Video: Building a 150-horsepower Milwaukee-Eight, Part 2, Crankshaft to Cylinder Heads

Crankshaft and Engine Case Assembly

In the last installment of our Weekend Wrenching project, we laid out our overall engine build covering the exact parts we used in our quest to squeeze 150 horsepower out of our street engine. And it is absolutely a street machine, meaning 91 octane pump gas, built with off-the-shelf components. We didn’t source exotic cylinder heads, monster throttle bodies, or push compression ratios to levels where the bike could only run on jet fuel.

But we did increase the displacement of the M8 powerplant to 131 cubic inches to help us move in the direction of sportbike-level horsepower numbers. And our first step in the direction of big cubic inches was to assemble the bottom end of our engine with an S&S Cycle stroker crank assembly that will be the foundation of our build. It not only increases the stroke of the engine from 4.5 inches to 4.625 inches, but also gives us a bulletproof bottom end to build big power with.

Other than a basic set of tools and a good torque wrench, here is a list of parts and specialty tools you will need to complete the crankshaft installation.

• S&S Cycle 131” Flywheel Assembly
• JIMS Crankshaft Bearing Tool
• A small hydraulic press

Big Bore Piston and Cylinder Assembly

Once you have assembled the bottom end of the engine, it’s best to install the cases back into the frame of the motorcycle before you proceed any further. Especially if you are working alone, the more of the engine you have assembled the tougher it will be to install as time goes on. To reach our final destination of 131 cubic inches of displacement, we need to top our S&S Cycle flywheel and rod assembly with larger pistons and cylinders. And we did just that using an S&S Cycle big bore kit that increased our piston size from a 4.0-inch bore to a 4.25-inch bore that, combined with our stroker crank, resulted in our final 131 cubic inches or 2,146 cc if you are playing along on the metric side.

For the cylinder and piston install you will need the following:

• S&S Cycle Big Bore Kit
• Piston Ring Compressor
• Small file

Cylinder Head Upgrades

On any engine build, the cylinder heads are an integral set of components when it comes to making power. They are the gateway for air to enter and leave the combustion chamber and the more efficiently you can do that the more power you can make. And there are definitely levels to the cylinder head game. Bigger valves, exotic and expensive port jobs all work very well but can be costly with long turnaround times. We didn’t want to leave power on the table by using our stock heads; we also wanted to keep the cost down and our build timeline in check. So we decided to use Harley-Davidson’s Screamin’ Eagle CNC ported  heads, which has what I consider a mild port job and it's an off-the-shelf part with no core charge. Meaning that you don’t have to swap out your stock heads and they ship right away. We also chose to upgrade some hardware and other components, like valve springs and roller rockers, while we had the heads off to add durability to our build.

For the Cylinder Head Upgrades you will need the following:

• Screamin Eagle Cylinder Heads
• A set of calipers
• S&S Roller Rockers
• Fueling Rocker Hardware
• Fueling Valve Springs
• JIMS Valve Spring Compressor

Once you have the cylinder heads on the engine you have knocked out the majority of the large components of the build. But we still have a ways to go, cam selection and installation, fuel system upgrades as well as addressing the durability of our driveline components in relationship to our newfound horsepower all lay ahead in our horsepower journey. Keep an eye out for the next installment documenting our big motor build.

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