Matt's RV-7 Project » Firewall Forward

Cowl corner flanges

Matt — Tue, 07 Sep 2010 00:44:29 +0000

While waiting to see if my epoxy will still cure properly, I drilled a bunch of #30 holes in between the existing rivet holes in the metal side flanges. When I later bond and rivet these parts to the lower cowl, the holes will help the epoxy get a good grip.

My epoxy turned out to still be good, so it's time for what will probably turn into several weeks of glass work. The first thing I want to do is build up some flanges on the inside of the lower cowl, at the outboard corners. Eventually these will be used for a single screw and nutplate per side, similar to this example. First I applied clear packing tape to the areas where I didn't want the epoxy to stick, and clecoed the two halves of the cowl together on the bench:

I laid up five plies of cloth from my box of scraps and stuck it to the inside of the cowl, after first cleaning the area of interest and wiping it down with epoxy.

This is how it looked after it cured, immediately after removing the top cowl:

I should be able to work with this. It needs to be cleaned up and trimmed/sanded to shape, of course. Then a bit of flox on the front to ensure a good fit, a few more plies on the back if it needs stiffening, and I'll have my flanges.

Horizontal cowl seam

Matt — Mon, 06 Sep 2010 02:09:06 +0000

I trimmed the upper edge of the bottom cowl along the lines I prevously marked. Then I spent about two hours sanding the cut edges straight and progressively taking off more material. You can't tell how it's going to fit until you have it on the airplane, fastened and/or taped into place, so it takes many repetitions of cowl-on, cowl-off, each time removing a few thousands with the long sanding block. The other thing that makes this frustrating is that the fit will change later once you fasten the top and bottom halves together, due to the way the cowl gets pulled inward along the horizontal split line. At this point, you're just shooting for a straight edge and a consistent gap, knowing that the spacing will change and need to be fixed again later.

At some point during this process, the aluminum pins that came with the side hinge material started to get kinked, as I knew they eventually would. The plans call for you to replace them with stainless steel wires, so I got out the torch and bent some new hinge pins.

Eventually I got to the point where I couldn't improve the fit any further, so I moved on to actually joining the top and bottom cowl halves. The plans specify more hinge material here, but I had that on my previous airplane and I had no end of trouble getting the long curved pins in and out. Instead, I intend to use quarter-turn fasteners and screws/nutplates just like I did along the firewall.

Since there is not a joggled flange molded into the lower cowl, I need to make my own. I gave serious thought to the idea of laying up some fiberglass flanges, but since it was a three-day weekend and I didn't have any of the glass tape I'd have wanted to use laying around, I decided to make aluminum flanges instead. These started out as 3"x30" pieces of 0.063" alclad, drilled for a double row of rivets and shaped to the proper contour with a rubber mallet.

After getting the shape right – another whole set of iterations – I drilled enough holes to let me cleco the flange plates securely to the bottom cowl. The upper fastener holes are spaced about 3 1/8" apart, starting 3/4" from each end of the flange.

Here you can see the cross section of the flanges. Compound curves in 2024-T3 aluminum are interesting.

After quadruple-checking that everything was fastened correctly, and with duct tape in strategic places to hold the cowl in alignment at the nose, I used the shop-light trick to drill through the upper cowl into the pilot holes in the flange.

The edges of the top cowl get pulled in once they're clecoed to the bottom cowl flange, so the gap between the top and bottom shrinks after you drill the fastener holes. So, it was another hour of try-and-fit work to get to the point where I ended up… a fairly straight seam, with a pretty consistent 0.020" gap for paint.

Big milestone… the two halves of the cowl are properly fitted and attached for the first time, even if it is just with clecoes:

Somewhere during all this, I noticed that there is only a finger-width of clearance between the alternator pulley and the inside of the bottom cowl. This is pretty typical for RV's… I will keep an eye on this, and maybe think about switching to a slightly smaller belt to let the pulley move inward.

I don't intend to undertake all the glass-finishing work at this stage, but I do need to carry out a few tasks with the gooey stuff. Since I haven't used the epoxy for a few years (!!!) I mixed up a test batch just to see if it's still good.

If this hardens by tomorrow, there will be more to report soon. If not, well, it's nice outside and I have the day off from work!

More lower cowl fitting

Matt — Mon, 30 Aug 2010 02:00:29 +0000

Not pictured in this update: several hours of tedious sanding and fitting! It's messy and fiddly and it doesn't photograph well. Just imagine lots of messing around with sanding blocks and dust getting everywhere and you'll get the idea. Anyway, the fit at the lower cowl to firewall junction is pretty good all the way around, and the bottom edges of the top cowl are straight and parallel.

