Ready to rivet after I disassemble one last time. This is a large post and will be the last on the Center Spar. I have reformatted and it is now in chronological order. Use this link Outer Spar Uprights install to go directly to the new information. I have added some navigation links. Drill Guide will bring you to the top of this page.

Drill Guide | Spar Spacers | Spar Uprights install | Outer Spar Uprights holes
Outer Spar Uprights install and Center Spar assembly

The Drill Guide

The Center Spar upgrade is going well. I believe in slow and steady. To drill out the holes for the 1/4" bolts a drill press is recommended, I have one, but remembered that I had purchased a drill guide to use on the 5/16" spar bolts originally. I never used it because it wouldn't work due to space constraints. Well, that's not an issue now.

If you don't have a drill press, here is an alternative. The Drill Guide was purchased from Woodcraft, part # #140876, $15. There might be a Woodcraft store in your area.

I use the drill bit to align the guide to the hole in the cap. Make sure it spins freely. Use sharp or new drill bits. I have a Drill Doctor that has paid for itself many times over. Start with the 1/4" drill to create a center. Then switch to a 3/16" drill and guide to drill out the hole. Slow and Steady. I seem to get off center anyway, not sure why. Switch to the 1/4" drill and guide and slowly remove the remaining material. I ended up with perfect holes.

Update: Figured out why the initial hole is off center. I was dropping the drill guides into the guide hole, the correct way is from the bottom up, there is a step on the drill guide. Bottom up locks it in place, top down allows for movement above the hole. I was able to compensate since I went slow and removed material. When using the final drill guide size, the 1/4" cap along with the guide kept it all lined up.

Almost messed up trying to use the drill guide the correct way, I was down to the last 3 holes. So, here is the best way, it has more steps, but it is more precise.

Set the Drill Guide using the final drill size, 1/4" or 5/16". Turn the bit over and twist by hand to mark the center. Remove the Drill Guide and center punch. Then I used a 3/32" for the pilot hole and worked my up to the final drill size. You end up with a small shoulder in the hole. Set the Drill Guide and drill to final size, there isn't much material left. Slow and steady.

The Drill Guide has notches and coincidentally fits perfectly between the clecos on some of the holes. This is a factory built Center Spar.

To drill through the top cap it was necessary to place it on top of the Spar web.

The Photo Guide recommends waiting to drill the 5/16" holes, I was comfortable addressing that with the drill guide. The holes were perfect and snug. I started with the 3/16", then 1/4" and finally the 5/16" drill. Slow and steady is important. I use a Milwaukee 18volt that has a very low gear and torque. I couple the design with operational aspects of light pressure and slow speed ;-)

The front Spar web with the cap on top.

With the cap inside and 5/16" spar bolt holes drilled.

This is my version of doing this part of the Upgrade. Pick your poison, as manufacturer you get to decide on the best path based on your skill set. You have to decide if setting up the Drill Press is easier. Mine has a 9"x9" table and I would need to add something to accommodate the 40" Center Spar. So, there is setup time to consider. Odds are either way probably takes the same amount of time.

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Drill Guide | Spar Spacers | Spar Uprights install | Outer Spar Uprights holes
Outer Spar Uprights install and Center Spar assembly

Spar Spacer Assemblies

I used RTV to attach the Penny Washers to the Spacer. Do Not use RTV between the Spacer and washers. Just a little dab will do ya!

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Spar Uprights Install

I installed the Spar Uprights, 6-ZU-2-4, a bit differently, had some alignment issues using the center line version in the photo guide. I decided to use a square to align the Spar Upright and then made reference marks to check if the 1/4" hole in the opposite end would be near the 10mm edge distance. My first attempt was at 9mm using the square, but farther off if I used the center line and holes I had drilled.

Here is the setup. I've aligned the Spar Upright using the square and make reference marks to check edge distance.

Next I checked the edge distance, it's a bit blurry, but I'm very close.

Clamped the Spar Upright and then I clamped the square to the upright edge, this will allow the 20mm shim to be flush. The Spar Upright is 19mm. Slide the shim under the Spar Upright and hold with finger pressure while you drill the top and bottom hole. Remove the square and finish drilling.

This is the other side representing the next step.

And it looked like this.

I'm going to use the Drill Guide to do the 1/4" holes. I'm pleased with the results, the bolts are snug and some need to be tapped in.

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Outer Spar Upright holes

Revelation 6-W-4-3, Center Spar Caps are created equal. Once you have aligned and drilled the Top Doubler, Seat Front Angle, Front Bottom angle and Rear Bottom angle, drilling additional holes for the Outer Spar Uprights is just a matter of stacking the parts and drilling as one unit. This ensures perfect alignment and eliminates the need to use long handled pliers to hold the spacer, etc. This will change pages 14-17 of the Center Spar Section Guide.

