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Old 05-17-2017, 06:57 PM   #1
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1972 29' Ambassador
Austin , Texas
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U-Nuts for Shell On, Belly Pan On Floor Replacement

Hi All,

First post here. I'm finishing up gutting my 73 ambassador 29', and planning to do a shell on, belly pan on floor replacement.

Before you ask why...
-because the belly pan is in great shape and my new layout will utilize existing gas connections which are also in good shape and intact under the trailer.
-I have no reason to suspect frame rust, as the front and back sections of flooring revealed a frame with original paint on it in most places (except in the back where the rear end separation had occurred. Even so, I believe that I will be able to assess the frame with the floor off from above anyways. So my plan is to move forward with out removing the belly pan unless something reveals itself during floor removal.
-It's nice to insulate from the top, as gravity is working with you.

I've been agonizing over how to replace the floor without getting my hands up under there to hold/ tighten nuts in the C Channel. I'm ok with using self tapping screws for the interior connections to the frame, but absolutely agree with the conventional wisdom that something more substantial should be used for the connection from the C channel, through the plywood. It must be something that cannot back out.

Now for my plan. U-Nuts. Here is the process.

1. Remove floor, clear out channel of all bolts and intrusions.
2. Cut and dry fit new flooring.
3. While new flooring is dry fit, drill 7/16" holes every 6-8" in the C channel...Perhaps just enlarging the existing holes. Holes go through the top of the C channel, though the plywood, and through the bottom of the channel. where they occur, outriggers also receive a hole.
4. Remove flooring and clip on 3/8" u nuts to the bottom section of the channel OR to the top flange of the outrigger where necessary.
5 Reinstall flooring, ensuring that nothing shifts,and bolt through the holes drilled in step 3.
6. All bolts are self retained (basically like a built in lock washer) and the floor is firmly fixed to the Channel.

Additionally, For the front and back sections where the channel sits on top of the plywood, my rock climbing mind led me to think about t nuts, which could be installed using the same pre drilling process after dry fitting the flooring.

Images of the concept below. Haven't found anyone else doing this yet, so looking for feedback. Mostly I'm wondering about strength of this system vs a typical 3/8" nut and bolt system.

Thanks!
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Old 05-18-2017, 06:59 AM   #2
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Greetings from the Florida Panhandle

Welcome to the Forums. We're glad to have you with us. That is a very comprehensive first post, Your graphics are very impressive.

Brian
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Old 05-18-2017, 07:04 AM   #3
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1955 22' Safari
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Nice graphics

Never heard of U nuts before. Are they available in stainless?
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Old 05-18-2017, 07:26 AM   #4
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1972 29' Ambassador
Austin , Texas
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Stainless U-Nuts

I haven't seen them in stainless. Does it matter?

Thanks for the complements on graphics. I'm graduating with my second architecture degree soon, so I've been well trained in 3d modeling software. I wanted to make sure I could convince myself with the 3d model before I move on to the real deal.

I would love to hear from others about your impressions of this system. I have ordered the U-nuts and T-nuts and will do a write up as I get them installed.

Ben
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Old 05-18-2017, 07:50 AM   #5
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1973 21' Globetrotter
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Welcome to the forums!

Well, you don't have to remove your entire bellypan and gas lines in order to access the undersides of the outriggers where the bolts and nuts are. You just have to remove the lower belt-line trim, and then drill out the pop rivets that hold the banana wraps and curved edges of the belly skin in place. If you have the original belly skin, it is only something like .025" in thickness, so will easily flex down to give you the access you need to the outer bolts. Those are really great drawings in your posting above, but you likely could have done the job I just described in the time it took you to draw those drawings.

Another benefit of releasing the edges of the belly pan is that you can seal the bellypan-to-outer shell interface with vulkem when you are reinstalling so that it does not funnel water into your bellypan like it does today. Furthermore, you can seal every pop-rivet that holds the bellypan to the shell and the beltline trim over the interface. Right now there is likely no sealant on these rivets and any leakage goes straight into the end-grain of your subfloor.

