Upgrading your M8 107 engine with a big bore kit is a popular path to increased displacement and power. But not all kits are created equal. We researched manufacturer specifications, industry standards, and aggregated owner feedback to evaluate seven of the most popular big bore kits for the M8 107. This guide breaks down performance, durability, value, ease of installation, and safety considerations to help you choose the right kit for your build.
| Photo | Product | Score | Buy |
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S&S Cycle 124 Big Bore Kit Top Pick 4.125 in bore, 124 ci, forged pistons, cylinder kit |
9.2 | 🛒 Check Price |
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Fuel Moto 117 Big Bore Kit Best Value 4.060 in bore, 117 ci, cast pistons, cylinder kit |
8.8 | 🛒 Check Price |
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Screamin' Eagle 128 Big Bore Kit Performance Powerhouse 4.250 in bore, 128 ci, forged pistons, complete kit |
9.0 | 🛒 Check Price |
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T-Man Performance 124 Big Bore Kit Quality Build 4.125 in bore, 124 ci, CP-Carrillo forged pistons, cylinders |
8.5 | 🛒 Check Price |
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Hillside Cycles 117 Big Bore Kit Budget Friendly 4.060 in bore, 117 ci, cast pistons, cylinder kit |
7.8 | 🛒 Check Price |
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DK Custom 124 Big Bore Kit Complete Package 4.125 in bore, 124 ci, forged pistons, includes gaskets and rings |
8.2 | 🛒 Check Price |
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Revival Cycles 117 Big Bore Kit Niche Option 4.060 in bore, 117 ci, cast pistons, basic kit |
7.3 | 🛒 Check Price |
1. S&S Cycle 124 Big Bore Kit: Top Pick
The S&S Cycle 124 Big Bore Kit is a well-engineered solution for M8 107 owners seeking substantial displacement increase. In our evaluation, this kit delivers strong horsepower and torque gains while maintaining reliability. The forged pistons and quality cylinder construction contribute to long-term durability based on owner feedback and spec sheets.
However, this kit comes at a higher cost and may not be ideal for riders on a tight budget. Those looking for a more affordable option or who do not need the full 124 ci might consider other kits. Installation requires mechanical competence, and we recommend professional tuning afterward.
- Proven 124 cubic inch displacement with high-quality forged pistons
- Includes cylinders, pistons, rings, and gaskets for a complete top-end kit
- Compatible with stock heads and throttle body for simpler installation
Pros: Excellent power gains across the RPM range; High durability with forged pistons
Cons: Premium price point; Requires proper break-in procedure
2. Fuel Moto 117 Big Bore Kit: Best Value
Fuel Moto’s 117 Big Bore Kit offers a solid middle-ground for M8 107 owners wanting more displacement without breaking the bank. Our research shows it provides noticeable power improvements while keeping costs reasonable. Owner feedback highlights straightforward installation, though some users note the cast pistons may not withstand extreme abuse as well as forged alternatives.
This kit is best suited for street riders and mild performance builds. If you plan to push high horsepower with forced induction, a forged piston kit would be a safer choice. The kit includes all necessary components, but we recommend a quality tuner for optimal results.
- Cost-effective 117 cubic inch upgrade with cast pistons
- Includes cylinders, pistons, rings, and gaskets
- Good balance of power and affordability
Pros: Great value for the displacement gain; Easy installation for experienced DIYers
Cons: Cast pistons less durable than forged under extreme use; Not ideal for high-boost or nitrous applications
3. Screamin' Eagle 128 Big Bore Kit: Performance Powerhouse
The Screamin’ Eagle 128 Big Bore Kit is the ultimate displacement upgrade for the M8 107, offering a massive 128 ci. In our evaluation, this kit delivers jaw-dropping power when paired with appropriate supporting modifications. The forged pistons and solid cylinder design ensure durability under high stress.
However, this is not a budget-friendly option. The kit alone is costly, and you will likely need to invest in head work, a camshaft, and a tuner to avoid bottlenecks. It is best for serious builders chasing high horsepower numbers. Weekend cruisers may find the expense and complexity unnecessary.
- Massive 128 cubic inch displacement for maximum power
- Includes high-compression forged pistons and heavy-duty cylinders
- Designed for use with Screamin' Eagle tuner and performance parts
Pros: Exceptional power potential; Factory-engineered for Harley-Davidson M8 engines
Cons: Very expensive; Requires supporting mods (heads, cam, exhaust) to realize full potential
4. T-Man Performance 124 Big Bore Kit: Quality Build
T-Man Performance’s 124 Big Bore Kit stands out for its use of CP-Carrillo forged pistons, a brand known for racing durability. Our research indicates this kit offers strong performance and longevity. Owner feedback praises the build quality and the company’s technical support.
