized pieces to stamp the shields from.
First step just squishes the smaller socket into the SS sitting on top of a scrap pine 2X4, making a round dent. Next, the dented SS is carefully centered on top of the larger socket and the smaller socket replaced in the dent it formed in step one. Placed in the arbor press, time to squash again. Pressed about an eighth of an inch into the larger socket forms about the size dimple required to cover half a firewall grommet.
It's pretty wrinkled up after step two, but plenty of malletizing with a small ball-peen hammer makes them mostly flat again. Sanded the edges back to get them in more of a pleasing and eye-catching shape before punching out the attach holes and a center hole.
The center holes will be widened as necessary depending what is going through it's respective grommet.
Oh yeah, Tony calls for a small disc of baffle material to be placed under the shield on top of the grommet when it's screwed in place as well as some sealant if the grommet doesn't tightly fit the wires (or whatever) going through it.

  Jul 20, 2021     Stainless Steel Firewall       Category: Firewall

Laid out the firewall shape on a sheet of stainless steel. Fabbed a full-scale half-firewall paper template and glued it to 1/4" foam core. Then transferred to the steel. Cut it to shape with the pictured "12 inch hand shear" which is what the label says it is, but it has a four-foot handle to operate said "hand" shear. I guess it refers to its method of operation as opposed to, say, "hydraulic/diesel" shear. No curved cuts with this baby, just lots of short, 1-1/2 to 2" length cuts, then rounded to smooth with hand files.
Put a machined or "turned" finish on the front surface using an angle die grinder. First laid out a 2" x 2" grid with a sharpie pen, then the Grinder was fitted with a 2" scotchbrite wheel and dabbed with valve grinding compound. Simply followed the grid by hand. It's not perfect or as neat as if I had used a drill press or mechanical indexing method, but I think good enough as only small portions of it will be visible at any given point.
Next steps will be to bend it forward 13.5 degrees at the level of the top of the top longerons. The edge will then be fitted with 3/4 x 3/4 angle around the perimeter.
Stainless steel angle is very stiff stuff to bend into the curve required for the firewall perimeter. Contemplated cutting/notching the edge to be riveted to the firewall or replacing with 0.062" extruded angle and shrinking to fit/form, but settled on the stainless. Made it work by first trimming the to be riveted edge to 1/2" width to allow easier forming, then heated/annealed same edge to allow use of fluting pliers (vise grips) to "shrink" the edge to conform to the firewall shape. I used 1/8" c'sunk rivets to produce a smoother firewall surface and allow for easier cleaning from an ongoing maintenance standpoint, plus the mating dimpled surfaces should allow for higher shear strength in the assembled flange.
I also used pro-seal between the flange and firewall and riveted it up while wet. This in hopes of mitigating any CO migration to the cockpit from the firewall-forward areas.

  May 10, 2021     Anchor points on the firewall       Category: Firewall

In preparing to get the fuselage out the door to the powder coaters, I needed to finish sticking the remaining tabs to the firewall... as soon as I figured out where they should go. Took the fuse off the rotisserie, mounted on the gear and had full access to put stuff where it needs to be.
The firewall itself is going to be constructed of 0.025" stainless steel sheet and does not tolerate having heavy stuff bolted to it, particularly in a high-vibration installation, without adequate support. Even small stuff simply bolted/screwed to a firewall will vibrate enough over the long run to produce cracking in the thin stainless. There will be more small stuff attached to the firewall, but backing plates and wide area washers help distribute the loads to minimize or eliminate the cracking problem. Pictured are the battery box, gascolator, master and starter solenoids and the oil cooler and duct adapter. There is also tabs for the engine to firewall ground lug and fuel line support tabs on the aft side of the firewall going to the gascolator.
Fourth picture shows typical jigging to hold a pair of tabs in the proper position for tacking/welding and wood bracing for the shaky ol' welder to rest his hand(s).
The bottom row of pictures displays a recent discovery: With the firewall in place, all those tabs I stuck to the firewall tubes are almost exactly 1/8" off the rear surface of the firewall. Screwing through the firewall to attach heavy stuff will likely suck the stainless in excessively and promote cracking and a funny look.
The culprit(s) are the four 1/8" steel disks I welded in place where the engine mount pads land. These will also hold the stainless that 1/8" off the tube surfaces. Pondering the fix, but right now looks like 1/8" spacers behind the firewall in the screwed-down areas... TBD
Update: Yup, added 1/8" thick aluminum spacers to each firewall penetration tab, riveted to the tabs with the nutplates.