Twin Cam A vs. Twin Cam B: What’s the Difference?

If you've spent enough time looking at used Harley engines, eventually you'll see somebody selling a:

“Twin Cam 88 motor.”

Cool.

But there's one pretty important follow-up question:

A or B?

From the outside, a Twin Cam A and Twin Cam B can look damn near identical.

Same basic engine family. Same two cylinders. Similar heads. Similar cam chest. Similar enough that if one is sitting underneath a workbench at a swap meet, it's easy to assume they're all the same.

They're not.

And if you're buying a complete engine, crankcases, crankshaft or lower-end parts, knowing the difference can save you from buying a very expensive paperweight.

The Easy Answer: A = Dyna/Touring, B = Softail

This isn't perfect for every obscure situation, but it's the easiest way to remember it.

Twin Cam A

The engine commonly called the Twin Cam A is the non-counterbalanced version.

You'll generally find it in:

• Dyna models
• Touring models

Harley introduced the original Twin Cam 88 in the 1999 Dyna and Touring families. Harley's 1999 documentation identifies those engines simply as the Twin Cam 88, rather than “Twin Cam A.” “A” has largely become the common way riders and parts sellers distinguish the non-balanced engine from the later B version.

Twin Cam B

The Twin Cam B is the counterbalanced version used primarily in Softails.

It arrived for the 2000 Softail model year. Harley's own 2000 Softail VIN documentation actually identifies the engine as the Twin Cam 88B — balanced.

So the quick version is:

Why Did Harley Build Two Versions?

The answer comes down to how the engine is mounted in the motorcycle.

A Harley-Davidson 45-degree V-twin naturally produces vibration.

That's part of the deal.

On Touring and Dyna models, Harley could use rubber engine mounts to isolate a lot of that vibration from the rider.

The engine can move around somewhat in the chassis while the rubber mounts prevent all of that movement from being transferred directly into your hands, feet and ass.

The Softail presented a different problem.

The Softail engine is rigid-mounted into the chassis.

Bolt an unbalanced big-inch 45-degree V-twin solidly into a frame and things can get pretty interesting.

So Harley added internal counterbalancers.

Thus:

Twin Cam B.

What's Inside a Twin Cam B?

This is where the engines really become different.

A Twin Cam B has two balance shafts inside the crankcases.

Those shafts are driven by their own chain and sprocket system and rotate in a way that helps cancel some of the vibration produced by the engine.

Harley's factory Twin Cam 88B parts catalog shows the additional:

• Front balance shaft
• Rear balance shaft
• Balance-shaft bearings
• Balance-shaft supports
• Balancer drive chain
• Front and rear sprockets
• Chain guides
• Tensioner components

None of that exists in the standard non-counterbalanced Twin Cam bottom end.

That's the big difference.

And it's why saying:

“It's all Twin Cam stuff.”

can get you in trouble pretty quickly.

Can You Put a Twin Cam A in a Softail?

Not as a simple bolt-in swap.

And the same goes for dropping a Twin Cam B into a Dyna or Touring bike.

The crankcases, mounting arrangement and bottom-end configuration are different.

There are custom frames, aftermarket cases and all kinds of ways people have built motorcycles over the years, so we're not saying somebody somewhere hasn't done it.

This is Harley we're talking about.

Somebody has put damn near everything into everything.

But if you're asking:

“Can I replace my factory Twin Cam B Softail motor with this Twin Cam A Touring motor I found on Marketplace?”

The answer is:

Not as a direct replacement.

Are Twin Cam A and B Parts Interchangeable?

Here's where the answer becomes:

Some are. Some aren't.

There is a surprising amount of overlap throughout the Twin Cam family.

Depending on year and displacement, you may find overlap in things like:

• Cylinder heads
• Cylinders
• Pistons
• Rocker components
• Cam-chest components
• Oil pumps
• Cam plates
• Covers
• Intake components

But don't take that list to mean they're universally interchangeable.

The bottom end is where you REALLY need to pay attention.

Things like:

• Crankcases
• Flywheel/crank assemblies
• Engine mounting provisions
• Balance shafts
• Balancer bearings
• Balancer chain components

are where the A/B distinction becomes critical.

Even Harley's performance parts documentation separates crank/flywheel applications. Harley lists different stroker flywheel assemblies for Dyna/Touring applications versus 2000–2006 Softails, which is exactly why you can't just assume a Twin Cam crank is a Twin Cam crank.

Here's a Real-World Example

Let's say you're working on a:

2004 Road King.

You find a complete 2004 Twin Cam 88 lower end for $500.

Same year.

Same displacement.

Looks basically identical.

Seller says:

“Came out of a Harley. Should fit.”

There's just one small problem.

It came out of a 2004 Softail.

That's a Twin Cam B lower end.

Your Road King uses the non-counterbalanced Twin Cam configuration.

Now that $500 deal isn't looking quite as exciting.

That's why when we're buying used Harley engine parts, model family matters just as much as displacement.

How Can I Tell If I Have an A or B?

If the engine is still installed in the original bike, this is usually easy.

Softail?

Generally Twin Cam B.

Dyna or Touring?

Generally the non-counterbalanced Twin Cam — commonly called the Twin Cam A.

If the engine is loose and sitting on a pallet, don't guess based on the heads or cylinders.

Check:

• Engine VIN / identification numbers
• Crankcase part or casting numbers
• Model year
• Case configuration
• Engine mounting points
• Seller's donor-bike information

And if you're buying a lower end, start looking for evidence of the Twin Cam B balancer system and case design.

What Years Were Twin Cam A and B Used?

As a general buying-parts reference:

Twin Cam A / non-balanced

• Dyna: 1999–2017
• Touring: 1999–2016

Twin Cam B / counterbalanced

• Softail: 2000–2017

There are year-to-year changes inside those ranges, so this does not mean every Twin Cam A part fits every A or every Twin Cam B part fits every B.

For example, Harley made significant cam-drive changes over the Twin Cam's life, and its own retrofit instructions distinguish between early 1999–2000 engines and later configurations.

Think of A vs. B as your first fitment check, not your last one.

Is One Better Than the Other?

Not really.

They were designed for different applications.

The Twin Cam A lets the chassis' rubber mounting system deal with engine vibration.

The B uses internal balancing because the engine is solid-mounted into the Softail chassis.

There are performance builders who have preferences one way or another, especially once you're talking about big-inch builds, crankshafts and high horsepower.

But for a stock or mildly modified street bike, I wouldn't look at it as:

A = good / B = bad

or vice versa.

They're simply different versions of the same basic engine family.

Buying Used Twin Cam Parts? Don't Assume Anything

This is really the point of the whole article.

If you're looking at a used:

• Engine
• Lower end
• Crankcase
• Crankshaft
• Cam chest
• Cylinder
• Head
• Oil pump
• Transmission-related engine component

don't buy it just because somebody wrote:

“Fits Twin Cam.”

Ask:

What year?

What model?

A or B?

What displacement?

What's the part number?

And whenever possible:

Measure the damn thing.

Because the $50 difference between buying the right used part and the wrong used part usually turns into a much bigger difference once you've paid shipping and it's sitting on your workbench.

Looking for Used Twin Cam Parts?

Skyway Cycle Supply carries new, used, OEM and aftermarket Harley-Davidson parts, including Twin Cam engine components.

If you're staring at a mystery Twin Cam part and you're not sure whether it's A, B, early, late, Touring, Dyna or Softail...

Send us the part number, engine number, measurements and some pictures.

We'll see what we can figure out.