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File this under stuff that probably didn't need to be done, but I'm doing anyways.

The original tub floor on my 90 was pretty beat up after 31 years. Against better judgement, I decided to replace it.

Step 0:

Acquired 36"x48" 6061-T6 0.125" thick aluminum sheet for the replacement tub floor. Ordered three new rear floor tub support top hats. Ordered stainless steel rear door thresh. These parts were from YRM.

Step 1:

Tear out the old floor. Every Defender seems to be constructed differently based on the year. In my truck (one of the earliest 90's) There were 18 button-head magnagrip fasteners holding the front of the tub floor. These are steel and are extremely difficult to remove without a grinder.

The rear tub thresh is held in place by 8 button-head magnagrips and then connected to the body of the tub by 5 flush-head magnagrip fasteners.

As they are steel, chisels do nothing to these fasteners. These fasteners will destroy chisels, actually.

Removing these magnagrip fasteners proved to be the most difficult part of the job so far. It took me literally hours plus using a variety of tools, including a two angle grinders, a bolt-cutter, a dremel with a cutoff wheel, etc.

Step 3:

Removing the tub floor can be accomplished in one of two ways:

1. The difficult way - peel up the tub floor from the thresh, and using a prybar, pop the spot welds on the floor to old top hats, eventually removing the entire floor.

2. The easy way - remove the old thresh by cutting out the 5 flush-head magnagrip fasteners in step 2.

Guess which way I took.

Step 4:

Clean as much as you can. I used a 120 grit nylon brush attached to a drill, which got rid of most of the dirt, dust, and oxidation on the original aluminum.

Step 5:

1. Using clecos, replace the thresh on the rear tub frame.
2. Place the three new top hats on the front tub frame.
3. Using clamps, clamp the new top hats to the thresh.
4. Mark the front holes on the top hat with permanent marker.
5. Mark the rear holes on the top hat with permanent marker.
6. Write "L", "M", and "R" on the top hats to signify which is on which side. As the Defender was made by hand, the holes differ for each top hat slightly and therefore you have to keep track of which top hat goes where.
7. Remove the top hats from the tub and place on a piece of wood. Since the new YRM top hats are very soft aluminum alloy (most likely to make bending easier) using a punch, punch the holes you marked with permanent marker. The punch will make a deep indentation, making precise drilling easier.
8. Drill the holes using a 1/8" drill bit, then move up to a 3/16" drill bit for the rivet.
 

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Discussion Starter #2 (Edited)
step 6:

place the drilled top hats back on the tub and fix in place with clecos from the underside of the truck.

Once cleco'd in place, place the new tub floor on top of the top hats, move to the correct position, and from underneath the tub, using permanent marker, mark the centers of the existing holes at the front and rear of the tub.

Do not attempt to drill the holes along the sides of the tub at this time.

step 7:

Remove the new tub floor and place on a pair of sawhorses. Using a punch, 1/8" drill, and 3/16" drill, drill out the holes you marked on the front and rear of the tub.

step 8:

Remove the top hats and thresh from the tub and place on the new tub floor (on the sawhorses.) Using 12 clecos, attach all these components together. There should be 6 holes on the rear of the new tub floor and 16 on the front of the new tub floor.

step 9:

Test fit by placing the new tub floor assembly back onto the old tub. The holes you drilled should line up. One at a time, remove and replace each of the 12 clecos.

Add 10 more clecos to the front of the tub, and add 5 clecos to the rear of the tub thresh to complete the test fit.

That's where I stand today.

------ Follow up post added August 23rd, 2015 10:24 PM ------

Just a note: I tried TIG welding the floor to the top hats but failed. I did not have a sufficient method to tightly clamp the top hats to the tub floor while welding and experienced heat expansion on the tub floor with pull-away. After this occurred, I have decided not to fasten the top hats to the new tub floor for the time being since I don't want to ruin the aluminum components.

