The 2017-later H-D Milwaukee-8 engines have become extremely popular for modifications, we’ve been testing and developing big bore engine kits for this motor since fall of 2016 when they were released. The Motor Company offers this engine in several displacements with assorted bore sizes and strokes, we want to provide some info to avoid confusion among these engines as the same displacement can be established with different bore & stroke combinations.
Let’s start with the bore & stroke of the stock Milwaukee-8 engines:
H-D Milwaukee-8 107″ = 3.937″ bore X 4.375″ stroke
H-D Milwaukee-8 114″ = 4.00″ bore X 4.5″ stroke
H-D Milwaukee-8 117″ = 4.075″ bore X 4.5″ stroke
Harley SE Stage III and IV kits:
Milwaukee-8 Stage III/IV 114 kits = 4.075″ bore X 4.375 stroke
Milwaukee-8 Stage IV 117 kit = 4.075 bore X 4.5″ stroke
Milwaukee-8 Stage III/IV 119 kits = 4.155″ bore X 4.375 stroke
Milwaukee-8 Stage III 122 kits = 4.155″ bore X 4.5″ stroke
Milwaukee-8 Stage IV 128 kits = 4.310″ bore X 4.375 stroke
Milwaukee-8 Stage IV 131 kits = 4.310″ bore X 4.5 stroke
Aftermarket big bore kits
You have quite a few options here, it all depends on your build objectives and if you are boring the OEM cylinders or you are replacing the cylinders with aftermarket. Note the factory cylinders are different between the 107 engines and the 114/117 engines, the 107 M8 cylinders can only be bored to 4.125″ (117″ CI), however the factory 114/117 cylinders have a thicker sleeve that can be bored to 4.180″ (124″ CI). The case spigot bore is the same between each engine. NOTE: we not longer bore factory cylinders.
Big Bore aftermarket cylinders have a thicker liner than the OEM cylinders yet still fit the case spigot without machining, this is the best route for big bore with options as large as 131″
Aftermarket cylinder examples:
Milwaukee-8 120″ big bore for 107 engines = 4.180″ X 4.375″ stroke
Milwaukee-8 124″ big bore for 107 engines = 4.250″ X 4.375″ stroke
Milwaukee-8 128″ big bore for 114/117 engines = 4.250 X 4.5″ stroke