What people actually change on a Honda Grom

The Honda Grom is a 125cc motorcycle that comes from the factory with basic components — a stock exhaust, standard suspension, and modest power output. Owners modify them for three main reasons: to make them faster, to make them handle better, or to change how they look. Most modifications fall into one of those categories, and the cost ranges from under $50 for cosmetic changes to over $2,000 for engine work.

The Grom's small size and straightforward design mean that many modifications are straightforward enough for someone with basic mechanical skills. That said, some changes — like engine tuning or suspension work — require tools you may not have or knowledge that takes time to build. Understanding what each modification does, what it costs, and whether you can do it yourself helps you decide which ones make sense for your bike and your budget.

Key Takeaways

  • Exhaust systems, air filters, and fuel tuners are the most common first modifications because they improve performance without requiring engine disassembly.
  • Suspension upgrades (springs, shocks, fork cartridges) cost $300 to $800 and noticeably improve handling, especially if you carry a passenger or ride aggressively.
  • Engine modifications like big-bore kits and higher-compression pistons cost $800 to $2,000 and require mechanical skill or a shop visit.
  • Cosmetic changes — grips, mirrors, seat covers, decals — range from $20 to $200 and require no mechanical work.
  • Many modifications void or complicate your warranty, so check with your dealer before ordering parts.

Exhaust, air intake, and fuel tuning — the entry-level trio

Most Grom owners start here because these three changes work together and don't require taking the engine apart. A stock exhaust is restrictive and heavy; aftermarket options like Akrapovic, Two Brothers Racing, or Yoshimura systems cost $200 to $400 and reduce weight while improving airflow. The sound is noticeably louder and higher-pitched, which appeals to some riders and annoys others.

An aftermarket air filter or intake kit (brands include K&N, Uni, and Pipercross) costs $30 to $80 and lets the engine breathe more freely. On its own, the power gain is small — maybe 1 to 2 horsepower. Combined with an exhaust, the effect is more noticeable. A fuel tuner or ECU reflash (like a Bazzaz Z-Fi or a custom map from a tuning shop) costs $150 to $300 and adjusts the fuel mixture to match the new airflow. Without it, the engine runs rich (too much fuel) and wastes some of the gains from the exhaust and filter.

Together, these three modifications typically add 5 to 8 horsepower to the stock 9.7 horsepower — a meaningful increase on a small bike. Installation takes a few hours if you're comfortable with basic wrenching; a shop charges $100 to $200 in labor.

Suspension upgrades — springs, shocks, and fork cartridges

The stock Grom suspension is soft and designed for comfort on smooth roads. If you ride two-up (with a passenger), carry cargo, or want the bike to handle more precisely, stiffer springs and better damping make a real difference. Suspension work ranges from straightforward to complex depending on what you change.

Replacement springs cost $80 to $150 per pair and are a bolt-on job — you remove the old spring, slide the new one in, and tighten the fastener. Aftermarket shocks (brands include Ohlins, YSS, and Hyperpro) cost $300 to $600 and bolt on the same way. Fork cartridge upgrades — which replace the internal damping components in the front forks — cost $400 to $800 and require removing the forks and disassembling them, which is more involved. Many riders send their forks to a specialist shop for cartridge work rather than doing it themselves.

Suspension changes are among the most noticeable modifications because they affect how the bike feels in every corner and over every bump. A stiffer setup also reduces the bike's tendency to squat under acceleration and dive under braking, which improves stability at higher speeds.

Engine modifications — big-bore kits, pistons, and cylinder heads

These changes increase displacement or compression ratio and require opening the engine. A big-bore kit (typically 140cc to 160cc) costs $600 to $1,200 and includes a new cylinder, piston, and gaskets. A higher-compression piston alone costs $150 to $300. A ported and polished cylinder head (modified for better airflow) costs $400 to $800. All of these require removing the engine from the frame, disassembling it, and reassembling it correctly — a job that takes 8 to 20 hours depending on your experience and the scope of work.

Most riders who do engine work either have significant mechanical experience or take the bike to a shop. Shop labor for a big-bore kit typically runs $800 to $1,500. The power gains are substantial — a 160cc big-bore kit can push output to 20 to 25 horsepower, roughly double the stock figure — but the cost and complexity mean this is a later-stage modification, not a first one.

