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  #41 (permalink)  
Old 11-28-2020, 06:27 PM
EM-0785's Avatar
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Location: Bellevue, WA
Cobra Make, Engine: Everett-Morrison 460, Toploader 4 sp, Jag IRS
Posts: 221
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Trailer Project
Serpent Express Retro Kit (1 of 3 posts)

Goals included:

● Small size
o Fit the side yard Serpent Den created for it
● Low height
o Sit at or below fence line
o Fit in garage
● Lightweight, sturdy flatbed
o Capable of long trips safely
● Quality top installation
o Maximize weather seal (given top style)
o Smooth/flat vinyl fit

Trailer Base & Serpent Express Top

I bought a flatbed car hauler to install a Serpent Express Retro Kit on top. Larry Johnson of Serpent Express was a ‘champ’ to work with! He fielded many questions, provided a customized kit with diagrams and perspective. He was very responsive throughout.

The flatbed is a 2016 16’ Aluma aluminum trailer with extruded aluminum floor, 2 - 3,500 lb. rubber torsion axles, electric brakes, breakaway kit, reinforced A-framed aluminum tongue, fold down rear stabilizer jacks, and pull out 6’ ramps.

Prep was needed to fit the Serpent top. The front corners were 45 degrees with a deck rail. To use the full 16’ for the top, the corners were filled in. A welder scavenged the deck rail to create corner frames and filled with flat aluminum. He also welded in a battery holder and winch mounting frame.

Starting Aluma flatbed trailer

Measured the deck, Cobra, and garage height. Got the dual rib frame for stability and extra doors (5 total) for easy access. Larry provided custom CAD drawings, frame cut sheets, frame & deck fitting plans, and detailed instructions with photos, all customized. He lowered the peak & rear frame heights for my specific max height.

I used weld-on aluminum upright deck fittings. Bought 1.75” OD / 1.25” ID 6061 bare aluminum tube. The welder cut 10 each 6” fittings, welding them perpendicular, located per the deck plan. They’re inset from the frame edge to allow 3/4” wide foam tape to run outside.

After welding, cleaned trailer before adding the top. Wand wash employees were testing a new pressure washer Suds Cannon. I asked how much to do my trailer…$5. They did a great job, I gave them $10, all were happy.

Suds canon

Removed manufacturer and dealer stickers and polished the outer deck frame and fenders, for a unique and clean look.

Before polishing

Polished with Blue Magic polishing cream, blue paper towels, and ample elbow grease.

After polishing

Restored rusted fender hold down bolts. Cut and added rubber fender mount gaskets to avoid metal-to-metal scuffing.

Fender bolts and rubber gaskets

Frame Structure

Assembled the frame structure. Got 10’ lengths of 1” EMT steel conduit from HD. Measured and cut lengths per cut sheet, marking for reference. Tried a miter box and hacksaw, assessed situation after one cut, bought a cut-off saw.

Got a 3.5 HP cut-off saw at HF, handled the task well, quite enjoyable. Neighbors confirmed I’m crazy, in the backyard after dark, shooting sparks across the yard. That was my cutting station, dutifully carried downstairs and out back nightly after work, generally raining, with a full complement of cords, lighting, safety equipment, leveling boards, etc. Then let the sparks fly.

Prepped cut ends with deburring tool, files, and steel wool. As overkill, I cleaned and coated each cut end with metal primer and Rust-Oleum to protect from corrosion.

Cutting, tools, coated ends, markings

Loose assembled the frame, front to back, with cut conduit sections & the kit’s connectors. Went together well, challenging getting the rear cross pieces in with the rest of the frame in place, with hard limits, like a Rubik’s cube.

Solution was family lifting rear frame off deck fittings to widen out at the connectors, w/o any piece exiting its connector and crashing down. Trust me, ‘crash’ is the correct term there. Idiot proofed (against myself) by tightening the connector set bolts most likely to crash outside of hand control. Success.

The 2nd iteration of that process actually did it. The 1st identified cross pieces requiring trimming to fit. Larry’s cut sheet wasn’t off, I’d assessed excess free room in each connector and added length to each piece for max extension if needed (mainly for uprights), made a couple too long initially.

