Fiberglass M416/M100 Military-style Trailer Tub Kit


x3 on the lid and RTT options. The mockups you have for the 6' see to be a bit tall but I've not thought or seen that design before.



Expedition Leader
x3 on the lid and RTT options. The mockups you have for the 6' see to be a bit tall but I've not thought or seen that design before.
When I get to the point in this project of deciding on what type of covers/caps to build, I'll post a bunch of designs I've come up with and we can have a discussion in this thread to decide what I should make molds for. First I'm going to get the base tub molds made, and mold up one or two tubs from those molds.


Expedition Leader
like with everything I've seen you post, I just want to know when/where to buy it. :)

Like every project of mine, I just build these for my own enjoyment :). However, some of my products do end up being licensed by companies who want to bring them to market (about 5 of my products have been licensed so far), and I'm hoping to find a company to pick up this kit. I've had multiple contacts from parties interested in this kit already, so hopefully that means there's a good chance this will get to production.


Expedition Leader
As I posted earlier, the basic tub design consists of 4 fiberglass panels - 2 ends and 2 sides, bolted or bonded together at the corners - here's a concept drawing of the 4' version of the tub on a Harbor Freight frame:


The overall dimensions are very similar to the original military trailers, this drawing shows the 6' version, although the only difference between the 6' and the 4' versions is the length.


Length - The molds and side panels are designed so that tubs of any length up to 72" can be made. The 72" length is the same as the original military trailers; a 48" length is perfect for putting on a Harbor Freight Mini Trailer Frame, which could be a very affordable solution for someone who doesn't need a full 6-foot trailer.

Top width - The top width (outside dimension) is 47.5", vs. 48" for the original M100 and about 51" for the original M416. This dimension is slightly smaller to make it easier to DIY-build a hard cover for the tub out of a 48" wide sheet of plywood.

Bottom width - The bottom outside width is 40 13/16". This dimension is a good fit on the frame width of the HF Trailer, and also matches the frame width of the original M416's. This width allows clearance for most "tow-vehicle-sized" tires on the Harbor Freight 5-lug axles without needing a wider axle or wheel spacers (more on wheel sizes/offsets/tire sizes later in this thread).

Tailgate Opening - The original M416 trailers don't have tailgates; one of the reasons for this is so the M416 can be watertight and float. You could build one of these fiberglass kits that way with two solid end panels, but I'm guessing most people will want a tailgate. The tailgate opening will accept any factory or aftermarket CJ-7/CJ-8, YJ Wrangler or TJ Wrangler tailgates, or the Dinoot smooth fiberglass tailgates can be used if someone doesn't want a "Jeep look" on their trailer.

Height - I've designed the tub about 2" taller than the original M416. One thing this does is provide enough height at the rear panel to fit a stock Jeep tailgate as well as providing enough room for the hinges to mount below on the panel the tailgate. I've shown 1.5 as the M416 dimension in the drawing, but that's roughly what it would be if you cut a Jeep-sized tailgate opening in an M416.


Expedition Leader
I'll be making the masters for the mold process mostly out of 3/4" medium density fiberboard (MDF). This material is very commonly used for mold master construction because it's so flat and smooth, it works easily, and it takes a nice finish. The goal is to build parts out of MDF plus other wood as necessary that replicate the parts desired in fiberglass, and then make molds of those MDF parts. The MDF mold master parts will differ from the final fiberglass parts in thickness of course, the MDF is 3/4" thick and the final fiberglass parts will probably be around 3/16" thick.

I've made up a sample section of the mold master for the tub side panel - I've done this sample so I can test and verify the exact dimensions and angles before I cut larger pieces to make the full side panel master. Here are a few photos along with an M416 fender. The tub section is sitting on a spare piece of Harbor Freight trailer frame rail, as it would be when the tub is installed on an HF frame.



With the fender mounted where it's pictured on the side panel, 33" tires will fit with no problem if someone wanted to run the same size tires that are on their Jeep. Of course these original military fenders are only 9.75" wide, so they wouldn't cover the full width of a Jeep-sized 33" tire, but the wider fenders I'll make will cover them just fine.

Those of you who are very familiar with the military trailers might notice a difference between what I've prototyped and the actual military trailer - on mine, the lip at the top of the tub faces in, while on the actual military trailers, the lip faces out. I'm doing the lip facing inward for these reasons:

- The outside width of the M416 is about 51", due to the outward-facing lip. I want it to be easy for someone to to DIY-build a hard cover for the tub out of a 4x8 sheet of plywood, so facing the lip inwards reduces the width to just under 48".