There are also a couple of large, beefy screws in the front, right behind the hold-down-plate, and a few in the back that hold down the rear angled plate. I am not sure I would trust these U-nuts in such a critical place as these.

Do you intend to retrofit in grey water tanks? Will you do this from above while replacing the floor? Just another thing to think about.

Now, if you are looking for the easiest way to do a floor replacement, then build the gantries, and lift the shell. It may sound like a lot of work, but there are a lot of benefits that come out of this approach. You are probably overestimating the ease of dry-fitting your plywood into the c-channels along the sides. Most people, if they manage to get the wood in place one time, are happy to leave it there for good.

Now for more unsolicited advice, which I offer after years of personal experience: These trailer rebuilds are fertile ground for scope creep and analysis paralysis. You can spend weeks agonizing over decisions, looking for the "best" way to do something or the easiest. There are the tried and true techniques described over and over on these forums. If you find yourself reinventing the wheel, maybe you are just trading a few hours of grunt labor for a few days of research, computer modeling, and vacillation over the decision.

good luck!
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Old 05-18-2017, 08:35 AM   #6
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1972 29' Ambassador
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Thanks Belegedhel,

I hadn't considered the possibility of dropping just the outer section of belly pan. Seems like somewhere I read that there are maybe hidden rivets after I remove the belt line trim and the visible pop rivets. Did you encounter that? Best way to get them loose? On reassembly,

I'll be adding a VTS 22 gallon grey tank in the rear area between two frame members and will use a Natures Head for sewage. Grey water tank will drop in fine from above.

Like I said, graduating soon and will be moving to L.A. to start a job in August. We already have a backyard with electrical and water waiting for us there to park the Airstream and go full time. So, my timeline is tight and I do worry about scope creep. I think I'll just put my head down and pull off the edges of the belly pan. Still have the benefit of keeping the gas system intact and being able to drop insulation in from above.

Ben
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Old 05-18-2017, 08:42 AM   #7
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Stainless

I guess it is my marine background, but whenever possible I use stainless hardware. Who knows what may get wet in the future and most hardware I have removed looks somewhat rusty.
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Old 05-18-2017, 08:56 AM   #8
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Easy! Just slide a putty knife between the pieces you want yo separate. But, usually the hidden rivets connect the shell to the c-channels.

Also, gravity is not your friend when it comes to insulating your floor. You want the insulation to cling to the underside of the floor, not to lie in the bottom of the belly pan soaking up moisture.
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Old 05-18-2017, 09:47 AM   #9
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I like your idea, but one thing to think about is the bolt loosening. From the factory the bolts were bent after assembly so as not to back out. As others have said, I would drop the outside belly wrap for easy access to everything.
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Old 05-18-2017, 05:09 PM   #10
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I think T nuts is a great idea, wish I thought of that, but don't be afraid to open the belly pan. Sometimes we make more work for our selvs by trying to save some work, I know I have!
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Old 05-19-2017, 10:00 AM   #11
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Wilde

Welcome to Airforums. Glad to have another free thinker with us.

I like your idea of unuts. It would be nice if they were available in stainless but I just removed some rotted flooring from the rear bumper area of my Tradewind and the fasteners were rusty but at least intact. I would use ss bolts and lock washers or locktite.

Dan
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Old 05-23-2017, 05:43 AM   #12
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I'm in the process of replacing some rotted flooring and repairing some minor rust on the frame.

Given my current personal experience, I would say you should probably remove the belly skin regardless of which fasteners you use, just to thoroughly inspect the frame, make any repairs, and at paint it.

Your trailer is older than mine, and at minimum there will be rust on the frame that you will want to clean off and repaint it. This is the best time for you to do that work.

BTW the fasteners I will be using are the old school elevator bolts with self-locking nylon insert nuts.
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