The connector set-bolts were kept loose to allow flexibility when later fitting the top, at minimum extension, then tensioned from there.

Frame assembly

Pipe Insulation

Pipe insulation was added to protect the vinyl top, particularly at corners. Measured and created a cut sheet for tube counts bought at HD. Opted to insulate the entire frame for a smooth surface & transitions.

Planned the insulation layout to ensure smooth corners, junctions and transitions. Cut to avoid junctions on connector pieces. Taped around each connector and at each cut end junction. Carefully trimmed each piece, particularly around corners.

Per Larry, used 3M Metalized Flexible Duct Tape 3350 UL181B-FX. Designed for flexible HVAC ducting, with wide heat range, durability, workability, and strong bond. Pulls off easily w/o disfiguring to rework rotations for overlap alignment. 3 tape rotations on each. Result is a clean look, full adhesion, hopefully long lasting.

Pipe insulation and 3M taping

Fitting Top

Prepped for fitting/fastening the vinyl top. Goals were a good weather seal, a clean outer frame edge, and a smooth top.

Weather seal is by self-adhesive foam tape around the deck frame just outside the upright fittings. Measured upright placement tight for the ” wide foam tape. Foam tape spec is ” thick, I opted for lower-compression, higher density foam, at 7/16” thick, which seemed durable for that environment.

The top has side pouches sewn in the lower edge. Cut ” EMT steel conduit ‘base bars’ and slid into those pouches. These set on the foam with inboard conduit clamps (via inside razor cuts) snapped over the conduit and bolted to the deck frame, creating a press down weather seal.

The side base bars are inserted into rear side openings. The front base bars are inserted via inboard razor slits made near the front corners, with the front flipped up for insertion from outside the trailer, then flipped back down.

Decided to cut base bar ends at front corners at angles where front and side bars met. The front and side pouches each ended at a 45 degree corner seam. The angle cut bars maximized filling that space for corner support and weather seal.

Welded the upright fittings tight to ” from edge allowing the outer weld beads to breach into that outer space a bit. Dremel ground a small amount off the outer beads allowing foam tape to lay straight and flat next to each upright.

Flex-Seal taped a couple spots under the foam tape to equalize surface levels, for a good weather seal. This was due to slight variation where the front corners were built. Then installed the foam tape.


Flex Seal for low spots

Foam tape

Initially set the base bars on the foam tape to mock-up the clamp location and accurately mark the holes for drilling. Instructions had clamp separation at 18” max., 12” for a stronger build, I used 9”. Used narrower spacing at the corners, base bar junctions, and ends, per instructions. Mocked-up pouch thickness and shape with paper sheets to align base bars passing by uprights, for accurate marking.

Used extending magnets a) below deck to locate the center point of the crossway channels, by feel, and b) above deck to match that location above for drilling. This allowed for marking by myself, with some Gumby stretching.

Base bar measuring & top frame drilling

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Last edited by EM-0785; 11-28-2020 at 09:39 PM.. Reason: Spelling
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  #42 (permalink)  
Old 11-28-2020, 06:32 PM
EM-0785's Avatar
Senior CC Premier Member
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Join Date: Nov 2015
Location: Bellevue, WA
Cobra Make, Engine: Everett-Morrison 460, Toploader 4 sp, Jag IRS
Posts: 221
Not Ranked     

Trailer Project
Serpent Express Retro Kit (2 of 3 posts)

Vinyl Top

Initial sneak peak of the top in its box, looking good! Laid out the top on the deck to acclimate with garage temp turned up. Set top over loosened frame for continued acclimation. Corners and peaks settled into place nicely.

Top in box

Top laid out

Top on frame loose

Fastening Top

My Aluma had 3 fastening typical scenarios, front, front corners, and sides. Front being most challenging.

The front frame is 2” x 5” aluminum requiring longer drill bits. Has a center channel underneath, with ” wide channel access exposing the true bottom an inch up into the channel. Also reinforced with internal structure, so drilling was more difficult.

It was critical to drop the bolts centered in the channel to get washers and nuts on w/o cutting the ” channels open. By aligning foam tape to the outer frame edge, that was a hard reference point, dictating the base bar and clamp, and therefore clamp hole, locations. I centered the base bars on the foam, snapped the clamps on, and marked the holes where they naturally fell.