I cut two pieces of 1/8" hinge material to attach the sides of the lower cowl:

After drilling a series of #40 holes with one-inch spacing, I put both halves of the cowl on the airplane and used a shop light to illuminate the holes from behind. It's pretty easy to locate holes in fiberglass this way, as long as you can work the light back behind what you're working on.

In between each pair of rivet holes, I drilled a 3/16" hole that will later help increase adhesion when the hinge is glued to the cowl.

I clecoed the cowl halves to the fuselage, carefully taped things where I wanted them to end up, and marked a line on the bottom cowl where it needs to be trimmed to fit against the top cowl.

I ran out of time to make these next cuts, but at least I have a nice line to work with next time I get a chance to do some work:

Lower cowl fitting

Matt — Mon, 16 Aug 2010 02:00:58 +0000

Now that I have the upper cowl fitting fairly well, it's time to start fitting the lower half. First I had to grind some relief notches into the outboard corners to clear the landing gear legs. This isn't spelled out in the plans but obviously has to be done in order to get the thing onto the airplane.

Duct tape helps keep things in place while you wrestle the lower cowl around. At this stage, the rear flanges are oversized and are overlapping the fuselage by around three-quarters of an inch. Note that I have the lower cowl tucked under the upper cowl – I found that things fit better around the front using that arrangement.

I used the now-familiar offset line method to transfer a cut line to the bottom of the lower cowl. Lacking any specific guidance on the matter, I chose to leave the cooling air exit ramp at its original length and make diagonal cuts along the sides to match the shortened underside of the cowl.

Several rounds of sanding and trial-fitting later, I had the bottom cowl edge fitting the fuselage well. I had previously drilled three #40 pilot holes in each mounting strip, which I transferred to the cowl using a strong light from behind.

Once the bottom edge fits well, you can mark and trim the sides. I used a shop light inside the cowl to double check the cut lines I laid out.

The lovely Mary appeared long enough to get her picture taken. Now that her residency is complete, she's promised to spend some time helping with the airplane project now and then, so you may start seeing her turn up in the occasional photo once again.

I got the initial cuts made and sanded straight, but I ran out of time to make it really fit well. It looks like it's pretty close, though. I left myself some little notes to remind me where to start sanding next time I get a chance to put in some airplane time.

Lower cowl mounting strips

Matt — Mon, 16 Aug 2010 01:00:38 +0000

Almost four years ago, I riveted hinges to the bottom firewall flange, as shown in the plans. But ever since then, I've felt conflicted about these hinges, since I've seen multiple RV's that have broken hinge eyes in this area, and I've read accounts from many other folks that have the same problem. It seems that some combination of airflow and exhaust pulses causes the underside of the cowl to get beat up more than the rest, and the aluminum hinge is perhaps not quite strong enough to put up with that amount of abuse over the life of the airplane.

The final straw was when I ran into an acquaintance who's just recently started flying his RV-6A… when I asked him how the airplane was behaving, he said "Great, except every time I fly it I find another hinge eye broken off the bottom of the cowl!" It wasn't very long after that before I found myself drilling the rivets out of the lower firewall flange, and cutting some solid mounting strips out of 0.063 alclad to replace them.

These got match drilled and dimpled to fit the holes that previously secured the hinges, and then I gave them a slight bend to follow the upward taper of the cowl.

I'll use either nutplates or camlocs here – I haven't decided. There should be just enough room to squeeze the rivets that will attach them to the firewall. I am keeping the hinges along the vertical sides of the lower cowl, since they don't seem to suffer from the same malady as the bottom ones (and also because the engine mount would have to come off to replace them!).

Upper cowl work

Matt — Mon, 16 Aug 2010 00:00:53 +0000

Between a long vacation, a week at Oshkosh, and hundred-degree temperatures in the garage, the airplane has been neglected for a little while. However, I have managed to make some progress on the upper cowl. This involved lots of iterative steps of sanding and fitting which don't make for good photos, but I'm fairly happy with how it's turned out.

I pulled the spinner out of storage and temporarily clamped it to its backplate so I could check the cowl-to-spinner transition. And also – I'm not going to lie – to look at how cool it looks with the spinner on there.

