So…what I’ve really been wanting to work on has been the firewall passthroughs for electrical stuff. Specifically, I wanted to get the fitting installed on the right side of the firewall so I could pull the wiring bundle through and start working on routing the ignition wiring back into the cabin.

When I looked at the fittings I bought, and read the instructions, I saw they specified #6 stainless screws for attachment, or stainless pop rivets. Given how tight things are inside the cabin where these fittings would attach, I really wanted to go the rivet route, and so I spent way too much time trying to find any 9/64” stainless pop rivets, to no avail. I also considered sizing up to 5/32” – a much more common size – but all the rivets I could find had too much grip length.

So I resigned myself to the screws. Before ordering from Spruce, I worked up a few other parts I needed, in a vain attempt to save on shipping. Critically, when it came time to place the order, I wasn’t sure the right screw length. But I thought to myself, “these are only going through two thin pieces of stainless, the shortest length is probably right.”

You can probably guess where this is headed. I got the attach holes for the fitting drilled into the firewall, grabbed one of my new screws, and…oh my, that’s way too short. The screws aren’t even long enough to get full engagement from a lock nut with no material in between, much less my two pieces of stainless and a washer.

Welp, time to move on to something else until I can grit my teeth and either order from Spruce again or just buy some hardware screws locally. Fortunately, I had another task I’d been thinking on – how to brace the baffles at the back of the engine. I’ve mentioned this a bit before – the baffle kit comes with a brace that attaches to the case bolts and holds the baffles in place, but I can’t use that because my ignition coil pack will be mounted there.

I’d ginned up a few possible ideas, none of which I liked much, but then a couple weeks ago I saw a photo with a brilliant solution. Instead of bracing from the front, using the case bolts, this person was bracing from the rear, using the top two bolts on the accessory case. The brace itself looked relatively simple to fabricate, and I’ve got some scrap .063 stock lying around that would be perfect for this.

The main challenge for fabricating and fitting this would be the work space – on the back of the engine, near the engine mount, and up at about my shoulder level with the plane on the gear. Visible here – with that annoying mount in the foreground – are the two bolts that will anchor the brace:

The first fun part if figuring out the spacing of those bolt holes, since obviously the brace has to be drilled to match them. Trying to measure this with all the stuff around was going to be a real pain, but after some more thinking, I came up with an idea: a simple shop-made measuring tool. I cut a couple strips of scrap, drilled 1/4” holes in one end of each, and a 1/8” hole in the other end. A cleco through the smaller hole acted as a pivot, and a third 1/8” hole in one piece anchored a final piece of stock. That final piece made a sort of “A” shape, allowing the tongs to be fixed in a relative position.

Then I just bolted the ends to the case, clamped my crossbar in place, and I had a template for those two holes:

Easy to measure this over on the bench, and…very nice of Lycoming to make the bolt hole spacing exactly three inches:

Using that measurement, I started working on a cardboard template. First step was to just locate the bolt holes and lay out the surrounding shape to clear the accessory case, then I did a trial fit to make sure that my bolt hole spacing and the surrounding shape would work out. Seems good to me:

That was the end of the night’s work. Next will be the more-fun task of roughly locating the two bends I’ll need to get this piece touching the baffle, and then will come the real fun of starting to replicate this thing in aluminum and getting it to fit right.

This post starts eerily last the last one…in the day or two after that last session, I spent an hour or two working on finishing the trimming of the rear side baffles. Never wrote anything up about it, since I’d probably be doing more the next day…or the next…or…well, you get the idea. Also, I apparently didn’t take a single picture while working on those baffles. Huh.

Anyway, here we are today, where I picked up on the task of trimming the rear baffles, which runs along the aft side of the engine. I’d previously drawn a first hack a trim line, way back when I put paper clips on the entire assembly, but I decided to fine-tune that a bit before making my first cuts. Since I had the rear side baffles done, I could use those to mark where the intersecting pieces should be cut at the corners. Basically I marked those junction points and sort of made a freehand hack at tweaking the trim lines. I was, of course, pretty conservative while doing this – as ever, it’s a lot harder to put material back than to take it off.

That first trim was, indeed, very conservative – I still wasn’t able to get the upper cowl all the way seated. Things were much batter after the second trim – I was able to get the cowl in place and the pins installed. A peek through the oil door shows all those paper clips:

After marking yet another trim line, then doing all the trimming/filing/deburring/etc, I reinstalled everything for one more test fit. Now we can see how the baffles have the specified 3/8” clearance to the cowl – this clearance will later be taken up by the rubber seal material:

So that takes care of most of the baffle trimming. Wait, that’s not all? Afraid not. There will be another vertical baffle section just behind the prop hub, which will also need to go through this whole paper clip exercise too. And once that’s done, I’ll probably paper clip the entire perimeter and do one final clearance check all around, just for good measure.