Fortunately, after drilling those front holes, they dropped through ‘dead center’ into the narrow channels below, allowing me to (barely) fasten them. It took fingertips, taping washers in place, and several tries to thread the nuts.

It took various wrenches to manage inside the channels, and taped wrenches until I got in place with both upper and lower wrenches, and various extenders, but managed all myself. A sense of accomplishment on those front bolts.

Front channel and holes

The front corners just required specific length hardware, yet were easily accessible.

Front corner holes

The sides were middle of the road challenging, and there were a lot of them. I computed hitting centers on the variable-width underside structural cross channels, by using 9’ hole spacing. Centering those maximized the tight fingertip access and allowed for large washers.

I set weight on the bolt heads to get 1-2 threads to protrude below to start the nuts, and avoid popping-up when barely touched. Taped washers on (using a guided missile type blind thrust with one fingertip, many tries on the most challenging locations) and the threaded nuts blind with fingertips. Tightening allowed more threads to pull down as the foam compressed.

Side holes

Used a roto-hammer in drill mode worked well, especially on the more challenging fronts with long bits. Used blue tape for marking on and keeping bits from traveling. Did 5-6 steps from small pilot bit up to final hole bit for the ” deck bolts. This kept the bits under control and achieved accurate holes/locations.

Started with big box store Cobalt bits and cutting oil, wasn’t doing the job and breaking smaller bits. Got fastener specialty store Cobalt M-35 steel bits and Boelube Paste, a Boeing developed drilling & cutting lubricant. That setup worked very well, no more broken bits, lubricant paste stayed on bit no mess and superb drilling effectiveness.

Roto-hammer, Boeing lube and instructions

Larry included extra clamps in the kit, for 50 total. I used all 50 given my spacing used. The top’s 5 access doors allowed doing all but a handful of the fasteners by myself. A lot of twisting and stretching, fit by feel, various extensions, seemingly always upside down, but it all worked.

Base bar clamps loose

Base bar clamps fastened

Base bars fastened – outer corners

Base bars fastened – outer sides

All hardware was stainless from McMaster-Carr. Was very happy with their stainless flanged nylocs with black nylon. I found a group of tools that managed the deck fastening pretty well, per photo.

Hardware & tools

Fastening Frame

With the base bars fastened down, it came time to drill, place, and fasten the bolts in the welded deck uprights. These go through both the fitting tubes and inserted conduit frame uprights, to hold the frame securely in place. There is a small amount of play by design.

One decision was whether to drill before or after the top was tensioned. If after tensioning it seems they would take their natural shape and have less fastened tension (maybe good with aluminum fittings). However, doing them first would make managing the remaining frame easier and more stable. After much back and forth, the answer presented itself.

I could tell I was going to need to use some extension all around to tighten/smooth the vinyl top. So, I did those deck fitting bolts first, pressing each upright to its outermost play position in the fitting while drilling (the corners pressed out at 45 degrees). This was done with a kneel pad and my head, both hands on the roto-hammer.

The 4 corner upright bolts are spec’d carriage bolts with smooth rounded heads to avoid damaging the top. Those ideally require squared holes. I used a Dremel and mini files to square out the corner fitting holes.

Corner round hole

Corner square hole

Dremel & mini-files

Bolted - corner

Bolted - non-corner

Initially I tensioned the upper frame slightly front to back, tightening connectors as I went. Not so tight as to over stretch it. Just to smooth it out some, I will retension the top after some trips with vibration and weather cycles. I’d left the top in its box for months before starting, so there were some fold lines. Many came out during acclimation and initial tensioning. The rest will work out with use and retensioning.

Top initially tensioned

Rear Door

I put a lot of thought into the rear door hold down. The standard spec is to drill holes in the side frame for the rear base bar to protrude through with trailer clips on the outsides. That works pretty well. However, it seemed to be a single fixed position that could be under/over tension depending on the fitting height or if the top stretches. The ones I’ve seen were almost over-tensioned, and tended to expand the holes in an aluminum trailer frame due to wear over time. Also, releasing the rear door can be a two person job at times, particularly if the bar sticks in the pouch, and puts added movement in the top and of the base bar in the pouch which all can tend to wear more over time. Granted, with a fixed position base bar, one can always re-tension at the upper rear frame, yet I was looking for a more real time flexible door fastening solution.

