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2003 Honda 400EX SporTrax (Specs and Review)

The 2003 Honda 400EX (a.k.a. TRX400EX or SporTrax) has become a household name among hobbyists and enthusiasts. Its popularity should be no surprise since the 400-cc SporTrax has always been regarded as one of the most capable and fun quads around. Ruggedly aesthetic and stark, this 4×4 introduced pure-sport 4-strokes into the ATV landscape and provided consumers thrills and a whole lot more – as you will soon discover in this guide.

The 2003 Honda 400EX (otherwise known as SporTrax) hails from the sport-ATV lineup that signaled the renaissance of the sport riding segment. Featuring an XR400R-adopted engine, 72-mph top-speed rating, and Pro-Link rear suspension, it has become a crowd favorite among sport-rec, 4-stroke quads.

Simplicity, versatility, and pure-sport ergonomics only form part and parcel of the 2003 Honda 400EX’ key selling points. Continue reading about the specs, features, and lowlights of this sport riding bigwig in this article to learn more.

Yellow ATV Sand Dune Jumping

About the 2003 Honda TRX 400EX

The 2003 Honda 400EX is the 5th installment of the Sportrax lineup since its inception in 1999. A sport-oriented machine down to its core, the SporTrax’s dedication to the revival of the sport riding segment became a rallying point for its loyal followers. Consequently, it also led to the Honda 400EX making a name for itself as a motocross-worthy four-wheeler.

Winning the Baja 1000 fresh out of the crate is immensely responsible for this remarkable feat. The machine would not have been able to achieve this had it not been for its XR400R-inspired engine and other race-ready components.

Ultimately, Honda wanted the ’03 model to dominate MX/GNCC arenas alongside powerful 2-stroke contenders already gracing the sport-ATV scene. But due to setbacks in overall design and scarcity of tech and feature upgrades, the machine was eventually resigned to being a serious trail tamer.

2003 Honda 400EX Specs & Features


The 2003 Honda 400EX is brought to life by an air-cooled, single-cylinder 4-stroke engine. Piston displacement is 397-cm3 (24.23 in3), the bore-stroke ratio is 85 × 70 mm (3.35 × 2.76 inches), and the compression ratio is 9.1:1. This racing-inclined power mill equips a Radial Four-Valve Combustion Chamber and a 38-mm piston valve with an accelerator pump. The 38-mm carb mates to oiled, urethane-foam air filtration and dry-sump lubrication and handles air-fuel mixture.

Given the SporTrax engine was adopted from the XR400R, its configuration will naturally lend to a 2003 Honda 400EX top speed of 65 – 72 mph (105 – 116 km/h) and a 28 RWHP (20.59 kW) horsepower. Overall, the quad’s powerplant is pretty robust. However, you will need to adjust the carburetor for high-altitude riding above 3,000 feet or 1,000 meters.

Fuel & Lubrication

It has a 2.64 US gallon fuel tank capacity. Using fuel with the appropriate octane rating is fundamental and reduces (if not eliminates) engine pinging or spark knock. Preventing contaminants such as dust, dirt, or water from getting inside the tank also helps.

Engine pinging results from the air and fuel mixture inside an engine cylinder igniting incorrectly. Hence, expect slight pinging when riding uphill or under heavy load, as these circumstances can potentially cause mistimed ignition. However, consulting a professional mechanic or immediately changing fuel brands is strongly advised if pinging occurs at sustained speeds or under normal riding conditions.

As for engine oil, capacity is 1.7 liters/1.8 US quarts (after draining) or 2.2 liters/2.3 US quarts (at disassembly) of SAE 10W-40 Pro Honda GN4/HP4 motor oil or its equivalent. Other viscosity grades – SAE 5W, 10W-30, 20W-40, or 20W-50 – are permitted for use following ambient temperature.

For excellent lubrication and unaffected clutch performance, use oil variants with a minimum API grade of SJ+ meeting JASO T903 MA/MB standards. Refrain from using non-detergent, vegetable, castor-based racing, or engine oils with graphite or molybdenum additives.