On the downside, this kit is priced at a premium and may be harder to find in stock. It is best suited for enthusiasts who want a high-quality forged piston kit and are willing to pay for it. If you are on a tighter budget or need a kit immediately, alternatives like Fuel Moto’s 117 might be more practical.
- Uses premium CP-Carrillo forged pistons for reliability
- Includes cylinders, pistons, rings, wrist pins, and gaskets
- Designed for high-performance street and race applications
Pros: Top-tier piston quality; Excellent customer support from T-Man
Cons: Higher price than some competitors; Limited availability may cause backorders
5. Hillside Cycles 117 Big Bore Kit: Budget Friendly
Hillside Cycles offers a no-frills 117 Big Bore Kit that focuses on affordability. In our evaluation, this kit provides a modest power increase and is one of the easiest to install. It is a good entry-level option for riders who want to experience a big bore kit without a major investment.
However, the cast pistons and basic construction mean this kit is less durable under sustained high-performance riding. It is not recommended for track days or aggressive racing. If you are a casual street rider looking for a mild upgrade on a tight budget, this kit may work. But we suggest saving for a forged piston kit if reliability is a priority.
- Affordable 117 cubic inch upgrade for budget-conscious riders
- Includes cylinders, pistons, rings, and gaskets
- Simple bolt-on design with no machining required
Pros: Very low cost; Easy installation process
Cons: Cast pistons may wear faster under hard use; Less performance gain compared to premium kits
6. DK Custom 124 Big Bore Kit: Complete Package
DK Custom’s 124 Big Bore Kit offers a balanced package with forged pistons and all required hardware. Our research shows it delivers solid performance gains and is easier on the wallet than some premium brands. Owner feedback generally positive, but a few users noted the need for careful measurement during installation to avoid clearance problems.
This kit is a decent choice for DIY installers who are comfortable with engine work. If you prefer a kit with a more established track record and fewer fitment concerns, the S&S 124 may be worth the extra cost. However, for the price, DK Custom provides good value for a forged piston setup.
- Forged pistons for improved strength over cast
- Includes all necessary gaskets and rings for installation
- Compatible with stock heads and throttle body
Pros: Good all-inclusive kit; Forged pistons at a reasonable price
Cons: Some owners report piston-to-wall clearance issues; Not as refined as S&S or Screamin' Eagle
7. Revival Cycles 117 Big Bore Kit: Niche Option
Revival Cycles offers a bare-bones 117 Big Bore Kit that is among the cheapest available. Our evaluation indicates that while it can increase displacement, the lack of included gaskets and rings means additional purchases are necessary. Owner feedback is mixed, with some reporting decent results and others experiencing issues with fitment and longevity.
This kit is best avoided by anyone seeking reliability or high performance. It may appeal to very budget-constrained builders who already have gaskets and are comfortable with potential compromises. For most M8 107 owners, we recommend spending a bit more on a complete kit with forged pistons.
- Entry-level 117 cubic inch kit at a low price point
- Includes only cylinders and pistons, no gaskets or rings
- Requires reuse of stock head gaskets or separate purchase
Pros: Very inexpensive; Simple design
Cons: Incomplete kit, must buy gaskets separately; Cast pistons and basic components limit performance and durability
Frequently Asked Questions
What is the best big bore kit for an M8 107 engine?
Based on our research, the S&S Cycle 124 Big Bore Kit is the top pick for its proven performance, durability, and overall quality. For those seeking better value, the Fuel Moto 117 Big Bore Kit offers a strong balance of cost and power.
Do I need to upgrade other parts when installing a big bore kit?
Yes, most big bore kits benefit from a performance tuner, high-flow air cleaner, and exhaust system. For larger kits like the 128 ci, head work and camshaft upgrades are often necessary to avoid restrictions and achieve optimal power.
Can I install a big bore kit myself?
Installation requires mechanical experience with engine disassembly and reassembly. Many kits are bolt-on, but precise measurement and torquing are critical. If you are not confident, professional installation is recommended to avoid costly mistakes.
What is the difference between cast and forged pistons?
Forged pistons are stronger and more resistant to heat and detonation, making them suitable for high-performance and forced induction applications. Cast pistons are cheaper but less durable under extreme stress. For most street riders, quality cast pistons are adequate, but forged is preferred for reliability.
Will a big bore kit affect engine reliability?
A properly installed and tuned big bore kit can be reliable. However, increasing displacement stresses the engine more, so using quality components and keeping up with maintenance is crucial. Kits with forged pistons and good cylinder construction tend to be more durable.
Our Verdict
After researching and comparing the top big bore kits for the M8 107, the S&S Cycle 124 Big Bore Kit stands out as the best overall choice for its excellent performance, durability, and owner satisfaction. For a more budget-friendly option that still delivers solid gains, the Fuel Moto 117 Big Bore Kit is our runner-up. Both kits represent excellent value when matched to your riding style and goals.
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