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EFI tuning and Exhaust Expert
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We field a lot of questions on engine size and big bore kits so we've put together a helpful article on our FM University site on Milwaukee-8 displacement on the various engines and big bore kits.

The Motor Company offers the Milwaukee-8 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

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. The aftermarket cylinders we are referencing have a thicker liner than the OEM cylinders yet still fit the case spigot without machining.

OEM bored cylinder examples:
Milwaukee-8 117″ big bore for 107″ engines = 4.125″ bore X 4.375″ stroke
Milwaukee-8 124″ big bore for 114/117 engines = 4.180″ bore X 4.5″ stroke

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

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