This piston kit is an 86mm standard-bore rebuild component designed specifically for the TRX400FW engine family from 1995 through 2003. It’s engineered to restore compression and power, delivering factory-level performance once again for daily riders and trail work. Featuring an OEM-referenced design (13101-HM7-000), this aftermarket replacement fits the stock cylinder and top-end components for reliable, consistent operation.

The kit targets common wear issues found in aging top-ends—loss of compression, diminished ring seal, and piston slap that erodes throttle response. By using a forged aluminum piston with a true 86mm bore, it delivers improved heat management and durability, helping maintain predictable power in rugged conditions and on the farm. The compression ratio for this bore size is typically around 8.2:1, aligning with the engine’s original performance characteristics.

Comprehensive Vehicle Fitment

Product Specifications

OEM & Aftermarket Cross-Reference Information

What's Included

This piston kit comes with the following components:

• Piston
• Rings
• Wrist Pin
• Circlips

Why Choose This Aftermarket Replacement?

• Direct-Fit Compatibility: Precisely matched to the 86mm standard bore TRX400FW engines, ensuring straightforward installation.
• OEM Cross-Reference: Designed to meet or exceed OEM specs with an exact cross-reference to 13101-HM7-000.
• Premium Materials: Forged aluminum piston chosen for strength, durability, and reliable heat management under load.
• Performance Restoration: Restores compression and throttle response lost to wear, delivering consistent power on trails and in field work.
• Reliability in Tough Conditions: Built to withstand dirt, dust, and temperature fluctuations common on trailers, farms, and rugged backroads.
• Cost-Effective Maintenance: A high-quality aftermarket option that delivers OEM-like performance at a competitive price point.

Installation Notes:

• Pre-assembly inspection: Inspect the cylinder bore for scoring or out-of-round wear and confirm it remains a true 86mm standard bore before installation.
• Cleanliness is critical: Thoroughly clean all mating surfaces, oil passages, and the cylinder interior to remove old carbon and debris that could affect ring seal.
• Ring orientation and gaps: Install the piston rings with the correct orientation and verify end gaps per the ring spec before seating the piston.
• Wrist pin and circlips: Ensure the wrist pin diameter matches the piston and install circlips securely in both ends to prevent pin migration during operation.
• Lubrication and break-in: Apply assembly lube to the piston, rings, and bore, then follow a proper break-in procedure to seat rings and establish a good seal.
• Initial run checks: After installation, perform a careful check for unusual noises, oil consumption, or loss of compression; address any scoring or binding before returning the engine to service.

Why Upgrade?

Running a worn piston kit leads to reduced compression, power loss, higher oil consumption, and increased risk of engine damage or seizure under load. Replacing with this 86mm standard-bore aftermarket piston kit restores compression and power, delivering durable performance and dependable reliability for trail rides and farm work.