I started out planning on doing a complete build series around Harley-Davidson’s Milwaukee-Eight engine with the intention of building a small, bolt-on, hot rod engine. That quickly got out of hand.

If you have spent much time around motorcycles or more specifically Harleys, you know there is a lot of bench racing and posturing at local watering holes among friends boasting how fast their modified Harley is. The reality is that making impressive horsepower numbers with a Harley engine can be quite difficult, especially if you want a real street bike. By that I mean one that runs on pump gas, that starts effortlessly no matter what weather or conditions, and can be your daily rider.

But when Harley introduced the Milwaukee-Eight engine platform a few years back, making bigger horsepower became a little easier. The four-valve head, increased compression enabled by oil and water cooling, along with stock cases accommodating more cubic inches, really opened opportunities for the aftermarket to make horsepower. As luck would have it, we happened to have a 2019 M8-powered Fat Bob in our shop and my itch to make some big Harley-Davidson  horsepower finally got the better of me. So I went to work.

I have done plenty of M8 cam jobs and what I consider other lighter modifications to M8s, but I really wanted to step things up with this build. I had a horsepower number in mind that in previous iterations of Harley motors would have definitely been possible but maybe not the most “streetable” of engines. I wanted to make 150 rear wheel horsepower.

I realize a dyno number is a fairly arbitrary thing. It varies by weather conditions, elevation, or even just from dyno to dyno, but I needed a target and even though it seemed a bit ambitious, that was where the goal was set. But just as important as my lofty horsepower goals was the ability for it to be attainable for someone to replicate in their own garage. No, I am not ignorant to the endless engine combinations of racey heads, giant billet throttle bodies, and unobtanium crank shafts that can be obtained from race shops all across the country. But I wanted to show that you can build this hot rod in your garage with a standard tool kit and a few specialty tools and off the shelf parts.

The great news is that not only is it possible but we documented the entire process. We will have an entire Weekend Wrenching series of videos to show you how to do the complete engine and drivetrain build and that will be releasing them over the next few weeks(?) taking you through the entire build process.

Let’s Get Started

The first step in this horsepower journey is to get the engine and driveline torn down. Outside of a general set of hand tools, it will be helpful to have a lift to put the bike on, a flat jack to prop up the engine, a T70 Torx bit and a bar to jam the primary. Drain the oil out of your engine and primary and break open the toolbox. It's time to get started building some horsepower.

Build Sheet

Here is the entire build sheet for the engine we built sans fluids. With greater power comes greater responsibility, as well as more stress on other parts on the bike, so we also did a few more things to our Fat Bob to complement the horsepower gain. And we also have installation videos for those modifications, as well. We upgraded our brakes to a set of Performance Machine calipers to help slow down our new beast. Next, we turned to Legend Suspension to help us get power to the ground via a Mono Coil rear shock. The install does require you to bleed your ABS system. We also upgraded our front suspension with a set of Legend AXO Suspension to get us through corners smoother and added a Memphis Shade Road Warrior Fairing to cut through the wind.

Here’s the list of the other parts needed for this build:


S&S Cycle 131” Flywheel Assembly

S&S Cycle 131” Big Bore Piston and Cylinder Kit

Screamin Eagle Cylinder  Heads

S&S Cycle Valve Spring Kit

S&S Forged Roller Rockers  

S&S Cycle Rocker Arm Shafts

Feuling Rocker Arm Stud Kit  

Feuling ARP Cylinder Stud and Bolt Kit

S&S 590G Cam Kit  

S&S Cam Install Kit

S&S Cam Plate and Pump Kit

S&S Tappet Cuffs

S&S Cycle Quickee Pushrod Kit

Fueling Short Travel  Lifters

Screaming Eagle 64 mm Throttle Body and Aluminum Manifold

Feuling 6.1 injectors


Rekluse Torque Drive Clutch

Evolution Solid Primary Sprocket Kit

Performance Machine Chain Conversion Kit


Bassani Road Rage III 2 into 1 Stainless Exhaust

S&S Stealth intake

Feuling Vented Dipstick

Legend Suspension 43mm Cartridge Kit

Legend Suspension Revo-A Mono Shock

Performance Machine Brake Calipers

Memphis Road Warrior Fairing

Memphis Trigger Lock Kit

Road Warrior Windshield


In addition to the parts, there are a few special tools you’ll need, in addition to what you already are likely to have in the tool box.

Crankcase Bearing Tool

Piston Ring Compressor

Valve Spring Compressor


Feeler Gauges

T70 Torx Bit

Torque Wrenches

Mainshaft Sprocket Locknut Wrench

Primary jam bar

Cam Sprocket Jam Tool

Cam Bearing Remover/Installer


Once you have the parts in hand and your toolbox sufficiently stocked, you're ready to fire up our video series and get down to building some American-made horsepower with your Milwaukee-Eight engine.