More cowl cleanup, hinge riveting

I’m back with more cowl updates, after a May that included vacation followed by all sorts of annoying adult issues, including a broken-down car, a broken-down home AC unit, and that one nasty storm we got to enjoy. But I’ve been back at work this past week, and today I ended up with some nice milestones taken care of.

Earlier this week, I picked back up on sanding down that cowl extension. Previously, I’d gotten rid of the extra protruding material, but the thickness in that area was preventing the upper cowl from fitting in place. So I spent some time working on that area and getting the thickness back in line. To do this, I set my calipers to the “factory” thickness, and used that as a sort of go/nogo gauge as I sanded excess material from inside the cowl. Once all that was cleaned up, I rehung the cowl again and verified that I could get all the hinge pins through.

At that point, I felt like there were no remaining excuses for not going ahead and getting all the various hines riveted in place. So I started working on prepping the holes, starting with a whole bunch of countersinking. Originally I was hoping to epoxy and rivet the hinges in place yesterday but then I did the wrong thing…I went online to get advice on this procedure. Before long I was swimming in opinions: Normal epoxy isn’t OK here, I should use special flexible stuff! Or maybe just Proseal! Also the normal rivets are too hard, they’ll crush the fiberglass, there are special soft rivets! And on and on it went…

Here’s the thing, the -8 plans and manual don’t even say anything about attaching the hinge pieces. Nothing about epoxy, they just mention drilling rivet holes and I assume after that they expect you to just rivet the things. Even my idea of adding epoxy for reinforcement is going the extra mile, but as per usual, I got all tied up in going with the right extra mile.

Anyway, I eventually mentally slapped myself in the face a few times and decides to just go with an epoxy/flox mixture. So today I got to work with the real prep – sanding down the mating surfaces (especially on the smooth fiberglass) for better adhesion and cleaning everything up. Well, the adhesion promotion issue brought forth another concern – the firewall joint hinges all have shims underneath them. Should I epoxy between each layer? I decided that was overkill, and instead went with a “flox rivet” technique. This is where you drill large-diameter holes in the hinges, and fill those holes with a thick flox putty, this providing some extra mechanical bond. So I also made those holes in the firewall hinges and then had to deburr those as well.

Finally, it was time for the fun. I mixed up a thick but still somewhat runny flox mixture – I want to have some bulk for adhesion, but I don’t want the stuff so thick that it doesn’t want to squeeze out. Next, I used a brush to paint the mixture into the mating areas in the lower cowl, then I got the hinges clecoed in place.

Now, originally my intent was to just cleco the stuff together and let the epoxy cure a bit, then come back and rivet. But i noticed pretty quickly that my epoxy squeeze-out was filling my countersinks, which would mean having to redo them, which seemed annoying. So I decided on the spot to go ahead and rivet while the epoxy was wet. Fortunately I’d already determined the rivets I needed and had them laid out, so this wasn’t a total scramble. It was still a sorta messy experience, though, and there was lots of tool cleanup afterwards.

I also discovered that some of my countersinks on the cowl split line were a little too shallow, leaving the rivets proud. Normally this would be a show-stopper, but in this case I went ahead and set them. I’ll come back later and sand them down, and eventually the whole cowl will be getting a skim coat of epoxy and primer that will smooth everything out. But I’m definitely going to recheck the countersinks before I repeat this procedure on the upper cowl.

So at long last, I have a lower cowl that’s entirely devoid of clecos:

I guess tomorrow or Tuesday I’ll get the upper cowl assembled, and then I can look forward to the fun of working on finalizing all the gaps everywhere. That’ll be followed by the real fun of getting back to work on matching up the spinner and forward corners, which I left off on so long ago…

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