I shortened the paper spacer slightly, lowering the height of the upper cowl behind the spinner. This gave me a nice continuous contour between the upper surface of the cowl and the spinner, and it also helped shift things around so I could fix a small fit issue at the lower outboard corners of the top cowl. The engine will naturally sag a fraction of an inch as the rubber vibration isolators wear in, so I wanted to slightly bias the cowl fit to take this into account.

Top cowl fitting

Matt — Mon, 21 Jun 2010 03:00:40 +0000

No more putting it off… time to start fitting the top cowl. Much like with the canopy, the instructions are frustratingly vague (although at least with fiberglass, if you take off too much there is usually a way to put more back on). I have been reading everything I can find on the usual RV sites, soliciting advice from friends near and far, and generally just trying to get an idea of where to begin.

One trick I discovered during my research was the use of a book as a spacer to set the height of the front of the cowl. I thought that was so clever I stole it for myself. As a rule I generally hate defacing books, but in this case an old outdated copy of the FAR/AIM had to give its life for the greater good.

I duct taped half the book to the top of the engine case between the spinner and the fuel injection distributor, and added and subtracted pages until I had the thickness I needed.

Here you can see why this works… the book-chunks on top of the engine naturally assume a shape that's helpful for centering and positioning the cowl, which has a similar contour.

There are no trim lines molded into the cowl, so it's hard to tell if you have it level, or if it's even symmetrical as-delivered. I leveled the fuselage and then used my super cool new self-leveling laser to find the centerline of the cowl and check the relative lengths of the sides. It turned out that one side is about 3/8" taller than the other. I'll use this tool again when I'm finalizing the top/bottom cowl split line.

Before you fit the top cowl to the fuselage, you have to make the top and bottom cowls fit each other at the forward end. On my cowl, I had to cut about a quarter inch off the lower cowl at the outboard corners in order to get the two halves to mate around the spinner opening. This is one of those frustrating problems where you don't quite know where to start removing material, and you get paranoid about cutting off something you shouldn't. I stewed about this for quite some time, and eventually just decided to start cutting and hope for the best. It turned out fine… just go slow and it will be okay.

The fit gets progressively better as you sand off more material…

Turns out I do have a use for that wooden copy of the spinner backplate I made – measuring the circularity of the spinner opening on the cowl. I ended up making the opening about an eighth of an inch taller than it is wide… the extra space will be at the "chin" of the spinner opening. This will hopefully give the spinner some room to sag downward as the engine mounts break in, without making the spinner-to-cowl transition look ugly.

The inboard side of the left inlet turned out pretty well. A little filling and sanding later on will make this practically seamless.

The right side fits okay, but not great. I will have to do some more involved work to get this to match up, but it shouldn't be too bad.

I locked the two halves together with a single cleco on each side. Later on I'll put real screws and nutplates here, but for now it's okay to let things float a bit.

Checking for side-to-side alignment:

The plans call for an idea gap of 1/4" between the forward face of the cowl and the aft edge of the spinner backplate. I'm going to shoot for 3/16" to begin with, then sand the cowl edges until I get closer to 1/4". That should keep me from accidentally making the gap too wide. Here I've positioned the left side of the cowl so a 3/16" spacer just fits through the gap:

The right side is a different story… the gap is more like 5/16" here. I guess that's not a huge amount of variation for such a big piece of fiberglass. I will have to even this up with filler later on, but I have done that before on another airplane and it's no big deal.

I drew a reference line on the fuselage skin exactly two inches back from the forward edge:

…then taped the cowl to the top skin to keep it from walking around:

I clamped the forward face to the spinner backplate, with a 3/16" spacer on the left side and a stack of scrap pieces on the right side as required to fill the wider gap. The key thing here is to keep the cowl from shifting around, not to try to force the gap to be even.

I very carefully marked a line on the cowl, two inches forward of the reference line:

Then I got out my cutoff wheel and trimmed the cowl back to the line. Since I was doing this solo, I didn't get a picture of the process, so you'll have to be happy with this self-portrait.

It doesn't quite fit perfectly all the way around, but it's nothing that hours of tedious trimming and sanding won't fix.

Giant sanding block

Matt — Sun, 20 Jun 2010 17:00:18 +0000

I made this two-foot-long sanding board out of scrap wood I had laying around, the better to sand the long straight edges of the fiberglass cowl. It's just a dowel screwed to a very flat piece of MDF shelving material.

I filled the screw holes to keep the paper from getting punctured and torn.

I attached some 60 grit 3M paper using spray glue. Purple, Mary's favorite.

