No updates for the last three weeks, I was at Oshkosh for the entire week of the show. Lots of fun and very motivational, as always. I came back and started working on the F-704 center fuselage bulkhead assembly. There are a lot of small things that must be done to these bulkheads for them to be ready for installation. First step was drilling holes for the electrical conduits; not a hard task, but I double-checked the measurements before starting in with the Unibit. Came out fine!

The next step was to fit the left and right center section side angles to the bulkheads for match-drilling. Here they’re clecoed in place, but the close-tolerance bolts that will eventually hold the wing spars, muts be inserted so that the corresponding holes in the side angles are closely aligned.

More random parts prep on the firewall…dimpling the brake doubler and firewall itself. For some reason, I take a lot of satisfaction in a simple thing like fitting that little plate.

Dimpling the firewall…gotta make sure that the dimples go the right direction. I wouldn’t be the first builder to screw this up.

Dimpling done…looks good!

I also finished countersinking all the stiffeners on the firewall’s back side. No problems, but no pictures.

The firewall is disassembled and I’m deburring parts. I’ve said it before…if you have a Dremel tool, you gotta have a #500 abrasive wheel for it. It’s a 1″ Scotchbrite 7A Medium wheel, and is the absolute berries for deburring in close quarters. Can’t always find ’em in the big box hardware stores, I find mine in the local neighborhood hardware stores. I’m sure you can order them online, too.

The next task for the firewall is fitting the recess that accommodates stuff hanging off the back. To put it bluntly, the fit of this part sucks – especially around the top. Apparently this is a common problem, some builders use a little “manual adjustment” with hammers and wood to improve the fit, others just do the best they can and accept a couple of sloppy holes at the top of the recess. I tried the manual adjustment route and had no success so I just drilled it in assembly with the firewall, making sure to drill from the side with the stiffeners so that those structural stiffener holes would remain unmolested. The result is a couple of slightly-egged holes in the recess, but it’s not structural – so I’m not gonna worry about it.

That finishes up, for now, the match-drilling required on the firewall.

Over the last two weeks I’ve been match-drilling the firewall components and fabricating a couple of additional brackets that bridge the firewall and fuselage floor stiffeners. There isn’t a lot of margin for error on these, and it’s easy to wind up with insufficient edge distance when match-drilling them to the existing stiffeners and gussets you see in the picture. I wasn’t happy with the first attempt, so I fabricated and re-drilled another pair of brackets – they turned out much better.

One other bit of fun is fitting this small doubler that reinforces brake line penetrations through the firewall.

Match-drilling the plate isn’t the problem, but drilling through stainless steel is. Follow Van’s guidance in the plans book and you’ll be fine, j ust remember to use slow drill speeds, and use boelube or some other cutting fluid when you drill. Also, use a Unibit for any hole larger than #30 – works like a champ and is less likely to leave burrs on the stainless. I also made sure to use the doubler as a backing plate, drilling through the stainless first and then the doubler – that also helped avoid burrs.

I’m making slow progress on getting the lower wing skins riveted on; I’ve chosen to do that work with a partner, even though it slows me down. So while I’ve had no riveting help, I decided to get started on the fuselage – specifically, on the firewall.

It’s a relatively easy place to start, all that’s necessary is a bit of plans study and some time to identify all the stiffeners, brackets and gussets that are required. The fit on some of these parts leaves a little to be desired, however.

Here’s one example where diagonal stiffeners nest into gusset at the upper firewall corners, all of which rest under steel brackets. Unfortunately, that little cutout in the gusset isn’t deep enough to allow that stiffener to fit correctly with prepunched holes in the firewall – so some adjustment is required. Vans’ plans say to tweak the end of the stiffener to fit but that would be too easy, wouldn’t it? To preserve edge distance on that part, I pulled out my trust Dremel tool and routed out the cutouts in the gusset. Problem solved, and I’ve kept an extra 0.0001 percent margin in the structure. I feel much better.

There are also a couple of 0.025″ shims that ride on top of those stiffeners. They took a few extra minutes to fabricate, no big deal. There’s also an extra stiffener that must be fabricated from 6061 angle. The stiffener itself is easy…finding the stock you’re supposed to cut it from is the challenge. There are three pieces of angle stock that could be used, just don’t choose the one that’s already cut to length for another part!

After getting the minutia squared away, I clecoed the firewall together – a nice bit of visual progress!