Firewall Forward

November 28, 2012

Adding a Doubler to the Fuel Fitting Pass-Through - 4.0 Hours

I've had something on my mind that's been bugging me for a while. Way back when I built the fuselage, I installed a doubler plate on the inside of the firewall where the fuel line passes through. Here's a picture of it. I installed an AN fitting and tightened it up so I could fabricate my fuel line that runs from the fuel pump in the cabin to this fitting. Well, it's one more of those little items that I didn't study the drawings enough for, or check out what my fellow builders are doing with this. Apparently, it's supposed to go on the engine side of the firewall and made a little wider, so you can encompass one or two of the rivets on the vertical stiffener angle that's right there. Being able to rivet this doubler to the stiffener angle would make sense. Much more strength than just riveted to the thin firewall. For a long time now, I didn't really want to change this because I don't want any aluminum parts on the engine side of the firewall. Especially connected to anything having to do with fuel. So what do you do?

Recently, I bought a steel fitting to replace the aluminum AN-fitting that I originally put in here, and it brought it all back to my mind again. For a while, I rationalized that it would be sturdy enough. Once the fuel lines are hooked up and everything is in place, no worries, right? Well, as I get ready to hang my engine, I can't get comfortable with this. A friend of mine on the VAF forums recently emailed me on this, asking me about it. So I grabbed the fitting and sort of wiggled it from side to side to see just how much it might vibrate around. It's pretty solid. But there is some movement possible. I could probably get away with it just as it is. But who wants some little item like this haunting your mind? Especially when there's a better way? What would a future FAA inspector say, during my airworthiness inspection? Who knows, but the final clincher for me was remembering that it took two stainless steel washers on the firewall side to be able to tighten the nut on this fitting. You can see them if you look carefully at this picture below. Why not remove one of those washers and replace it with a steel plate that's riveted to the firewall and the vertical stiffener angle, like it's supposed to be?

Okay. So where do I get a steel plate that's appropriate for this piece? I ended up in the aviation isle of Home Depot, looking at all kinds of Simpson steel ties and angles that are used to nail joists and boards together. I couldn't find anything suitable. Full of discouragement, I went over to the electrical dept. and found just what I needed! A plain 4" square plate made of galvanized steel, used as a cover on a 4" square electical box. My cost... 60 cents! When I got out into the shop, I found that it was the perfect material I was looking for. I could cut it to the shape and size I needed. I ended up cutting it the old fashioned way... lots of elbow grease and an ordinary hack saw while it was clamped to my bench. It turned out to be sort of a unique shape, so that I could make the inboard end of it the right size and shape so as to capture two rivets instead of one (unless I drilled additional holes, which I didn't want to do). Had it been a simple rectangle, I would have only had one rivet on the stiffener. Here's my finished product, all riveted in place. The upper-left rivet actually holds the platenut in place, as well as going through the stiffener. No problems, though. I cleaned it up and shot some Lycoming Gray paint on it, to match my engine and the battery box. I had to remove the starter contactor solenoid temporarily. That was actually the worst part of the whole job.

The trickiest part of this was getting it to fit right under the leg of the starter contactor, without interfering with it. Some careful filing was needed here, removing a little at a time until the fit was just right. Here's a closeup, showing how it all came together: It just fits!

Once everything was back in place, I installed the fitting. Sure enough, only 1 stainless washer is needed now. I tightened it up and wiggled it firmly as I had done before. It is DEFINITELY stiffer than it was before. So it's worth a few hours in the shop to take care of it and move on. I'll sleep much better.

Here's another shot of the inside, with the fuel line hooked up finger tight, just to check everything. I left this doubler on, too, primarily because if I took it off, my fuel line here would be about 1/16" short. That's probably not a big deal, but why not leave it on? The whole installation is super strong now.

I finished one more little task tonight in the shop. Those of us building A-model RV's need to drill an access hole through the firewall under the nose wheel gear leg bolt (tapered pin in my case) to make it possible to install or uninstall the bolt that holds the gear leg in place. If you ever need to remove the gear leg in the future, you need a hole here to push the bolt through, to remove it or replace it, so you can take the gear leg on and off. It's that close to the firewall when it's in place. If you don't have this hole, you'll have to take the whole motor mount off the plane to get the gear leg off! Not a nice thought. After mounting the plane on her gear, this hole is plugged with a stainless steel plug and sealed with a high-temp sealant.

I'm sure there will be more items installed on the firewall before I'm through, but at this point I can't see anything else that MUST be done before hanging the engine in place. Once again, I'm considering riveting the firewall recess cover in place. I'll do some reading and think about it for a few days, then decide. I'm glad I didn't rivet it on a while back. This work today would have been a bear if I had done it earlier. Being able to reach right through made this job so easy.

<< Previous | Home | Next >>

Contact me: swayze "at" (replace "at" with the @ sign... no spaces... you know the deal)