Power travels to the ground via a 5-speed constant-mesh transmission and a cable-operated, wet, multi-plate clutch assembly (with a 1-N-2-3-4-5 gearshift sequence) sans reverse. An RK520 SMOZ10S or DID520V6 O-ring sealed chain (96 or 120 links + joint) – replaceable by a DID 520ERT3120RB 520 ERT3 Series Exclusive Racing Chain (view on Amazon) – handles wheel spin.

A gear-driven counter-balancer reduces unwanted engine vibration, making for smooth maneuverability regardless of terrain difficulty. Plus, its fording depth limit of 10 inches (25.4 cm) allows the quad to traverse shallow water crossings with ease.

Primary Reduction Ratio2.826 (23/65)
Final Reduction Ratio2.533 (15/38)
Transmission Gear Ratio – I2.917 (12/35)
Transmission Gear Ratio – II1.938 (16/31)
Transmission Gear Ratio – III1.474 (19/28)
Transmission Gear Ratio – IV1.182 (22/26)
Transmission Gear Ratio – V1.000 (26/26)


Placement of most of the controls – such as the clutch and parking brake levers, start button, headlight/dimmer/engine stop switches – are located on the left handgrip and has been this way for the majority of the 2003 Honda 400EX year models. The fuel valve, gear pedal, and choke lever are on the same side of the engine.

Throttle levers, front brake fluid reservoir, front brake, and clutch lever are on the right handgrip. The neutral indicator lamp and ignition switch remain on the center console, while the rear brake fluid reservoir and operator foot peg are on the right side of the 4×4.


An ICM (or Ignition Control Module) and electric starting system with an idle speed of 1,400 ± 100 RPM brings the ’03 SporTrax to life. A single-phase (SCR shorted, full-wave) alternator with a rated output of 0.147 kW @ 5,000 RPM serves as its charging system.

It uses a maintenance-free 12V 8Ah/(10 HR) YTX9-BS battery with dimensions of 150 x 87 x 105 mm for powering electronic accessories. It also requires a Nippon Denso X24GPR-U or NGK DPR8Z spark plug with a 0.024–0.028 inch gap and torque spec of 1/8 – 1/4 turn (old plugs) or 1/2 turn (new plugs) after the plug seats. For extended, high-speed riding, switch to an NGK DPR9Z or Nippon Denso X27GPR-U spark plug instead.

Tires & Brakes

Front 22 x 7-10 M/R 101 and rear 20 x 10-9 M/R 501 Ohtsu radials (each with a 4/110 bolt pattern) mount on steel or aluminum wheels. These knobbies pair with all-wheel-drive triple hydraulic discs (174-mm dual front discs and a single 220-mm rear disc) with dual brake calipers, providing the 4×4 a 70:30 percent ratio of its stopping power.

Front wheels are 10×5.5 AT mags, while rear ones are on 9×8.0 AT rims (view on Amazon). Aftermarket tires like Maxxis Razr 4-Ply Sport Tires (view on Amazon) are excellent replacements for stock rubber in case of wear or damage.

Front and rear cold-tire pressure should be 27 kPa. Depending on the terrain type, airing down or inflating tires within the range of 23 kPa to 31 kPa is permitted. Regular tire tread inspection is important, as a depth of at least 0.1875 inches (4.0 mm) warrants immediate tire replacement.


Enclosed in a double-cradle steel frame (6.5° caster angle, 28-mm trail) are independent, front double-wishbone with 5-way preload-adjustable shocks and a Pro-Link rear swingarm with a fully adjustable Showa mono-shock. Each setup offers a respective wheel travel of 209 mm/8.2 inches and 230 mm/9.1 inches.

The 48.4-inch/1,230-mm wheelbase, 4.3-inch/110-mm ground clearance, and 3.2-meter/10.5-feet turning radius remain unchanged as in previous-year models. All in all, the machine’s suspension components make for reduced unsprung weight, tighter cornering, and improved traction.


