The Barn Find Build

[Eric Page]

Thanks, Brett!

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I started off today by getting the second door stripped of fiberglass, Lexan and its latch bracket. Since I learned yesterday that a #28 drill does a nice job separating the head of a 1/8" rivet, that process went a lot faster. Removing the latch bracket was quicker as well, since I didn't bother with the grinder and went straight for a propane torch. Heating the bracket for about 20 seconds boiled the powder coat under it and it knocked off easily. Another two 1/4" holes and a couple of minutes' shaking got the rivet stems out, so I just need to strip the powder coat and they'll be ready for welding.

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Next on the agenda was getting the small parts of the control column mounting block out of the paint stripper and cleaning them up. When I dry fit the parts together I discovered that the spacer and mounting flange were different shapes, which wasn't going to satisfy my pilot brain, so I took them to the belt sander and with them clecoed together, sanded them to match one another. Some surface sanding and Scotch-Brite buffing got them ready for Alumiprep and Alodine (along with the hinge plates from the second door), then I pulled my first rivets and was rewarded with a rehabilitated assembly, ready for installation!

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Next up was getting the rudder torque tube assemblies riveted together. I ducked into town for a pot of bearing grease, smeared a modest amount on the inside and outside of the four PVC bearings, slid the tubes together and riveted them in place. Having no experience with such things, I was impressed with the smoothness added by the greased bearings; it's a noticeable difference from the dry parts. I just wonder how many years the grease will last before I'm drilling out 24 rivets to reapply.

I figured I might as well finish prepping the rudder tubes for installation, so I started work on the four outer bearings. Two of the originals were ruined in disassembly, so I had two fresh ones from Kitfox and was anticipating a lot of sanding to get a good fit. They must be smaller diameter than the originals, because they were a perfect fit right out of the box. I had to run a 1/4" drill through the bore to get the AN4 bolts through, and I shortened them slightly so I'd have a couple of threads outside the nut. The two usable old bearings were still too big, so I sanded their ODs for a smooth fit as well, and ended up with the whole mess ready to go into the fuselage.

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The final task for today revealed a bit of a problem. I removed the pin retaining straps from the control mixer, extracted the pins and pulled the assembly apart. The reason it was so stiff became immediately obvious: the -416L washers had been embedded in powder coat and the whole thing seemed to have been assembled without lubrication.

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I used the Scotch-Brite wheel to remove powder coat from the mating surfaces and to clean the pins (I'll use new ones for final assembly), then tried a dry-fit to check for better movement. I had a very difficult time getting it back together, as even with the powder coat removed there's only room for a washer at one end. I was only able to get the second washer installed my really gritting my teeth, and once together it didn't move much easier than it had in the first place.

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As you can see from the second photo, I need about half a light washer thickness, plus a few thou for grease. Is this lack of space a common problem in the control mixer? Are builders removing a few thou of steel from each of the four mating surfaces to make room?

Finally, a couple of questions about rudder pedals:

1. The flanges on all four of mine are not bent to a 90° angle, so the bolt and nut have to pull the flanges inward when they're tightened, creating drag on the pedal's rotation. Should I compress them in a vise to achieve a full 90° bend before assembly, or is this normal and acceptable?

2. The plastic bearings that run through the "tee" at the top of the pedal tube are very tight on the long AN3 bolt. I have to apply a lot of pressure to get the bolt to go through, so it doesn't rotate after assembly; the bearing isn't functioning as a bearing. Do I need to run a #12 (0.189") drill through them to open them up a tad, then apply grease?

[alexM]

A 3/16" reamer should be all that is needed there

Bend the pedals to 90 and mess with washers to shim as needed.

Looking great!

[PapuaPilot]

FYI the correct size drill bit for doing 1/8" rivets is a #30. It's best to use the #30 for removal of rivet heads too. Using a #28 drill bit would make a larger hole that could lead to loose rivets if you are using the holes again. I think you said you were going to have a welder fill these hole, so this isn't a problem in this case.

Use a reamer, avoid the temptation to use a drill bit to finish any bushings.

[alexM]

FYI the correct size drill bit for doing 1/8" rivets is a #30. It's best to use the #30 for removal of rivet heads too. Using a #28 drill bit would make a larger hole that could lead to loose rivets if you are using the holes again. I think you said you were going to have a welder fill these hole, so this isn't a problem in this case.

Use a reamer, avoid the temptation to use a drill bit to finish any bushings.

I was going to say that also. #30 is a bit you already have (a few of probably), will cut the rivet head off perfectly and have zero risk of oversizing the hole.

[Eric Page]

FYI the correct size drill bit for doing 1/8" rivets is a #30. It's best to use the #30 for removal of rivet heads too. Using a #28 drill bit would make a larger hole that could lead to loose rivets if you are using the holes again. I think you said you were going to have a welder fill these hole, so this isn't a problem in this case.

I was going to say that also. #30 is a bit you already have (a few of probably), will cut the rivet head off perfectly and have zero risk of oversizing the hole.

Sorry guys, I should have mentioned that I know better. In this case the material immediately under the rivet heads was scrap, so I just wanted to shear the heads off as quickly as possible. In any case, I didn't push the bit all the way through the hole. I pulled off the fiberglass and Lexan, used a punch to tap the stems through the holes, then shook them out through an enlarged hole.