- The inward-facing lip makes it easy to attach a tonneau cover like the one that's available on the Dinoot trailer. These are really nice covers, I have them on both of my Jeep-tub trailers, and I think they'd be a great add-on for the military tub kit:


Now that I've verified all of the dimensions and angles with the short sample tub master section, I'll begin making the actual side panel mold master. Photos to come.


Expedition Leader
There being no further comments, suggestions or criticisms from anyone of the features and dimensions I've posted, I'm going to begin making the mold masters this weekend. I'll post some photos of the construction along the way.

In the meantime, here's a drawing I did to show ways in which the parts I'm about to make might be configured...

Because the design of this tub kit is modular, several different configurations could be assembled. The original M100/M416 trailers didn't have tailgates, so if a tub to match the configuration of an original was desired, one could be assembled with two solid end panels. If you did this and sealed up the joints well, I'm sure the trailer would float, just like the originals did.

More likely though, I'm expecting most people would want a tailgate, which is the middle option in the drawing below. But tailgate panels could also be put at both ends, which isn't as strange as it might sound - see the photos at the bottom of this post...


The three tubs in the top row above have "stock width" M416-style fenders, but the stock fenders are only 9.75" wide... since many people want to run the same tire/wheel combination on their trailer as they have on their tow vehicle, and those are typically wider than the original trailer tires, the stock M416 fenders don't cover them, so I've designed 12.75" M416-style fenders. I've got a manufacturing source to make them in sheet metal, and the initial pricing quote is quite nice. And, for the M100 fans, on the bottom right is a tub with round M100-style fenders. I've already found a very reasonably-priced source for those fenders in a 13" width, which also would cover "tow-vehicle-sized" tires.

I'm probably going to test both the wider M416-style and the wider round M100-style fenders on the prototype trailer(s) I'll build.

The two-tailgate-end configuration turns out to be pretty handy - I've got tailgates on both ends of my large Jeep-tub trailer, and use them both regularly for longer cargo like my 16' long extension ladder :):



Expedition Leader
Looks great!
Can't wait to see the actual run.


I'm making good progress building the mold masters for the military tub kit... I've been posting photos of the mold master construction in the main project thread in the Trailer Adventuring Forum: I'm using that as the main project thread, so I won't clutter up this thread on Expo with every detail of the mold master construction and fiberglass work. I will of course post updates on the build here as well, but I'll cover more details there than here.

And if you would like to see this kit get to market, you could visit this link and offer your comments...


Expedition Leader
I've finished up the woodworking phase of the side panel mold master, here are a few photos:




The side panel is sitting on a piece of trailer frame rail as it would be in the completed trailer, and that's an N.O.S. original M416 fender alongside of it.

The next steps for this master is to put a finish on it that's impervious to the solvents in the polyester resin used with fiberglass. Once that's done, I'll make a mold from this master.

I'm also beginning the woodworking for the end panel master now.

You may notice in the photos above that the fenders on the scale model seem to be wider than the original 1:1 scale M416 fenders. More on fenders for this kit soon.

There are more photos and text in the Trailer Adventure forum thread:


Expedition Leader
I'm now working on the mold master for the tub end panel, photos soon.

I've designed the top outside width of the fiberglass tub to be 47.5" so that a single sheet of plywood can easily be used to DIY-build a hard cover. The original M416's have a lip on the outside of the tub that increases the width to about 51", and I believe the M100's are just over 48" outside width. Here are a few simple hard covers drawn on the M72 1/4 scale model, these would be easy to DIY-build from a sheet of plywood:



Here's a set of instructions for building a nice plywood hard cover for a Dinoot Jeep-tub trailer, complete with gas struts to support the cover when it's open. The covers I've pictured above could be constructed in a much the same way, differing only in details and dimensions:



Expedition Leader
As I'm building the mold master for the tub end panels, it occurred to me that I could also do a wider version end panel mold to create a tub with more space. It would take the same side panels, just wider end panels.


The standard width tub size is very close to the same dimensions as the original military M416, and can be installed on an original M416 trailer frame. To install the standard tub on a Harbor Freight frame, the HF frame would be narrowed, which is a pretty simple job - cut the frame cross-members to the shorter length, drill new bolt holes in the ends, and bolt the frame together. The spring seats on the axle must be moved inward as well.

The M72-W wide tub would be 48" wide at the bottom, which is the stock width of the larger Harbor Freight frames, so it would install directly on a stock-width Harbor Freight frame. The top width of the tub would be just over 55". The stock HF 21" tires on 12" would clear the tub just fine because the tub is the same width as the frame, but to fit "tow vehicle sized" tires/wheels, it's likely either spacers or a longer axle would be required.

The work-in-progress on the end panel mold masters is posted in the Trailer Adventure forum thread:
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I just want to buy the 1/4 scale M416. What a great match to my full size one. It can go behind the grandkids electric powered jeep!!!!

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