Today was all about making an aluminum threaded grounding bolt insert for the firewall engine ground bolt.

Before I started that however, I pulled all the forms off the wing root heat shield mounting tabs.  I have some minor gaps on a few of them at the corner 90° bends, but besides that they came out really good.  My plan is to actually mount the #6 screw platenuts before I trim the mounting tabs, which I’ll do once I pull the engine off the bird (way easier then).  In addition, I’ll construct the forward heat shields and glass in those tabs once the engine is off as well. I’m just getting everything done as much as I can with the engine on, in my attempt to get the engine installed and engine compartment configured.

At some point in the recent past —I believe when I installed the throttle cable through the firewall— I realized that to remove the engine ground strap from the firewall in EZ fashion that I had the main ground bolt going the wrong way: inserted into the firewall towards the nose.  This meant every engine or ground strap removal required me getting a wrench up into the busy hell hole on one side while unthreading the ground bolt from the firewall side… too much hassle for my lazy self.

Now, at first I was scratching my head and just could not remember exactly why I didn’t just reverse the bolt (vs the big hole?) and add an extra nut to secure it to the firewall, then mount the grounding strap terminal with a final securing nut… then I finally remembered that for some reason in both the B&C instructions and Bob Nuckoll’s book “AeroElectric Connection” the grounding bolt went through a stack of washers, which were the thickness of the firewall, to secure the engine ground strap on the firewall side and the ‘forest of tabs’ on the hell hole side.

Thus, my plan is to replace that stack of washers by embedding a threaded port on the firewall to secure the 5/16-18 brass firewall ground bolt into, from hell hole side aft, without having to mess with any tools in hell hole once it is initially installed: meaning threading the bolt into the hell hole ground block (‘forest of tabs’) through the firewall threaded insert, once, and then be ready for the engine ground strap on the engine side [which can be taken on and off at will without needing hell hole access].

Yep, should all be EZ-PZ once complete.

The initial issue was the shoulder on the 5/16″ brass bolt that came with the forest of tabs, as with many bolts, would not allow me to thread the bolt into the firewall grounding point without a 1/2-inch or so sticking out.  I needed the threads to go up to the head of the bolt.  So I chucked up the bolt in my lathe (pic 1) and hit it with a 5/16-18 die to add some threads to that sucker (pic 2).

Ahhh, much better!  Now we have threads nearly all the way up to the head.

I then removed the bolt and chucked up a 9/16″ diameter 6061 rod into the lathe and hit it with a center drill to get a good starter hole for drilling.

I drilled into the end of the 6061 rod with a letter F (0.257″) drill bit ~1/2″ deep (pic 1) before tapping threads in the hole with a 5/16-18 tap (pic 2).

I then tested the newly formed threads with my brass grounding bolt.  Looking good so far!

It was now time to cut off the threaded nub at 0.38″, which is the approximate thickness of the firewall depth (it varied depending on what side of the hole I measured due to overlapping corner BID tapes).

Once the threaded firewall ground nub was removed, I roughed up the perimeter with the Dremel tool and tested the fit of my freshly cut threads… on both nub and bolt.

And test fit it into the firewall.  Again, so far so good!

I then prepregged 2 separate square plies of BID, wet those out and slathered up the firewall threaded grounding bung with flox (with Saran wrap in threads to protect it).

I then put the threaded grounding bung into the floxed firewall hole (pic 1) and laid up a ply of BID over top of it on the firewall side (pic 2).

I then laid up a ply of BID on the hell hole side over the firewall threaded grounding insert, and then peel plied it.

Finally, I then peel plied the firewall side layup as well.

Hopefully I’ve put the saga of the firewall grounding block AND the engine ground strap configuration to bed.

My last task of the evening was to add another fire block to the wing root at the aft end, where I scrounged around and found a thin scrap carbon fiber piece which I cut to make the small tabs (about an inch square) for the aft end of the wing root cowling flanges, for both sides. Yes, I realize that a lot of builders close these gaps up when glassing the wing root and/or when creating the cowling flanges… better late than never I guess!

Regardless of my lack of planning (wink) I used my leftover epoxy in the cup from my firewall grounding bolt threaded insert flox install/layups to whip up some flox and secure these minute tabs into place on the end of each set of wing root cowling mounting flanges, for both the left (pic 1) and right (pic 2) sides.

As par usual, I had planned on getting a lot more accomplished today… it’s amazing how much time these seemingly small tasks often take to complete.  Still, it’s done and I’ll continue to push forward.