Top cowl mounting strip continued

Matt — Sun, 20 Jun 2010 02:00:56 +0000

More work on the top cowl mounting strip, to which the camlocs will eventually be attached. The spaces between adjacent camlocs need to be notched out so the remaining aluminum can be shaped to conform to the tapered cowl shape. I taped over the pilot holes for the camlocs – to keep myself from accidentally enlarging them prematurely! – and started each notch with a hole whose edge is about a quarter inch away from the end of the fuselage skin.

A lot of cutting, filing, and shaping later, here's the finished product. If you can get to this point in building the airplane, you should be able to make this part from raw stock with no problems, so I won't bore you with a detailed step-by-step.

I found that it was handy to have a variety of rotary files and scotchbrite wheels to chuck up in the drill press. And yes, you can cut all the notches on a small bandsaw if you plan carefully.

I will have to make finer adjustments later, but for now I just roughly bent the two most outboard tabs on each side to generally match the cowl shape. The easiest way to do this is to put it in a vise with padded jaws and wail on it with a rubber mallet.

The finished product clecoed in place, one of many trial-fittings it took to get to this point. You'll notice that I also trimmed down the side pieces that go below the bend at the upper longeron. I resisted the urge to put one more camloc down the side, since there needs to be a gap between the camloc strip and the side hinge in order to insert the hinge pin. Luckily I remembered now and not after finishing the top cowl!

Top cowl mounting strip

Matt — Mon, 14 Jun 2010 01:00:51 +0000

Wow, whatever I had really knocked me for a loop, but thanks to modern medicine I am mostly over it at last. Irregularly-scheduled blogging will now (sporadically) resume.

I pulled the forward top skin out of storage and clecoed it to the fuselage:

The cowl on my last airplane was attached with a combination of quarter-turn camlocs and piano hinge, instead of using hinges everywhere as the plans show. That seemed to work all right, so I've decided to use a similar setup on the current airplane project. The bottom cowl will be attached to the firewall with straight sections of piano hinge, and I'll use camlocs to attach the top cowl to the firewall and along the horizontal seam between the top and bottom cowl halves.

The camlocs need a metal backing strip attached to the fuselage, to which the camloc receptacles will be mounted. I cut a 3" strip off the end of a sheet of 0.062" alclad (gee, I really wish I had a big metal shear) and shaped it with hand pressure until it conformed to the curve of the firewall.

While clamping it carefully in place, I match drilled every other hole through the forward edge of the top skin and the firewall flange. If you plan it carefully, you can use a single piece with no compound curves that runs from a point just above the upper longeron all the way over to the corresponding spot on the opposite side of the firewall. Then you add small filler pieces below the firewall bend line, which I'll show in a bit.

To mark a trim line a fixed distance forward of the firewall flange, I drilled a 1/16" hole at a suitable location in a piece of scrap aluminum, stuck a pen through it, and dragged it along the mounting strip. Simple.

Lots of cutting and filing later, I had the mounting strip trimmed down to size.

It's hard to see here, but I've now marked and drilled #40 pilot holes where the camlocs will eventually go. You can also see one of the extra filler pieces that brings the line of fasteners all the way down to the horizontal cowl seam.

The instructions recommend a maximum spacing of 3.5"-4" between adjacent camlocs; however, since I remember seeing the cowl "pillowing" between fasteners at high speed on my last airplane, I decided to reduce the spacing to 3 1/8". That also helped keep the hole pattern away from the split between the main piece and the two smaller pieces.

Going in circles(?)

Matt — Tue, 01 Jun 2010 01:00:54 +0000

Bleargh. I have been sick for a week. The only thing I've had the energy to do is to trace a circle onto a piece of MDF and cut/sand it to make an accurate copy of the spinner backplate.

I'm not even sure why I did this. It seems like it could be useful for cowl fitting later on.

I'm going to go lay down now.

Oil door cutout

Matt — Mon, 17 May 2010 01:00:21 +0000

Well, there's no sense in avoiding it any longer… I have to start on the cowling sometime.

The first step is to mark and cut a hole in the top cowl for the oil door. I ignored the dimensions in the plans and cut a hole that's more of a square than a trapezoid, since I have in mind a different arrangement for the hinge and latch.

I spent some time sanding the edges smooth, because I'm fairly certain I'll be spending a lot of time reaching my tender arms through here.

Left side firewall passthrough

Matt — Sun, 21 Feb 2010 23:32:53 +0000

It's been pretty darn tough to get excited about working in the garage this winter… it hasn't been much above freezing down there since Thanksgiving or so. Still, I managed to finally do a little straightening up and miscellaneous work this weekend. I drilled holes for a second wiring passthrough on the left side of the firewall:

Here's the finished installation:

A view from the aft side of the firewall… all the transducer wires will go through that snap bushing, to the EIS box in the next rib bay.