My goals included no side-to-side material movement to open, and an easy, one person, variable tension fastening method. Stainless steel Terry Body hold down springs & hooks were used to hold the rear door down with a firm and flexible tension approach in basically a straight down hold fastening solution. I used a shop for winch mounting and rear door fastening solution. They came up with this rear door solution without hesitation as I was explaining the goals!

As opposed to the ” hollow conduit (which is a bit over 11/16” OD) as typically spec’d, I first planned to use a ” solid aluminum round bar in a 6061-T6-cold finish which has 0.50” OD. However, as the shop recommended, we used flat aluminum stock with dimensions 1-” wide x ⅛” thick. That used more of the tall rear pouch for added rigidity. It was thinner, so dropped the bar/pouch perfectly into the rear channel in my trailer design without being tight and scuffing/wearing the vinyl pouch.

My Serpent door was specifically designed to extend a bit past the deck to drop into that channel given my trailer’s design. This all worked to create a pretty good natural weather seal for the rear door. Also, the mounts used for the tension fasteners bolt through the flat bar putting the downward force on the bar. The ends of the bar don’t extend past the pouch so it has a clean appearance, plus the bar won’t slide out with the through bolting.

My goals for opening and holding up the rear door were a simple, competent means that’s easily workable by one person. Again, the shop came up with an elegant solution. It was achieved using 24 lb neodymium magnets and nylon straps on each side. One magnet is attached to the upper inside frame with a nylon strap attached to it and another magnet attached to the loose end of the strap. The door is rolled up and the straps wrapped around with the loose end magnet attaching to the fixed frame magnet, magnetically clamping through the vinyl top. It holds firmly and is simple to manage in seconds by one person.

The rear access holes in the side base pouches, and the hollow side base bar ends seemed exposed to weather and grime. I installed metal conduit caps to protect those. That left small pouch openings above the base bar. I may fill with a clear sealant, maybe Flex Seal. I’m not going to worry about it on the rear door base bar pouch. There is a slight open area at the rear corners below the end of the door zipper. I’ll monitor that and either not worry about it or may cut some foam or such to insert on the inside to block when the rear door closes for a complete weather seal.

Rear door - flat bar hold down with tensioners

Rear magnetic door holder straps

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Last edited by EM-0785; 12-08-2020 at 09:02 PM.. Reason: Remove word
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  #43 (permalink)  
Old 11-28-2020, 06:37 PM
EM-0785's Avatar
Senior CC Premier Member
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Join Date: Nov 2015
Location: Bellevue, WA
Cobra Make, Engine: Everett-Morrison 460, Toploader 4 sp, Jag IRS
Posts: 221
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Trailer Project
Serpent Express Retro Kit (3 of 3 posts)

Tie Downs

Goals here included over-capacity strength and quick ease of use. I mulled over several approaches to tie downs. Wheel nets were not as accessible given my trailer. E-tracks to suspension arms were a concern over strength given side angles and intermittent welded solution being considered.

In the end, I went with the trailer’s 4 existing swivel floor mount flush tie down ring wells that correspond nicely to the new tie-down rings welded on the front of the Cobra and given the rear suspension. They ended up being fairly ideally placed for my car. Some additional structure was added to the front of my car to both support adding an anti-roll bar and attaching tie-down rings in easy reach.

The existing swivels provide similar strength regardless of horizontal clock orientation. The front Cobra welded rings are further forward than the suspension arms we’d otherwise considered reaching to strap around, so very quick to clip to.

This solution requires short straps, all 4 easily accessible from my trailer doors with a quick easy reach to simply clip the hook on the welded front tabs and an easy loop around the rear inner half shafts. Granted this does bind down the suspension, but for the limited time usage in the end that was acceptable to me.

I bought some 6061-T6 aluminum E-tracks from Mac’s that I’d planned to have tig welded to the deck, but opted for the above solution instead. After some trailer usage, I’ll determine if there are areas for gear that it would be helpful to have some e-track welded on for, I prefer to minimize bolts/holes through the deck.

