Category Archives: countersink

Tank attach brackets

I finished up countersinking the fuel cap brackets and moved on to the tank attach brackets. These have a bearing, some shims, and three different kinds of nutplates that need to be attached.

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As always, first I had to find everything.  I had put the shims by the brackets themselves when I was hunting up everything at the start of the chapter, but it took a while to dig out all the nutplates.  The MS21051-L08’s in particular were a bear to find.  They are in bag #3015.  They are the only nutplates in there.  The spreadsheet of parts was very helpful! I separated the shims on the bandsaw and carefully deburred the holes (the instructions note to do a good job with this, particularly on the #8 screw holes).

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The shims have to be trimmed to match the ends of the attache bracket.  The narrow shim in particular has a very slim edge clearance to the outer nutplate hole.  I carefully sanded them down on my sanding disk to get the a close clearance.

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I marked the “primer line” on the attach brackets and started clecoing the nutplates in as I found them.  I’m substituting K1100-08 for the K1100-08D because I’m going to use “oops” rivets instead of dimpling the tiny shims.  I’m afraid that the shims will warp badly in the dimpler.  The hole on the end, in particular, is way too close to the edge for my to think about dimpling.

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I filed the cut marks off of the brackets before running them through the Scotch  Brite wheel.  The initial edge was a little sharp.  So I donated a few more drops of “Blood, toil, tears, and sweat” to the project.

The RV-12 was feeling a bit neglected.  The early low clouds had finally risen.  My son Ryan came down for the afternoon, so we fired up 3EN and did some pattern work.  He is getting ready to start his flying lessons, so I let him do one of the takeoffs and one of the landings.  He did a great job!

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I primed up the external parts of the brackets and shims and countersunk the brackets.  The picture shows a test fit.  The bracket lays tight to the skin and the shims have sufficient clearance.

2017-07-01 16.10.12

The final step for the day was to rivet the flange bearings onto the brackets.  This is where I really like working both right and left together.  I’m able to make sure I’ve got everything set up correctly.  The nice mirror image gives me some assurance that dyslexia didn’t bite me here.  I don’t show the riveting, but it came out pretty nice.

I have a few more things to finish up before diving into the Proseal.  I have to rivet some nutplates to the shims and then the shims (with more nutplates) to the brackets. I’m using the NAS 1097 rivet trick here rather than dimpling to avoid warping the shims.  I got the light countersinks done, but then had to jet to meet my wife for BBQ at Killen’s.

So many holes…


I didn’t fully appreciate how long the countersinking on the spars would take!  There are SOOOO many holes.  116 nut plate holes for mounting the fuel tanks, nut plate holes for mounting access plates, and 300 skin attach points — for EACH WING.

I first countersank the nut plate holes as called out in the plans.  You are trying to remove just enough material to get a AN426-3 rivet to sit flush.  I checked each countersink as I went along.  It felt like it was going fast, but then I turned around and saw how many holes were left to go!

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Luckily my son came out to help.

He did a great job on the skin counter sinks for the very visible left upper side.

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I made a test dimple in a scrap piece of .032 aluminum to make sure that the skins would eventually sit flush.  You can see it in the close up below.

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After finishing the top left, my son took off (can’t spend too much time with the old man in a cold garage after all!).  So I was on my own to finish the countersinking for the left bottom.  That was it for the day.  No build time on Sunday because I actually went flying instead!

With the left spar fully drilled, I hoped to move on to riveting the nut plates so I could finish Section 13.  I decided that it would be better to finish drilling and countersinking the right spar to keep the wing builds roughly in sync.

I was building on Friday (normally my “honey-do” day since I’m only working 4 days a week as a consultant) because I’m headed off to Houston to visit my brother.  I woke up early with dreams of hitting the shop at 8 and working all day.

Didn’t happen!

My wife woke up and I ended up doing a couple of small honey-do items anyway 🙂

Then I had to mail off a package and fill the minivan with gas before the trip.  The first station was closed, so I went directly to the post office instead.  A huge line for mid-morning Friday!  Then off to an alternate station to fill up (but no left turns allowed to exit).  I finally dragged myself in to the shop only to discover that I left my heat gun at home.

My shop gets really cold.  And it is “power” challenged (one shaky 15 amp circuit shared with the rental house).  And if the power blows, it takes an hour to get it back on again (breaker panel in locked basement only accessible by landlord).  So, I have to baby my compressor to get it started by preheating the block.  So, another 50 minutes was spent heading home to find it and bring it back.

Then my son called to see if I wanted to grab lunch with him between classes.  So we had a quick lunch, but at least I gained another set of hands.  I finally hit the shop around noon.

The j-channel was already set up on the right spar.  It went much faster with a second pair of hands to put the clecos in.  With that done, my son headed back to class and left me alone with my countersinking.  At least that’s all done now.  3 build sessions to take two expensive spars and to transform them into something that looks almost identical to what I started with.  Sigh.  At least next time, I can actually do some riveting (before finishing the final countersinks into the nut plate holes).


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