Mounted dummy prop hub

Matt — Mon, 25 Jan 2010 01:01:59 +0000

I primed the spinner backplate and doubler, then riveted them together:

I had to buy these spacers from Van's for five bucks each, since the dimensions aren't shown on the plans (and I don't have a lathe either).

Using hardware-store bolts, I attached the spinner backplate to the dummy prop hub. The plans call for an AN960-416 here, but I think they must have meant -616 for a 3/8" bolt.

Another view:

Then I bolted the backplate and hub to the engine crankshaft. There's only enough clearance to turn the captive prop bolts about one flat at a time, so this part took a little while. Removing the prop to replace the alternator belt is going to be annoying.

I plugged the oil passage to keep moisture out of the inside of the crankshaft. The hole isn't threaded, but I managed to find a plug that fits somewhat snugly and used fuel lube to seal it.

Just for grins, I threw the top cowl on there just to see how it looks. Pretty cool:

Oh wait, I forgot about this part… I may need to finish the top skin before I can work on the upper cowl attachment stuff. Argh.

Back to wiring I guess…

Prop hub and spinner backplate

Matt — Sun, 17 Jan 2010 23:40:30 +0000

Another month without much work on the airplane. I have an excuse, though: it's been really cold in my garage!

I have a whole list of wiring chores left to complete, but I need to take a break from running wires through the fuselage before I go cross-eyed. I thought I might work on some firewall-forward tasks for a while, which will actually help with wiring in a roundabout way; before you can do much wiring forward of the firewall, you need to finish the baffles, which means you need to fit the cowl, which means you need to fit the spinner (the pointy bit on the very front of the airplane), which means you need to fit the propeller. Plenty of work to do there.

The plans suggest making a spacer out of wood or PVC to stand in for the propeller, but since the consequences of getting the cowl/spinner gap wrong are so ugly to contemplate, lots of folks end up using a real prop instead. However, instead of using my very expensive propeller as a fitting template, I had the idea to use a junked prop hub instead – that way there's no questions that the dimensions will be right.

I put out a call for an unairworthy Hartzell prop hub to use for fitting the cowl and spinner, and received the following hunk of grungy aluminum from a generous forum member (thanks, Joe!). It's only the rear half of the hub, but that's all I really need anyway. It is the older C2YK model instead of the C2YR that I have, but the critical dimensions for spinner-fitting are the same.

A little solvent and elbow grease cleaned it up pretty well. The key metric here is that we want to avoid leaving a big grimy smear on the clothing of any wives who happen to be walking through the garage. It just seems like everything will go so much more smoothly that way.

Digging into the pile of remaining airplane parts, I pulled out the rear spinner bulkhead and its associated doubler ring.

As I expected, the spinner bulkhead is noticeably dished, although it appears to be otherwise true and symmetrical. This appears to be a common occurrence, so I will probably just ignore it.

With some hardware-store bolts, I temporarily attached the doubler to the bulkhead:

…then drilled and clecoed the holes for the rivets that will eventually attach these two pieces together:

If you have a Hartzell prop like I do, you need to cut out a significant portion of the spinner bulkhead. I transferred the shape of the hole in the doubler ring:

Initial cuts were made with my air nibbler, which is great for this kind of work. Going slowly and carefully, I was able to get within 1/8" of the line.

Then it was time to file, file, file. The corners look lumpy here, but that's how the cutout needs to be in order to clear the prop hub.

More filing and finish work… I made plenty of aluminum shavings.

I spent a lot of time making sure all the edges were well and truly deburred.

I need to prime these parts before I rivet them, but it's way too cold and dark outside to do that, so it will have to wait on the weather.

Also: What are these?! I haven't looked at the plans for over a year, I think… ever since I got to the step that said something like "Now might be a good time to think about wiring and an electrical system, if you want one."

And speaking of the spinner backplate… I realized that this piece will literally be moving the fastest of any of the parts that I've fabricated: in addition to however fast the plane is moving through the air, the perimeter of the 13" diameter spinner will also be doing about another 100 mph as the crankshaft is turning at 2700 RPM. Neat.

Desiccant plugs

Matt — Sun, 10 May 2009 02:00:36 +0000

I put in a new set of desiccant plugs in the engine this weekend… after two years of service, the old ones were starting to lose their magic ability to dry out when I put them in the oven every few months.