I am investigating:

1. Whether the top hats actually have to be fastened to the tub floor anywhere other than the front and rear of the floor. My suspicion based on the design is no, but if someone else knows for sure, please let me know.

2. Whether the top hats should be spot welded to the tub floor as per the original parts.
 

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Discussion Starter #3
In the meantime, take three 3 steel tub floor supports, galvanized and weathered, and paint with primer of choice. I used SW pro-cryl self-crosslinking acrylic primer with zinc phosphate.

I also replaced the rubber pad rivets with countersunk stainless m6 screws and nuts.

I am hoping the combination of sealing primer and galvanized steel will protect against galvanic contact corrosion of the new top hats.
 

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I appreciate the write up Ed. It looks really good.

That's pretty much what I did but I wish mine had been a little more...for lack of a better word, uncomplicated. One of the POs (I'm 3rd owner) of my 82 Siii had replaced the floor with ungalvanized diamond plate. Guess what happened?? Galvanic corrosion ate up the lip the floor sits on and corroded about 1/2in up the sides of the wheel boxes. It was really a mess...wrecked havoc on my poor tub. You could call it a worse case scenario...and an undertaking and learning experience... :)
 

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Just a note: I tried TIG welding the floor to the top hats but failed. I did not have a sufficient method to tightly clamp the top hats to the tub floor while welding and experienced heat expansion on the tub floor with pull-away. After this occurred, I have decided not to fasten the top hats to the new tub floor for the time being since I don't want to ruin the aluminum components.

I am investigating:

1. Whether the top hats actually have to be fastened to the tub floor anywhere other than the front and rear of the floor. My suspicion based on the design is no, but if someone else knows for sure, please let me know.

2. Whether the top hats should be spot welded to the tub floor as per the original parts.
I am right at this stage on my floor replacement. I am on my 4th style of rigging the top hats and floor to stay in place. I will get a great weld pass every now and then but some of them look pretty boogery.

The center top hat I might just pop rivet as I will not get a clamp to keep it down pretty soon. What are your thoughts on that?
 

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x2 on the solid rivets. If you absolutely insist on using a blind rivet, at least go with a sealed unit for slightly more strength. Not much of a comparison between them and solid rivets, though.

Floor looks good, Ed!

-Ash
 

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Discussion Starter #10
Thanks for the advice guys. I am also glad to see Don doing the same.

Where I am so far. Next is to cleco the 5 front thresh holes that are missing in the picture (can't do this and still have the door close) Then mark the side holes from underneath the tub to drill.

Based on what I have seen so far, with a 1/8" thick floor panel, it doesn't seem necessary for the tub floor to be welded to the top hats. In fact, I drove around with the 1/8" tub floor supported only by the rear and front of the tub, no clecos, no top-hats, and it could easily support the weight of all my tools and other junk. Impressive.

I'm investigating which rivets to use. I'm also contemplating the use of ultra-corrosion resistant 10-32 fasteners instead of rivets.
 

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Discussion Starter #11
Also I have circled the one place where i attempted a weld. There is a small "bump" on the top side of the sheet and a blow-through on the top hat. This is due to thermal expansion of the sheet during the weld and incomplete clamping on the two structures.

I terminate the welding procedure upon first sign of this and do not plan to continue to weld or otherwise fasten the top hats to the tub floor at this time.

Maybe Don and I can share notes after a year and see what the differences are in actual use.
 

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I was just curious if you were going to do any corrosion prevention prior to assembly. I would suggest a thin coat of zinc chromate. Simple and inexpensive. NAPA usually caries the stuff and it's green. One can would do this project. I'd hate to see a good job go bad.
 

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i have been thinking about this. i am too scared to use anything with hydrofluoric acid (aka fluorozirconate, fluorotitanate, and trivalent and hexavalent chrome with straight hf) and am currently investigating moeller zinc chromate as you suggest.

how in your experience has zinc chromate held up?
 