Use a reamer, avoid the temptation to use a drill bit to finish any bushings.

Yeah, probably a good idea. That's what I get for posting updates at 10pm after standing in a cold garage all day.

[Eric Page]

Today didn't seem very productive. It was cold and foggy all day, and I couldn't seem to get the garage warm enough. Anyway, excuses...

I started with the control mixer. I put a fine sanding disc on the die grinder and slowly sanded the bearing surfaces I mentioned yesterday, just until I had room for the second -416L washer. Once that was done, I cleaned out the pin bores with acetone and cotton swabs, greased the new hardware and the bearing surfaces, and reassembled. It's not what I'd call floppy loose but there's not enough friction to hold up its own weight, and it's very smooth.

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Next I made a start on removing powder coat from the door frames. My compressor can't keep up with the die grinder long enough to do that job, so I put the Scotch-Brite pad in a cordless drill instead. I burned through one slightly used battery just doing one side of the bottom rail, so it's going to be quite a project. No pictures yet.

Finally, I went over to a friend's shop and pressed the four bearings out of the control column weldment. Two felt like they had fine sand in them and the other two felt like they had stones in them. Two had obvious corrosion on the races and seals (naturally, the side facing the table in this photo; sorry!), so SkyStar clearly didn't supply stainless bearings. This would be something to check on an annual inspection if you have a plane of this vintage.

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For anyone who's interested, they're marked 7R4 and they measure 0.250" ID x 0.625" OD x 0.196" wide. I found double-sealed stainless steel replacements at Fastenal. Part number 4126716.

I wish the control mixer used those instead of washers and pins...

[Eric Page]

The new bearings arrived a couple of days ago. They look great, feel smooth and (with some work on the control column) fit perfectly.

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The sockets they press into were a bit of a mess (sigh...). The first builder seems to have pressed the original bearings into place without removing any powder coat, which had two effects: 1. the bearings didn't seat all the way, so the control stick pivots had room to move fore and aft on the axle bolt; and, 2. the bearings raised a lip of powder coat and steel in the bottom of each socket, preventing the new bearings from seating all the way either.

I had to clean out that lip with a small burr in the Dremel tool, then I used a sanding drum to open the ID of each socket just a smidgen until the bearings had a good, snug fit. That allowed the bearing to seat fully, but there was still a little slop with the pivots installed. I found that putting a -10 washer in one side and a -10L in the other takes up the slack, giving a nice fit-up.

There was some missing powder coat in a couple of places and some rust had started in the area around the bearing sockets. I removed the rust and scuffed the surrounding areas with Scotch-Brite, then primed and painted. Here it is, with new hardware, ready to assemble tomorrow when the paint is completely dry.

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Over the last two days, I've spent several hours at a friend's shop, using his sandblasting pot to remove powder coat from my door frames. What a miserable job! It's been chilly and windy here lately, with on-and-off rain. Running a sandblaster in 45° weather, with a 20mph wind, is not my idea of a good time. By the time I was done I had gone through about 130# of sand, and I was freezing to death. Then I realized my car had been parked downwind the whole time, so I got to hose that off when I got home (remember the wind?). Anyway, the door frames are ready to go to the welder to have all of the rivet holes closed up.

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I tried to reinstall the control column mounting block in the fuselage yesterday, without success. The holes in the angle piece are too close to the bend radius to allow the bolt head to sit tight against the flat part of the "L" and the holes in the fuselage tabs are too close to the welds to allow any nut to thread on. Being as careful as possible, I still managed to butcher a nut. I also noticed that the tabs are welded to the fuselage tube too low, so there's a gap between the bottom surface of the "L" and the tabs.

I had a good think about the whole problem and decided to try something a bit different. I'm going to take up the space between the "L" and the tabs with washers, then I plan to secure the joints using 3/16" steel pop rivets instead of AN3 bolts. I can grind a little bit from one edge of each rivet head if needed to make it sit flat on the "L" and the mushroom underneath will form itself to the surface it finds, so the weld radius won't interfere.

Given that the whole assembly is very rigid already with just the two AN3 bolts through the rectangular section, I'm comfortable that the rivets will be adequate to provide whatever additional stability it requires. Can anyone think of a good reason not to do this, apart from, "the manual says to use a bolt?"

[jiott]

Eric, I am afraid that welding up all those rivet holes on the same side will cause massive warping of the frames unless some kind of precaution is taken.

[Eric Page]

I have that fear as well, Jim, but the guy I'm taking them to is an artist with a TIG torch, so I'm cautiously hopeful that he can overcome physics.

[alexM]

Eric, I am afraid that welding up all those rivet holes on the same side will cause massive warping of the frames unless some kind of precaution is taken.

I tend to agree with this. I only have my own doors to compare with, and I need to revisit them because I see the lexan material sitting smooth one day and serious puckers on other days - and that's just sitting on the shelf in my hangar. My doors have what looks like black 3M VHB which would cover all those "lightening holes" you have now.

I'm real sure VHB would be the wrong material to use since it would defeat the oversize holes in the lexan to allow for expansion/contraction, but some kind of gasket material still seems in order.

Anyway as long as you don't figure 8 one of those existing holes I think they could be ignored.

As for your control column piece I know the spot you're referring to all too well. Mine is just barely doable as is and it is obvious that a hole drilled even 1/16" off could result in the condition you're experiencing.