Upgrading your Harley-Davidson Milwaukee-8 engine with a big bore kit can deliver massive horsepower and torque gains. However, installing a 124″ or 128″ big bore kit requires more than just larger cylinders and pistons. To achieve maximum performance and reliability, you need the right supporting components, proper tuning, and a solid installation plan.
This guide focuses on 124″ and 128″ big bore kits, which can be installed on 2017-later Milwaukee-8 engines without case boring. There are two main approaches to these builds:
- Street Touring Kits – These prioritize low-to-mid RPM power, often retaining stock cylinder heads, throttle body, and bolt-in cams.
- High-Output Kits – These create maximum horsepower and typically include headwork, a larger throttle body, upgraded injectors, and high-performance exhaust systems.
For the best results, it’s important to select high-quality components, match them properly, and ensure precise tuning.
Choosing the Right Exhaust System
A high-performance exhaust system is essential when increasing engine displacement. The stock exhaust or systems designed for stock engines often restrict airflow, limiting horsepower.
- If your build targets 130+ HP and 140+ TQ, you need an exhaust system proven to support those power levels.
- Many aftermarket exhausts work well on stock engines but fail to deliver on big bore applications.
- Avoid excessively large open-baffle mufflers, which can negatively impact performance and throttle response.
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 and 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
A larger engine needs more air and fuel to make power efficiently.
- Stock 55mm throttle bodies work for builds up to 125 HP, but upgrading to a larger unit yields gains—sometimes 10+ HP on 140+ HP builds.
- Larger injectors are required. We typically use 5.5gm injectors, which support up to 160 HP.
- Your air cleaner must support the engine’s power level. Many stock or aftermarket air cleaners lack enough surface area or have restrictive covers that limit airflow.
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 Tuning: The Most Critical Step
Proper EFI tuning determines how well your big bore kit runs. The best EFI tuner is the one that:
- The tuner or dyno shop is most comfortable using.
- Offers reliable tune support and a strong tuning toolset.
- Allows for pre-build tuning strategy planning to ensure a smooth tuning process.
Tuning is crucial because fueling, ignition timing, and throttle response must be adjusted to match the increased engine displacement.
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 and peripherals that are available to you. Plan the tuning strategy before you do your build, start with the closest map possible for your combination.
Clutch Upgrades for Big Bore Applications
More power means more clutch pressure is needed to prevent slipping.
- The FM Duplex Clutch Kit will support 160+ TQ
- There are multiple options for AIM SDR clutches that can support your build
- Always use a non-synthetic primary lube. Bel-Ray V-Twin Primary Lube has been our preferred choice for years.
We generally use the FM Duplex kit or an AIM SDR lockup with heavy duty springs to support 160+ ft. lbs. of torque. The Duplex kit fits both Touring and Softail models. AIM offers both Touring and Softail specific lockup clutches. 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.
Final Thoughts on Installing a Big Bore Kit
A successful big bore build requires careful component selection, precision tuning, and expert assembly. Pairing the right Milwaukee-8 cam specs with a properly tuned EFI system, upgraded throttle body, and a matching exhaust ensures maximum power gains.
Ready to start your build? Check out these resources:
Fuel Moto Dyno Charts – See real-world big bore power results.
Fuel Moto Milwaukee-8 Big Bore Kits – Find the right kit for your build.
EFI Tuning Products – Get the best tuning tools for your setup.
Clutch Components – Upgrade your clutch for increase torque.
Cam Plate’s & Oil Pump’s – Ensure proper oil flow for high-performance builds.
For expert advice and top-tier performance parts, visit www.fuelmotousa.com!
