What's a Big Twin?

New to Harley? You may see fitments specifying that an item fits "Big Twin." But Harley-Davidson never made a model called the "Big Twin." If you're trying to figure it out, you're in the right spot. "Big Twin" describes the engine in some bikes.

Up front is the quick answer, and then if you feel like reading, a little more explanation. In years gone by, Harley-Davidson models traditionally broke down largely into "Sportster" and "the big bikes." 

TL;DR: Today, we carry on that legacy and "Big Twin" generally means, "air cooled, internal combustion engine, not found in a Sportster that's platform-agnostic with the final drive on the left." 

Here's a chart you can use as a cheat sheet.

For many of you, that will suffice. For the dedicated learners out there, keep readin'!

A bit more on this (admittedly confusing) system

This system used to make a ton of sense. But it's gotten messy as time has rolled on. Every year brings new models, but the old hogs still prowl the roads, so we need to account for a pretty large range of bikes.

Now, we identify models (Wide Glide), platforms (Dyna), engine (Twin Cam) and letter codes (FXDWG in the case of our Twinkie example). Some motorcycle parts are naturally more fit-specific. 

But especially with engine items that are used across platforms, "Big Twin" is still often the most accurate and efficient way to describe something that fits all the air-cooled bikes that aren't Sporties. 

As Harley's model range grew, the aftermarket came to realize that some platforms don't lend themselves to heavy modification like the V-Rod, Street, and liquid-cooled Sportster. Generally these are excluded from most aftermarket fitments, which touches on the first hallmark of a "Big Twin" fitment: air cooling. Even this is changing—Twin Cooled Touring bikes and air-and-oil-cooled Softails are indeed Big Twins, but for many years, this was an important delineator.

The most important characteristic is a divorced transmission. If you are familiar with Triumphs or metric motorcycles, "unit construction" may be a term you are familiar with. It means the engine and transmission are housed in the same box. If you need to take the engine out, the transmission must come with it, and vice-versa.

If you can unbolt the transmission and the engine stays in the bike (or the flip-flop of that, trans in and engine out), it's divorced construction. Unit construction makes for a more compact drivetrain and adds needed stiffness and rigidity. Divorced construction lends itself well to chassis modification and serviceability aspect of things.

The Sportster has always been of unitized construction since it was born. Big Twins have not, though in recent years the trans is bolted to the engine in order to try and get some of the benefit of unit construction without destroying the serviceability.

As with all things Harley, this isn't a hard-and-fast rule. The Servicar (G) and Solo models (W) are divorced construction, but at just 45 cubic inches of displacement, they aren't counted as Big Twins, mainly due to the fact that they literally are the smaller flathead, physically and in terms of displacement.

Confusingly, this problem with the flatties occurred again in the '80s: the little guys and the Big Twins had the same colloquial names when the engines hit the streets. The Sportster has an "Evolution" engine and so does a bigger bike like a Dyna, and they are totally different powerplants. 

This is where the drive chain can help you ID what you see. If you see a stock-ish Harley-Davidson with a drive chain or belt that's not on the jiffy-stand side of the bike, it's almost certainly not a Big Twin engine. 

Why the aftermarket uses the system still

It's shorthand because we don't like typing. Seriously, it's just the best way to communicate fitment in as concise a format as we can. Here, let me give you an example.

Some of the engines lasted a long time across many platforms, and the engines ran side by side in many cases. For instance, the Big Twin Evo was found in the 1984-1994 FXR, 1985-1987 four-speed bikes (Fat Bob and Wide Glide), 1984-1998 Touring, 1991-1998 Dyna, 1984-1999 Softail, and a plethora of aftermarket choppers. Remember that in '84 you could still buy a Shovelhead-powered motorcycle or one with an Evolution, and in 1999, the Dyna and Touring bikes were available with Twin Cam engines, but Softails were only available with an Evolution. To make matters worse, Harley-Davidson reintroduced a small run of FXRs in 1999 and 2000 as CVO bikes that used Evolution engines.

For something like a rocker box, it's far simpler to write "1984-1999 Evolution Big Twin." (CVO bikes are usually excluded from fitments, but trust that anything listed for that would probably fit a 2000 FXR4 just fine.)

For the older machines, there was a ton of crossover even on non-engine parts. For instance, the only differences between any 1936 to 1947 Flattie and Knuck of the same year are the oil lines, exhausts, engine, and fuel tanks. Every other piece on the bike is the same. 

You probably have the hang of it by now. If you don't—give us a jingle. We can help.