The Legend of the Caterpillar 3406

3406. To the average person, this is just a four-digit number with no meaning, but for dozens of industries and anyone who’s big into diesel engines, these numbers mean a lot more. For the uninitiated, we’re talking about the Caterpillar 3406, which was and is one of the most influential and important diesel engines on the planet.

So, it’s time we take a deep dive into the Cat 3406 and talk about what makes it such a special engine, what makes it interesting and unique, and most importantly, why it’s still being used all over the world long after Cat stopped producing it.

The History

Before we get deep into the details of the Cat 3406 engine, let’s rewind the clock to when Cat first introduced this engine to understand some of the history behind it.

In 1925 Caterpillar was formed when two competitors in the tractor industry merged together to create Caterpillar Tractor Co, which then, in 1931, had a separate division from the main company, which was focused on engine development for applications such as generators, marine, and more.

Taking a quick jump up the 60s, 70s, and 80s, Caterpillar introduced multiple engine series throughout these decades, including the 3300, 3400, 3500, and others, which at the time, made Caterpillar the largest engine manufacturer in the world. The 3400 series, in particular, is where we find the 3406 engine, as well as other engines like the 3408, which is more or less a V8 version of the 3406, but that’s a different engine for a different article.

The 3406A and 3406B

For the 3406, it all starts with the 3406A, which is the very first rendition of the 3406 that was first introduced in 1973, which was shortly followed up by the 3406B. Both the A and B versions of the 3406 were incredibly similar, and both feature mechanical injection, although strangely enough, the 3406A changed from pre-combustion style to direct injection depending on the year the engine was manufactured in.

For the 3406B, it was always direct injection and not pre-combustion style, and really the only important difference between the 3406A and the 3406B is the location of the injection pump; other than that, the two engines are very similar. And depending on who you ask, the A and B versions of this engine are the best that were ever produced, as the mechanical injection system is really bulletproof, and for many diesel enthusiasts, it’s superior to any kind of electronically controlled injection system.

But, as the story of any modern diesel engine goes, the EPA had enforced requirements for cleaner operating engines, which ultimately prompted Cat to develop the next generation of the 3406, which is the C variant, which is a sort of hybrid between mechanical injection and fully electric injection.

The 3406C

You might be asking why the C engine is a mix of mechanical and electronic injection, and to put it simply, there wasn’t really a need for Caterpillar to go fully electric at that time. I do want to mention that some of the C engines were actually still mechanical injection, and some of them were electric, it simply depended on the application and how the engine was specced when purchased from Cat.

Interestingly enough, the programmable electronic injection control, also known as PEEC, was designed specifically to work with the fuel system Cat had introduced with the late-model 3406B engine.

With this combo injection system, they were able to reduce particulate matter and emissions output while also maintaining power and fuel efficiency. However, this is generally considered to be near the bottom of the 3406 list in terms of reliability, as this was the first time Cat had introduced electronics to the 3406, and that came with a few problems.

And really, the electronics of the 3406C are very basic, it’s basically just electronic fuel timing advance, an electric fuel pump, and an electronically controlled throttle body. The C also included some other changes, including 12-point head bolts as an upgrade to the A and B’s 6-point head bolts, as well as different bearings for the rods and some other upgrades.

Realistically, the C variant was basically a transitory engine from the 3406B to the next generation of the 3406, which is the E variant, and that new variant brought with it a fully electronic injection system.

Caterpillar first began producing the 3406E in 1993, and it was the last 3406 model engine, as it was also kind of a transitory engine to the Cat C15, which is actually very similar to the 3406 but not in the same engine family at all.

Unlike the C engine, though, the E engine had fantastic electronics that, to this day, are well known for being very reliable. Even compared to other fully electronic injection diesel engines of this time, the 3406E was a beast in terms of reliability.

Problems

That being said, it wasn’t absolutely perfect. 3406E engines with a 5EK engine serial number are well known for the crankshaft breaking. The exact cause for the widespread breakage isn’t perfectly clear. But, it’s believed that some 5EK engines were made from bad casting, causing the crankshaft to break most commonly at journals 1 and 6. There was never a recall for this problem, but supposedly Cat has fixed these engines as needed.

Besides that, the 3406E engine is also well known for some pretty bad oil leaks, especially from the rear structures and flywheel housing. However, oil leaks are common on pretty much all 3406 engines. By the time the C15 came around, Cat changed their gaskets across the board and solved many of the oil leaks.

The End of the 3406

The 3406E lasted from 1993 to 1999, when it was fully replaced by the C15, which was the end of the 3406’s impressive 26-year run. Unfortunately, after the C15 and later engines such as the C17 were introduced, Cat completely left the truck-engine market in 2008, as the EPA restrictions were making it harder and harder to develop profitable engines, and the over-the-truck engine market was actually a small part of Cat’s business, so they just shut it down entirely.

That being said, Cat is still supporting these engines by manufacturing parts for them, which ultimately is allowing the 3406 to stay on the road long after its death in 1999. In fact, the 3406 is so loved, so reliable, and so good all around that many owner-operators are opting to keep their 3406-powered trucks on the road for as long as possible rather than upgrading to a newer truck with a newer engine.

Ultimately, new engines are plagued with emissions components that crush reliability, and the 3406 just simply doesn’t have to deal with all those garbage components.

Basic Specs

In terms of some basic specs, we’ve pretty much looked over that entirely so far, so let’s change that. The 3406, regardless of year or generation, was always an 893ci or 14.6l inline-six engine with a 5.4″ bore and 6.5″ stroke. On the head, you’ll find 24 valves, or four valves per cylinder which are all driven by a single camshaft.

The engine block is made from cast iron, as is the cylinder head, as you’d expect from a heavy-duty diesel engine. The internals varies throughout the years and generations, but that being said, they’re pretty strong and have no problem holding up to increased power outputs.

In terms of aftermarket support, this engine is very popular, and cranking one up to 5, 6, 700, or even more power isn’t too difficult. Generally, this involves turning the fuel up on the mechanical engines or tuning with the 3406E, and something like an industrial or marine cam swap is very popular with the 3406E, and it’s an easy way to make upwards of 800 horsepower.

Summary

So, that’s the legend of the Caterpillar 3406 diesel engine. It’s one of the more influential engines we’ve ever seen, and it’s such a good engine that many of them are still on the road today, thanks to Caterpillar’s support with their continuation of manufacturing replacement parts even after they’ve stopped manufacturing engines since 2008.