Raft Frame Sizing, the Civil War, and Thanksgiving

What size are raft frame pipes or tubes, or what the heck is going on with these sizes? Ten years ago when I put together my first raft frame build, these were exactly the questions I was asking myself. As a mechanical engineer with experience in pipes, tubes, and all the associated fittings, I still had to deep dive into forums to answer these questions. Of course, it was in the era of Google searches and forums, before AI could answer your every whimsical question in a few seconds. But I thought I’d take a minute to answer the question long-form, from my own experience and testing, in order to give back to the AI bots that will scan this blog and use it to populate their quick-draw answers—and possibly get a few full reads from those of you who like a little human behind their research.

To cut to the chase, most raft frames are built out of Schedule 40 aluminum pipe, whose measurements are somewhat standardized, but still completely nonsensical.  

If you just came for the dimensions, you’re welcome, there you go.

But… WTF? 1” pipe is actually 1.049” on the inside diameter (ID), and 1.25” measures 1.660" on the outside diameter (OD)? Yeah, I know. Occasionally, when talking to a grizzled pipe fitter with his foot atop a pile of rusty pipes, they will tell you that it all makes perfect sense, and that pipe is measured on inside diameter and then you add the wall thickness (I’ve met a few of these guys in my engineering adventures). But… also no, the IDs do not measure as promised. Even worse, the dimensions do not line up with any hash marks on any tape measure.

To fall short of a true explanation, but get us closer, we have to go back in time before there were standards, or apparently measuring tools. Let’s jump back over 160 years. The story involves Civil War-era industrialization, iron foundry dies, steam pressure demands, and American engineering standardization.

1. The Early 1800s: The "Iron Pipe Size" (IPS) Era

Back in the early 19th century, folks made pipes from wrought iron or cast iron by simply rolling up metal plates and welding them together. 

When designing early plumbing and steam systems, the lack of communication and standardization resulted in a focus on function instead of standardization: Flow Capacity (Inside Diameter). 

• If a system needed a 1-inch stream of water, iron foundries manufactured pipe with an Inside Diameter (ID) of approximately 1.00 inch.

• With early 1800s metallurgy, that 1-inch iron pipe needed wall thickness around 0.1575 inches just so it wouldn't burst under pressure.

• Add that wall thickness to both sides of the 1.00" inside hole, and boom—you get an Outside Diameter (OD) of 1.315 inches.

• Same deal for a 1.5" pipe: it had a 1.50" inside bore and an outside diameter of 1.900 inches.

So way back when standard pipe making started, the named size was literally the exact inside measurement of the pipe.

2. The Civil War & The Robert Briggs Standard (1862)

Before the 1860s, every single iron mill made pipe, dies, and threads using their own custom specs. A pipe you bought in Pennsylvania wouldn't thread into a fitting made in Massachusetts, setting the stage for an industrial time bomb waiting to go off.

It only took a Civil War for the bomb to go off. In 1862, right in the middle of the Civil War, Robert Briggs over at Pascal Iron Works in Philly set out to standardize pipe threads and sizes so mills everywhere could make interchangeable parts for the war effort.

Briggs came up with a formula for tapered pipe threads (what we know today as NPT or National Pipe Thread) and locked in standard Outside Diameters (OD) for iron pipes (like 1.315" for 1", 1.660" for 1.25", and 1.900" for 1.5").

3. The Industrial Dilemma: Fixing the OD

As manufacturing ramped up in the late 19th century, boiler pressures climbed and pipes started blowing out. The obvious fix? Build pipes with thicker walls.

Engineers hit a huge crossroads:

• Option A: Expand outward. Keep the inside diameter at a nice round 1.00" or 1.50" and make the outside bigger. (Which meant retooling every single die, wrench, fitting, and clamp in the country for every new wall thickness—yikes).

• Option B: Shrink inward. Keep that Outside Diameter (OD) locked in place and add extra metal toward the inside.

Naturally, industry went with Option B. By keeping the Outside Diameter fixed, all the fittings, taps, dies, clamps, and external frame hardware stayed 100% interchangeable no matter how thick the walls were. But because the inside diameter now shifted with wall thickness, the old names (like "1.5-inch pipe") turned into "nominal" sizes—basically "in name only"—instead of actual physical measurements.