My friend Larry, who built a Serpent for his Cobra, gave me a gift of custom heavy duty straps he and his friend Scott had designed for his Serpent setup. That was a real honor, and I will always use these straps in honor of my friend Larry! They offer e-track capability as well, by design, if using a trailer with e-tracks.

Car strapped down


Welded two hollow aluminum winch mounting bases to the front of the deck. These served to raise the winch to reduce the chance of the rear deck interfering with the cable, and as a base to achieve mounting of the winch plate and winch, while hiding wiring connections underneath.

Replaced the new winches’ metal cable with a nylon cable for ease of use, smaller winding, and softness in the event it rubs on anything.

Due to pull angles for loading, the car will be towed (winch cable attached) through the grille. This is achieved with two hidden tow rings inside the mouth of the Cobra that a cross strap attaches to for a winch attachment point in the center of the grille.

Winch mounted (with nylon cable & wireless remote added)

Inside Lighting

Installed 4 ea long swivel dual-tube LED light fixtures with a switch accessed inside the front right door near the battery. These are oriented front to back at each corner. Due to the angled down shape of the top/frame, each is angled to project light in toward the center of the trailer. This provides ample lighting throughout the trailer and coming above from all directions to help avoid dead zones.

Tow Rig

Bought an eight year old 2011 Ram 1500 Big Horn with 8K miles in like new condition. It came with a SnugFit canopy. The prior owner towed it behind their RV to have a vehicle when they took trips. I added a Tekonsha P3 brake controller.

Hitch & Scale

Went with a Convert-A-Ball 10,000 lb shock absorbing cushioned ball mount with a 6” drop. Added a Weight Safe hitch ball with built in scale for dialing in tongue weight. Bought these from

Side Yard – Serpent Den

One major thing I had to do was create a place to keep the trailer. While we designed the trailer to fit in my garage, I still intend to store it outside most of the time. My side yard is just barely big enough for the trailer and was overgrown. I cleared that out and lowered the pad by a foot of soil to level it out. Added a new fence and ⅝ minus crushed rock pad with heavy duty landscape paper. Added a short chain link fence on the back end to keep out little dogs in the backyard.

A main goal was a sturdy gate that would not sag over time like so many around here. I spec’s a heavy duty or commercial style hinge for overkill. The fence company delivered and used an aluminum frame for the gates as well. Also maximized the fence height to keep the trailer from being seen over the fence.

Extended the crushed rock out to the street, with landscape timber borders. Replaced the water main with a flexible one (we’d last done copper many years ago) and dug it deeper this time. Re-routed the sprinklers in that area. Moved the two sprinkler boxes to align with the water meter box and get more out of the way of the tire paths. Replaces boxes with stronger, drive over versions.

Serpent Den development

Serpent Den in use

Trailer Cover

Due to the unusual shape of the Serpent trailer, I toyed with the thought of a custom cover, or using tarps and just wrapping the excess on the short height end.

Yet I found an ADCO Tyvek All-Climate + Wind RV Cover for Pop-Up Camper up to 16’ Long on in gray. It had almost the same dimensions as my Serpent except some expected excess in the front that I’d bungee down. I liked the all weather capabilities, with material that breathes with moisture and protects against the sun.

Ends up it fit almost perfectly in the front, and the rear fit was nice and tight just a few inches short in back (as expected). It fit better even than expected, so I’m pleased with it.

Logo Decals

I hadn’t planned to place any logos on the trailer. However, given Larry’s superb assistance and support of our hobby with the Serpent Express small business, I still may proudly display the Serpent Express logos once the top is fully smoothed out after some use. That could also increase visibility to other drivers.


That, Cobra aficionados, is how I built my Serpent trailer. Perhaps soon to be a long-lost art as they now offer a hardtop enclosed version in the same shape, without all the work! However, I am very comfortable with the sturdy base flatbed I used that should meet any of my trip plans/needs. This will remain a very satisfying mechanical endeavor for me, right to my limits!

See you on down the road...
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Last edited by EM-0785; 12-14-2020 at 06:45 PM.. Reason: Add photo, correct spelling
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