What do I need to install a big bore kit on my H-D Milwaukee-8 bike?

In this article we are going to discuss some of the associated components and peripherals that are required for successfully assembling a big bore kit on a Milwaukee-8 engine. We are going to primarily discuss the 124″ & 128″ variety which can be installed on 2017-later engines without case boring. There are really two types of combinations & objectives when it comes to these kits; 1) Street Touring kits that focus primarily on low-mid range power, these kits generally use stock cylinder heads & throttle body and in most cases use a short bolt in cam. 2) High Output kits, these are going to be more elaborate combinations that are configured to make considerably more horsepower, they generally include headwork, larger throttle body, and a host of other components that are necessary at these higher power levels. The following information is about giving the engine what it wants, not telling it what it needs. The best combinations with the best components, combined with the most precise tuning will yield the best results, every time.


Exhaust System

You will need an exhaust system that’s proven to support the power level of the kit you are working with. For example; if it’s a 130Hp/140Tq build you need a pipe that’s known to support 130+Hp/140+Tq. Most exhaust systems on the market for these bikes are designed around stock engines, unfortunately many/most of these exhaust systems are not going to work. It’s not even the manufacturers fault, if you are are adding 50% or more power you are asking the pipe to do something it was not designed to do in the first place. There is a relatively short list of exhaust systems on the market (although growing) that will support big bore applications (especially the higher output combinations), and as power levels increase exhaust design is paramount. The excessively large, open baffle mufflers that many are running simply will not work in many big bore applications. See our exhaust test. A short statement is many exhausts that work really well in a stock or mildly modified engine may not work well in a big bore application. Talk to the shop that is doing the build, talk to the manufacturer. Tell them you need a pipe for your build and your objectives, ask for examples and dyno charts; an answer “that should work” is not going to cut it. If the exhaust is not right for the application it’s not just a matter of making less power or less torque, the wrong exhaust for the combination can completely affect the entire outcome & it’s tunability, it may put a hole in the power curve, and we’ve seen examples where the motor wouldn’t even want to take throttle. Choose your exhaust system wisely.


Throttle Body, Injectors, & Air Cleaner

For mild big bore kits many times we utilize the stock 55 throttle body, they will generally work on builds up to about 125Hp, however there are still good gains by upgrading to a larger unit on these builds. If you are assembling a higher output combination a larger throttle body is pretty well mandatory and will make a significant difference in this application. In early testing we found gains as much as 10+Hp with larger throttle body on 140+HP builds. Larger injectors are required in all of these builds, in most M8 applications we typically use 5.5gm injectors which will support up to about 160Hp. You will also need an air cleaner that will support the power level of the engine you are building. Again, most of the air cleaners on the market are designed for stock or mildly modified engines; many of them do not have filter elements with sufficient surface area, some have covers that partially shroud the element, etc.. it all comes down to having an air cleaner assembly that will flow enough CFM for the given application. Some brands offer larger elements for this purpose, otherwise you will need to utilize a different unit that will support the engine you are building.


EFI Tuner

Tuning the EFI is one of the most (if not the most) important facet of the big bore installation. There are assorted EFI tuning products on the market for tuning your factory ECM as well as stand alone replacement ECM’s that will work. The best advice we can give is to use the product the person doing the tuning is most comfortable tuning with; this goes for either a professional dyno tuner or if you are tuning the bike yourself. Use the product you can get the best tune support on, and the product with the best tuning toolset & peripherals that are available to you. Plan the tuning strategy before you do your build, start with the closest map possible for your combination.



In a big bore application the clutch will need more holding power. We generally use the AIM SDR lockup & heavy duty springs and this will hold 160+Tq. Unfortunately due to the design of the primary & derby cover on Softail models the SDR will not fit due to clearance. On the Softails we use the heavy duty springs which will generally hold up to 150Tq. It’s also extremely important to use a non synthetic lube in the primary, Belray V-Twin Primary lube has been our go-to for many years.


Quick Links:

Fuel Moto dyno charts
Fuel Moto Milwaukee-8 Big Bore kitsĀ 
EFI Tuning Products
Clutch Components
Cam Plate’s & Oil Pump’s