I don’t think I’m going to do that right now, though. I think it’s time to finally remove that forward upper skin…along with what I’m sure will be some really stubborn duct tape adhesive residue. That stuff’s been on there for a couple summers now, and there’s no way it’s going to come off without some liberal application of acetone. But once that’s off, I can resume work on electrical systems, which I’m kind of looking forward to as a change from fiberglass and baffle fiddling.

So…as I start typing this, it looks like I missed documenting some work a couple weeks ago. There was some time spent fabricating a conical gusset to close out the square corner I mentioned at the end of my previous post, but I guess I didn’t take any pictures. I’ll have to try and sneak them into a later post, I guess.

After doing that work, I almost let myself get distracted by the air intake snorkel, which has to meet up with the left inlet ramp…but then I remembered the main reason I’m in here, which is to get the baffles trimmed so I can work on FWF wiring. And so I moved on to working on that trimming work.

The basic deal is that the upper portion of the baffles need to be trimmed to provide 3/8-1/2” of clearance with the upper cowl. Later, rubberized seal material will be riveted in place, which will, well, seal against the cowl. This is a somewhat annoying task in any case, and more annoying in my case since I don’t have an RV-8-specific baffle kit. My baffles are intended for a side-by-side aircraft, which means there’s even more trimming to do than in a normal case.

Anyway, there’s a pretty universal approach to this trimming task, which involves putting a bazillion paper clips across the top of the baffles, then laying the cowl in place, then using the disturbed paper clips to see the contour of the cowl and work on trimming the baffles to fit. The pictures I’ve seen have people doing the entirety of the baffles at once, but this seemed overcomplicated to me, especially for the initial trim.

So I started with just the forward side baffles. These need the most trimming of anything; they need to match the curve of the upper inlet ramps, which means 1) removing a lot of material and 2) matching a surface that’s way more curved than the rest of the cowl.

Here’s a pic looking in the front of the cowl from my first fitting attempt. You can see that the top of the baffle is straight, unlike the curved ramp, and as a result only a small area of the paper clips have been pushed down. You might also notice the huge amount of open space to the right, between the cowl halves, The upper cowl is nowhere close to sitting in place:

After a couple iterations of trimming those forward side baffles, I decided that having hacked out most of that curved ramp area would make it OK to try fitting the whole thing. So I installed all the baffles and then put paper clips all the way around. In case you’re wondering, there are 190 of them. Yes, I counted:

The really fun part is that 15 minutes after taking that picture, I removed all those clips again, after marking the first conservative cut line all around… Also, because the cowl was back to not being anywhere near sitting in the right place, and especially because that made it hard to get the orientation right, I knew these cuts wouldn’t get me far.

So after this iteration, I went back to doing a sort of piecemeal fitting, starting with getting those forward side baffles trimmed to the proper length (as opposed to “kinda close” like before). Below you can (hopefully) see the little hash marks I made at each paper clip location; this is the beginning of laying out the cut line:

A nice little game of “connect the dots” results in a trim line:

I think it was after cutting to that line – or maybe there was one more iteration, I can hardly keep track – I was finally able to get the upper cowl in place. Peeking in the inlet, we can see that there’s still more trimming to do; the gap at the very front of the inlet is good, but where it curves up, it’s still too close:

I did decide that this was the right time to add the rear side baffles into the mix. The left rear side baffle shows the contrast between the trimmed and untrimmed state:

I did go ahead and get the initial trim done on those rear side baffles, but that was when I called it a night. I’d done enough repeated installation and removal of paper clips for one day.

Next up I’ll get those rear side baffles trimmed to where the cowl will sit in place, then I’ll move on to the rear baffles…and after that I guess I’ll do paper clips all around again so I can get the final trim done.

The baffle journey continues! The overall theme here was to refine the interface between the inlet ramps, the forward side baffles, and the cowl inlets. As seen below, as provided, the ramps sit well below the lower portion of the inlet:

This isn’t what we want. We need those ramps to fit closely to the inlets, so they can eventually seal together. So I got to get even more practice taking the cowl on and off while tweaking the bends of both ramps. At least I was able to bend the ramps in place, instead of having to remove and reinstall them repeatedly; that at least reduced the tedium a bit. After a few iterations, I had both ramps resting gently against the bottoms of the inlets.