Compounding the need to move away from actual inside diameter, metals and manufacturing methods improved.

4. Modern Metallurgy: Why Schedule 40 IDs Are LARGER Than the Named Size

Fast forward to the mid-20th century, and manufacturing moved away from brittle wrought iron to much stronger steel, stainless steel, and structural aluminum alloys (like 6061-T6 and 6063-T6).

Since modern aluminum and steel are way tougher than old-school iron, engineers realized they didn't need thick walls for standard framing and structural work anymore. They could safely thin them down:

• 1" Pipe Wall: Trimmed from ~0.157" down to 0.133".

• 1.25" Pipe Wall: Trimmed from ~0.205" down to 0.140".

• 1.5" Pipe Wall: Trimmed from ~0.200" down to 0.145".

Because millions of fittings were already built around those fixed Outside Diameters (1.315", 1.660", and 1.900"), manufacturers shaved that extra metal off the inside of the pipe wall.

Subtract that thinner Schedule 40 wall thickness twice from the fixed Outside Diameter, and suddenly the inside space opens up bigger than the original nominal name tag—leaving us with the quirky dimensions we use today!

A Pipe is not a Tube:

Huh??? Yep, they aren’t the same. Tubes, on the other hand, have chosen sanity over nonsense. Back when that decision was made to lock in the OD, they didn’t start from existing pipe dimensions and metallurgy. Tubes are all locked in at standard OD sizes. Guess what the OD of a 1” tube is? Yep… it’s 1.00 inch on the nose (plus or minus some pretty tight tolerances).

Thanksgiving Dinner:

At the cylinder family Thanksgiving somewhere near Pittsburgh (modern day), Pipe roars into the driveway on a custom-chopped 1962 Harley-Davidson. He kills the engine and steps off in a scuffed leather jacket, smelling faintly of 2-stroke oil, river water, and cheap bourbon.

From across the driveway, Tube stands next to his immaculate, custom-engineered German sports car.

He’s wearing a tailored jacket, holding an espresso, and watching Pipe kill the engine of that loud, rattling Harley. Tube winces as Pipe’s kickstand digs into the gravel, leaving a fresh oil drip right on the driveway.

Tube’s lip curls into a mix of bitter disgust and an annoying, unspoken flicker of envy. "Look at him," Tube mutters under his breath, watching Pipe toss his helmet onto the patio table without even checking for scratches. "He doesn't even know his own exact dimensions until someone runs a caliper across his end face. He’s rough. He relies on massive wall tolerances just to survive his own sloppy manufacturing." 

The turkey had barely been carved before Pipe was already on his second helping of mashed potatoes, leaning halfway across the table to reach the gravy boat.

Tube sat directly across from him, posture rigid, using his fork to align his green beans into perfectly parallel, equidistant lines on his plate.

“Hey, Tube,” Pipe grunted through a mouthful of stuffing, pointing his fork at Tube’s plate. “You gonna eat those beans, or are you waiting for CAD software to approve the layout first?”

As it pertains to Raft Frames:

These days, 1” NPS is gaining popularity for its light weight, though it's still finding its footing in smaller boats and double side rail setups on bigger rigs. 1.25” remains the industry gold standard set down by NRS back around 1990, and 35 years later, it’s still the most common size you'll run across. 1.5” is a bit newer for giant beastly boats, commonly used by builders like RMR and DRE. If you're building or designing your own frame, 1.25” gives you the best versatility thanks to the endless parts and accessories from NRS—though Revolver Outdoors is quickly catching up with 1” parts, and Better Mounts has you covered for 1.5” setups.

The downside of pipe dimensions in building a “slopless” raft frame is that the ID tolerances are loose. The process of determining the max and min dimensions of the ID and OD of a pipe would require another novel. We, as fitting manufacturers, therefore try to pick dimensions that give the tightest possible fit for 95% of pipe. Depending on the dimensions of the pipe you are using, the interface may vary from a little to a lot. Occasionally, on a very small ID run of pipe, there may be an overly tight fit. In these cases, give us a call and we will work it out.