Engine modifications also carry the highest risk of creating reliability problems if something is assembled incorrectly or if parts are mismatched. A reputable shop or experienced builder reduces that risk significantly.

Wheels, tires, and brakes

Lightweight aftermarket wheels (brands include Galespeed, Marchesini, and Rotobox) cost $600 to $1,200 per set and reduce unsprung weight, which improves acceleration and handling. They're a bolt-on upgrade if you have the right tools and knowledge to balance and true them. Many riders have a shop do the installation.

Tires are a consumable that you'll replace anyway; upgrading to a sportier compound (like Michelin Power RS or Pirelli Diablo Rosso) costs $80 to $150 per tire and improves grip and feel, especially on a track or in aggressive riding. Brake upgrades are less common on a Grom because the stock brakes are adequate, but a braided steel brake line ($40 to $80) or a radial master cylinder ($150 to $250) can improve brake feel and modulation.

Cosmetic and comfort changes

These modifications don't affect performance but change how the bike looks or feels to ride. Aftermarket grips cost $20 to $60 and can reduce hand fatigue on longer rides. A custom seat or seat cover costs $100 to $300. Mirrors, fenders, headlight bezels, and decal kits range from $15 to $150 each. Clip-on handlebars cost $50 to $150 and change the riding position. A tail tidy (a smaller rear fender and light assembly) costs $80 to $200 and gives the bike a more aggressive look.

These changes require no mechanical skill beyond basic bolt-tightening and are a good way to personalize the bike without affecting warranty coverage or reliability.

Warranty and reliability considerations

Honda's warranty on a new Grom typically covers the frame and engine for three years or a set mileage limit, whichever comes first. Aftermarket exhaust, air filters, and fuel tuners may void the warranty on the engine, depending on Honda's interpretation and your dealer's policy. Suspension and wheel changes usually don't affect warranty coverage because they're separate systems. Engine modifications (big-bore kits, pistons, head work) almost certainly void the engine warranty.

Before ordering parts, call your Honda dealer and ask whether a specific modification will affect coverage. Some dealers are strict; others are more lenient. If warranty coverage matters to you, stick to cosmetic changes and suspension upgrades. If you're past the warranty period or willing to accept the risk, you have more freedom to experiment.

Reliability also depends on the quality of the parts and the skill of the person installing them. A cheap big-bore kit from an unknown brand, installed incorrectly, can cause engine failure. A quality kit from a reputable manufacturer, installed by someone who knows what they're doing, typically runs reliably for thousands of miles.

Frequently Asked Questions

How much horsepower can I realistically add to a stock Grom?

With bolt-on modifications (exhaust, filter, tuner), expect 5 to 8 additional horsepower, bringing the total to around 15 to 18 hp. A big-bore kit and head work can push it to 20 to 25 hp. Beyond that, you're looking at exotic builds with significant cost and complexity.

Can I do these modifications myself if I've never worked on a motorcycle before?

Cosmetic changes, exhaust, and air filter work are beginner-friendly. Suspension springs and shocks are intermediate. Engine work (big-bore kits, pistons, head porting) requires mechanical skill and specialized tools. If you're unsure, a shop can do the work; expect to pay $100 to $300 in labor for bolt-on modifications and $800 to $1,500 for engine work.

Do I need to modify my Grom to have fun on it?

No. A stock Grom is fun to ride and reliable. Modifications let you customize it to your preferences and riding style, but they're optional. Many owners ride stock Groms for years without any changes.

What's the most popular first modification?

An aftermarket exhaust is the most common first change because it improves sound and performance, costs less than $400, and is straightforward to install. Many owners pair it with an air filter and fuel tuner shortly after.

Will modifications make my Grom faster in a straight line?

Yes, but the gains are modest because the Grom is light and the stock engine is already fairly efficient. A big-bore kit with tuning can add 10 to 15 mph to top speed. Exhaust and filter alone add 3 to 5 mph. Suspension and handling improvements matter more than raw power on a bike this size.