Overall dimensions of the 2003 Honda TRX400EX are 72.2×45.3×43.7 inches. Seat height conveniently accommodates medium-built to taller riders at 810 mm/31.9 inches and is perfect for ripping through flat surfaces. The GVWR estimate is at 635 lbs., including the maximum load capacity of 243 lbs., the curb weight of 392 lbs., and a full gas tank.


The ’03 SporTrax was one of the earlier models that had yet to receive Team Honda-inspired graphics. The overall styling is simplistic and great for customization while robust enough to safeguard the engine and other internal components from the elements. The front cowl and factory fenders can benefit from racing decals and aftermarket 2003 Honda 400EX parts to add a touch of aggressiveness to the quad.

2003 Honda 400EX plastics were available in red and yellow, with body-matching, high-clearance fenders, and heavy-duty urethane skid plates that protect the wheeler’s underside and rear brake-disc/sprocket. You can keep the body panels as damage-free as possible by spray cleaning them with running water and mild detergent. Refrain from using a power hose, as it can cause abrasions and lead to corrosion.

2003 Honda 400EX Price

The 2003 Honda 400EX retained its list price of $5,699, cheaper by $400 against the 2000 MSRP. This competitive pricing gave it an edge over its same-class counterparts. Furthermore, it was a sensible decision on Honda’s part, as virtually nothing has changed in the machine’s framework and overall design. Like production models from the previous year, the ’03 SporTrax only offered standard trims – perhaps, a means for the Japanese firm to keep the quad’s retail pricing in check.

As for resale values, prices range from $1,805 to $2,375 per Nada Guides data. About the same price range – from $978 to $3,000 – shows in auction listings, Craigslist, and other trader sites. Most of these pre-owned units are in decent, running condition, and frequently sold in the UK, California, Texas, and North Carolina. Near-mint quads would naturally fall on the higher end of the price spectrum and come equipped with performance-enhancing add-ons, raising the average resale price by at least $500.

Honda 400EX Starting Issue

Bore increases and fuel/electrical system problems aside, a no-start issue is innate with the ’03 Honda SporTrax. And usually, the first thing you will notice is your starter motor attempting to turn your engine over without any luck. When this happens, check the spark plug tightness, engine stop switches, and battery while ensuring the latter has a full charge.

Likewise, determine if any bore modification was done and if this was performed correctly. If these components check out, you may begin inspecting other components to pinpoint if the culprit is engine flooding, fuel starvation, or some other type of breakdown.

Other problems to watch out for are head gasket leaks, flimsy swingarm, overheating, and intermittent stalling. The clutch and gearbox are reportedly problematic – especially for heavily modded quads or those used for racing applications. Stock tires are also not as puncture or damage-resistant as owners would have liked.

More often than not, these issues stem from driving habits and do not necessarily mean the 4×4 falling short on durability. Thankfully, your service manual provides relevant information on how to address these hiccups.

About Honda

Honda Motor Company Ltd. is a Japanese conglomerate known worldwide for its ingenuity in automotive design and a dominant force in the ATV and motorcycling industries. Established in 1937 as Tōkai Seiki or Eastern Sea Precision Machine Company, Honda began as Toyota’s supplier of mass-produced piston rings.

Since then, the Japanese firm has come a long way, diversifying its product offerings to include surplus engines, motorized bicycles, mini pickup trucks, and eventually venturing into ATVs like the 2003 Honda 400EX.

Red ATV Riding Up Sand Hill

Conclusion – 2003 Honda 400EX Review

It took another two years for Honda to give the 2003 Honda 400EX suspension upgrades. But even then, the four-wheeled machine still was not at the receiving end of GNCC-worthy modifications. Most old-timers may think the company’s show of restraint did it more harm than good.

But for enthusiasts with a clearer understanding of the future, the quad’s starkness certainly gave us, owners, more freedom to make the vehicle their own. Highly-skilled operators and casual riders can agree that the ability to customize a machine can dramatically improve one’s riding experience – whether on wooded trails or hard-packed roads.