Use this information at your own risk. This is not ZAC approved and I have not had any discussions regarding this modified procedure. I am not an aeronut engineer or machinist, nor do I have any additional letters after my name. I am a Zenith Independent Manufacturer and submit this to be used or discarded, your choice.

Start by stacking in the correct order the Seat Front Angle, Back Spar Cap and Front Spar Cap. The shape of the Top Doubler will create clearance issues on the Drill Press, this will be back drilled later along with the Spar Webs. Insert bolts in existing holes to maintain alignment, clamp to the Drill Press. Start with a 3/32"/#40 drill bit, followed by 1/8"/#30, #20, 3/16" and finish with 1/4". You can start with the 1/4", but I like to remove material and work my way up. Use a cutting fluid like WD-40.

Notice that the next picture has both Spar Caps. This is when I had the revelation.

It should look like this.

Back drill the Top Doubler and Spar web. Stack the Spar Cap on top of the Top Doubler and use the Drill Guide. I don't have a 1/4" center punch, so after I align the Drill Guide using the appropriate drill bit, turn it over and twist by hand to mark the center. Remove the Drill Guide and use a center punch. I start with 3/32"/#40 bit and drill freehand. If your center punch is right on, you can work up to a 3/16" bit and leave a 1/32" shoulder to be removed by the 1/4" drill. Do Not allow the remaining material to guide the drill. My Milwaukee weighs 6.5 pounds and I need to lift to keep light pressure while I shave off the remaining material. Slow and steady.

Repeat for the bottom. Rear Bottom Angle, Spar Cap, Spar Cap and Front Bottom angle. Use some scrap wood to compensate for the flange.

And this takes us back to the first photo.

Use the same procedure I describe below to install the Outer Spar Uprights. I will probably attach the Spar Spacers using RTV to help keep things together while assembling.

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Drill Guide | Spar Spacers | Spar Uprights install | Outer Spar Uprights holes
Outer Spar Uprights install and Center Spar assembly

Outer Spar Uprights Install

The 1/4" holes are done and I need to back drill through the Spar Web and Uprights, 2 upper holes and 4 bottom holes. Marked the holes, this is upside down, the 4 marked holes are the bottom. The top outside holes to be drilled with a 3/32"/#40 only, these are the bottom holes on the Outer Spar Uprights.

I decided to pre-drill the remaining Spar Uprights, fronts are on the right.

Here is the Template, the "T" is the top rivet on the shim.

Then I realized that the Spar Uprights were different. The shim is the same length, but the back and front have a different hole postions. So, you will see the new marks.

To the Drill Press. I used a piece of 3/4" plywood to support the piece and held the upright by hand. Pre-drilled using 3/32"/#40.

Same process as the Spar Upright Install.

This is the lower Outer Spar Upright hole. I drilled to 3/32"/#40 and it will serve as the pilot hole when back drilling the Upright.

Spar Uprights installed and pre-drilled. Then drilled to #20.

Next I back drilled the Rear Bottom Angle 6ZU2-8. I clamped the angle to the spar web to get a tight fit before drilling.

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Time to assemble the Spar to back drill the Seat Front Angle to the Top Doubler, I had already back drilled through the Spar web/Front Seat Angle. First I used RTV to attach the Spar Spacers, this was a really good idea.

Before I back drilled, I made sure that I had the right gap for the wing spar. Used shims and a clamp, not shown.

I wasn't looking forward to back drilling all these holes or the additional holes at 20mm, so I drilled 8 holes to get the alignment.

Then I turned it over and set the rivet line for the Top Doubler and pre-drilled to 3/32"/#40.

Then I removed the Top Doubler and Seat Front angle, turned upside down, clecoed and back drilled the remaining holes.

And it looks like this. Set the 20mm rivet pitch and drilled using 3/32"/#40.

Reinstalled the Top Doubler and Seat Front Angle using the 8 back drilled holes and new Top Doubler rivet line holes. Back drilled the new 3/32"/#40 holes in the Seat Front Angle. Then drilled to #30 and then to #20.

I'm going to use the Bottom Doubler as an install template, but need to make sure I have the correct width for the wing root. Using the remnants of the wing jig with shims, I can set the correct spacing for the front Center Spar rivet line.

Drill Guide | Spar Spacers | Spar Uprights install | Outer Spar Uprights holes
Outer Spar Uprights install and Center Spar assembly

Use this information at your own risk. This is not ZAC approved and I have not had any discussions regarding this modified procedure. I am not an aeronut engineer or machinist, nor do I have any additional letters after my name. I am a Zenith Independent Manufacturer and submit this to be used or discarded, your choice.