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I find it is the best and easy to apply. It's been used in aviation for nearly 100yrs. A good clean surface and good for years. Easy to touch-up too. Best prep I've used was MEK. Wear eye protection when whipping down w/MEK. Trust me on this. A hot poker in the eye would have felt better.
 

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Welding these hats has been an utter pain in the ass. I am using 3/32" stock so it doesn't have to get as hot as your 1/8" stuff. But the hats are thinner and melt quick if the torch angle isn't perfect. I've been clamping things down, then pre-heat, then try and get a good puddle blending the 2. Once that gets going I just move down the line keeping it hot so the 3/32" floor will keep its puddle. I add filler to the hat to replace the material that melts. For my meager skills it is like a roller coaster ride.

For a small tub like a 90 - I think if you use thicker gauge sheet you might not even need the hats. And with thicker gauge you could use countersink bolts and tie them right to the metal cross pieces - maybe just use a spacer so you don't torque it down too much.

Really surprised Jonathon Ward didn't spend more time on the inner tub construction. Like I said in my thread - there could be a much more elegant design to it.
 

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This may sound a little cheap or non rover like but just throwing it out their to chew on. Their are some amazing epoxy' out their to join metal. Aluminum specific and they do not come apart.
 

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This may sound a little cheap or non rover like but just throwing it out their to chew on. Their are some amazing epoxy' out their to join metal. Aluminum specific and they do not come apart.
I think the contrary is true. The epoxy's are much more expensive than welding and a lot stronger too.

The use them pretty exclusively on super cars these days, and when cured are stronger than the materials they join together.
 

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Nice writeup, although I think mine below are better :finger

Yes I think if you don't want to rivet, you should epoxy or bond the hat channels. Time will take it's toll. You know I have the buck and hammer drill for solid rivets right?

Steps Ed should have taken for this project

1. Drive to tractor supply and pick some stall mats
2. Cut to size and install.
3. Drink
 

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Discussion Starter #19 (Edited)
Good info on this thread.

I did the additional clecos and marked my holes from underneath the tub this evening.

I used 36" wide clamps to hold the sidewalls of the tub against the floor.

C$ - may take you up on that after I evaluate my options

I like the idea of adhesives. Dow Betamate is apparently used in Lotus vehicles and is the majority structural adhesive for the Aluminum F150. According to Aston Martin Betamate 2700 is a viscous two-part polyurethane adhesive. They do sell containers for vehicle body repairs. I am considering.
 

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Welding these hats has been an utter pain in the ass. I am using 3/32" stock so it doesn't have to get as hot as your 1/8" stuff. But the hats are thinner and melt quick if the torch angle isn't perfect. I've been clamping things down, then pre-heat, then try and get a good puddle blending the 2. Once that gets going I just move down the line keeping it hot so the 3/32" floor will keep its puddle. I add filler to the hat to replace the material that melts. For my meager skills it is like a roller coaster ride.

For a small tub like a 90 - I think if you use thicker gauge sheet you might not even need the hats. And with thicker gauge you could use countersink bolts and tie them right to the metal cross pieces - maybe just use a spacer so you don't torque it down too much.

Really surprised Jonathon Ward didn't spend more time on the inner tub construction. Like I said in my thread - there could be a much more elegant design to it.
I think the trick is stitch welding to keep the HAZ minimal. That thin gauge material is very prone to warping so it's going to be difficult.

As for fasteners, even on my floors with the 0.090" material it's hard to use much more than a countersunk rivet -- you'd really need to get into the thick stuff to be able to run a proper bolt through it and then you're talking about weight and $$$.

I have a NAS 90 coming in at the end of September for a full tub rebuild using the same components and construction method as our 110 tubs, so once that's complete and I have the CAD work all done I'll be able to offer you guys a much higher quality floor that is a (mostly) drop-in replacement to save the headache.

-Ash
 
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