This led to the next tweak: since the inlets aren’t perfectly flat, we need more bending. The idea is to mark the spot where the ramps contact the inlets near the spinner, and draw a diagonal line outboard. Then the ramp gets bent along this angle, until the outboard portion of the ramp sits up nicely against the inlet as well. Unfortunately this did require taking the ramps on and off a few times (along with the cowl, of course).

The result is a nice close fit across most of the width of the inlet:

Next up, the ramps and side baffles need to be trimmed. They can’t actually overlap the inlets as seen above; if they did, then installing the cowl would require starting an inch or so forward of the final position and sliding it backwards. That can’t work once the prop is in place.

Instead, the ramps are trimmed until they stop just short of the aft edge of the inlets. This will allow the cowl to be lifted vertically into place, so it can work within the space behind the prop spinner. Eventually there will be flexible material added to the cowl, which will seal against the ramps…but that’s later:

With that trial-and-error work done, the next step was to fabricate a support bracket for the left ramp. This will prevent the forward board edge of the ramp from being pushed down by high-pressure air entering the cowl. The bracket consists of a flat plate that screws into the engine block and a thick angle to tie that plate into the ramp. Both pieces are precut, but the plate needs to be trimmed to fit. I ended up cutting off about half an inch or so to get it to clear the ramp.

Then the plate gets screwed into place, the angle is clamped to the ramp and against the plate, and the four holes get match-drilling. Here are the two pieces prior to drilling:

Then, after demurring and such, they get riveted together. The angle also needs to be drilled to the ramp. Curiously, the plan images show the angle having three prepunched holes in this spot, but my angle had none. The instructions say to only rivet the two outboard holes, and not the middle one – I think it ends up being in common with another attach angle that will be added later. So I just laid out and drilled the two outboard holes for now – I’ll figure out the middle one later.

Here’s the support angle after drilling the ramp attach holes:

The next step would be to rivet the ramp assemblies together, but I decided it was a bit late for riveting (these will require use of the rivet gun). Instead I jumped ahead to adding some bends to the forward side baffles. Previously, after finishing all that tweaking, I traced the edge of each ramp on the intersecting side baffle. These lines then are used to bend the lower flanges inboard; this bend provides some more support for the forward outboard corner of each ramp.

The instructions sort of gloss over the details here, but I had to do some trimming of both pieces to accommodate the bends. One of them I was able to do on my little bending brake, which was nice, but the other one I had to do the old manual way – clamping it with a piece of wood and beating the crap out of it with a hammer.

Once the bends were done, I remounted the side baffles on the engine; they’ll be ready to receive the ramps again once I get those assemblies riveted together:

Next session will involve some fun scratch metal working, as I address those square corners between the ramps and the side baffles.

Not much in the way of shiny content today. Mostly I’ve been working on getting those tapered cylinder extensions fitted in place and drilled for rivets. Part of what made this fun is that for the most part, there’s just no good way to clamp the extensions to the adjacent baffles, especially with everything installed on the engine – which is what I wanted, so I could ensure the extensions laid across the cylinders properly.

The one exception to this was the #1 baffle, where I was able to get a cleco clamp through the large hole for the prop governor oil line. I wasn’t so lucky with #2, but that’s still at the front of the engine, where there’s decent access, so it wasn’t so bad. #3 and #4 were quite a bit more challenging, since I had to work around the engine mount. I ended up using some creative methods involving scrap wood and popsicle sticks to sort of hold things in place while I marked one hole at a time, then drilled it on the bench, then went back to mark the second, and so on.

Here are a couple photos of the #1 and #2 extensions in place; the extensions are the little wedge-shaped pieces just barely visible near the cylinder bases I didn’t even bother trying to get similar photos of the other two, because 1) it would have been a lot tougher to frame and 2) I was more in a “just get it done and move on” mode.

With those in place, I went back to the instructions and started…following them (a rare luxury at this stage of the build). The ensuing work was just getting the two inlet ramps baffle assemblies deburred and match-drilled as needed. There were a couple pieces to get riveted together, but for the most part everything is staying clecoed for the time being.

So those ramps are back in place, and I have the lower cowl hung again right now. The next step will be tweaking and trimming the baffle pieces in this area so they’re in the right spot relative to the cowl inlets. There’s also some fun to be had up here to get things fitting nicely to the rounded corners of those inlets…stand by for fabricating some fun conical aluminum pieces…