Engine ground straps

Matt — Wed, 17 Sep 2008 02:36:21 +0000

I installed a braided ground strap between the back of the engine and the firewall ground block. This is the heavy-duty conductor that carries the hundreds of amps of starting current back to the negative terminal of the battery. It's fastened at both ends by adel clamps, so there's no strain on the actual terminal ends.

Closeup of the attachment to the engine… it's secured to one of the auxiliary alternator mounting studs with a nut and lockwasher, and is clamped to a neighboring stud.

Then, because I have equal love for both belt and suspenders, I installed a second one. If the first one were to somehow come off, the full current of the starter would flow back to the negative battery terminal through all the various sensor wires, control cables, etc… quickly turning them into smoke. Thus, a backup ground strap. This one goes between a sump bolt on the left side of the engine, over to a nutplate on the lower firewall. I couldn't find a way to provide strain relief for the ends, but it's only a backup.

There's so much stuff back there that it's hard to get a good picture, but you can sort of see where I put some heatshrink tubing over the second strap. There's no danger of a ground wire shorting to ground, obviously, but I wanted to guard against it accidentally rubbing against the engine mount. It doesn't touch it currently, but it's possible that it might flop around and tear up the powder coat.

Propeller arrived

Matt — Mon, 15 Sep 2008 02:00:54 +0000

Eight and a half weeks after placing the order, the propeller arrived. It is a thing of beauty. Heavy, expensive beauty.

The box is like eight feet long and weighs ninety pounds. With it sitting like this, there's barely any room to walk. Or store empty beer bottles.

I dragged it over by the wings, and raised it up on some boards so the cardboard doesn't soak up moisture from the concrete garage floor.

For reference, here's the current status of the fuselage:

And the rest of the garage… what a mess.

Ordered propeller

Matt — Thu, 10 Jul 2008 14:21:22 +0000

Last night I sent Van's a big chunk of change for a propeller – specifically, a Hartzell C2YR-1BFP/F7497-2. That's an aluminum constant-speed prop with a pair of 72" blended-airfoil blades specifically designed for the two-place RV's, and will look something like this.

I kind of wish I'd ordered the prop sooner, since it has a 12-16 week lead time and I find myself with the following chicken-and-egg scenario: In order to finish the FWF plumbing and wiring I kind of need to have the baffles in place, for which I need to have the cowl fitted first, which means I need the prop to fit the cowl. Actually I guess that's a whole series of chickens/eggs. Theoretically you can fit the cowl without the prop if you make some kind of spacer, but even Van's suggests you wait for the real thing to arrive if you have a constant-speed prop like I will. Apparently there's something different about how the spinner is mounted…? We'll see.

Meanwhile, I suppose I will continue to work on interior fuselage systems, when I have time.

Sniffle valve

Matt — Mon, 30 Jun 2008 01:42:20 +0000

Fuel-injected Lycoming engines can sometimes experience a phenomenon wherein excess fuel drips down the induction tubes and pools in the intake manifold after shutdown. This can be a problem, since it could cause a troublesome fire during the next start. To alleviate this, you install a simple little one-way check valve in the bottom of the intake, which lets the fuel drip out of the engine onto the ground. For whatever reason, this item is colloquially referred to as a sniffle valve.

Here's a bad picture of the one I bought from Airflow Performance for a few bucks – it's basically just a normal AN fitting that's been drilled out and had a ball bearing and retainer clip inserted. Reduced pressure inside the manifold causes the ball to be sucked up to close the valve when the engine is running, but it drops down and lets the fuel drip out when the engine is shut down.

I knew from previous reading that the sniffle valve is too close to the exhaust pipes on an IO-360 with horizontal induction, so I installed it in a 45-degree brass street elbow to clear the pipes. The exact orientation of the valve is said not to matter much, as long as the ball is free to drop when the airplane is at rest. I also attached a 1/4" hose barb to 1/4" AN flare adapter, similar to the one I used for the fuel pump drain line.

I ran another 1/4" aluminum line from the firewall up to the sniffle valve, using a length of 1/4" rubber fuel hose as a flexible coupling. The forward (engine) end of the aluminum tube is ever so slightly flared to give the clamps something to work with – using a real beading tool would have been best, but I can't afford one and this is not a critical application. The aft end of the drain tube is attached to the firewall flange with an adel clamp, just like its twin the fuel drain line.

One more of the million firewall forward tasks is complete… and with that, I'm off to try and make a dent in the beer and ice cream still left over from Saturday's party.