This OS 0.75 oversize piston ring kit is engineered to refresh the 400cc four-stroke Foreman platform’s top end. It fits the TRX 400FW Foreman models produced from 1995 through 2003 and uses an 86 mm bore with a +0.75 mm oversize to compensate wear. The result is restored compression, reduced oil blow-by, and renewed power for trail work or farm duties.

Designed as a reliable aftermarket replacement with geometry matched to the 86 mm bore, this OS 0.75 kit targets common wear like compression loss and rising oil consumption found on high-mileage engines. The tailored ring set delivers a durable seal under varying load and temperature, making it a dependable choice for rugged trail riding or farm duties where downtime isn’t an option.

Comprehensive Vehicle Fitment

Product Specifications

OEM & Aftermarket Cross-Reference Information

What's Included

In the box you’ll find the essential components for a complete piston-end seal refresh on your TRX 400FW:

• Piston Ring Set (0.75 mm oversize)
• Top compression ring
• Second compression ring
• Oil ring

Note: This kit does not include the piston, wrist pin, circlips, or gaskets.

Why Choose This Aftermarket Replacement?

• Direct-Fit Compatibility: Precisely matched to an 86 mm bore with +0.75 mm oversize for reliable seal and proper ring end-gap alignment on the TRX 400FW Foreman platform.
• OEM Cross-Reference: 1:1 references to OEM part numbers 13041-HM7-003 / 13041-HM7-004 with documented cross-compatibility, offering a factory-like rebuild at a fraction of OEM price.
• Premium Materials: High-grade alloy steel rings with hardened, chrome-faced surfaces to resist wear and maintain compression across temperature ranges.
• Performance Restoration: Restores original compression and reduces oil blow-by, helping to prevent top-end failures during demanding trail or farm work.
• Reliability in Rugged Conditions: Engineered for durability in dirt, dust, and loading scenarios typical of 4x4 work and off-road use.
• Cost-Effective Rebuild: Aftermarket replacement that delivers OEM-style fit and performance without OEM pricing, enabling timely, repeatable engine refreshes.

Installation Notes:

• Verify the cylinder bore is 86 mm and confirm the oversize matches +0.75 mm before installation.
• Orient the top compression, second compression, and oil rings correctly; stagger end gaps per the service manual to minimize oil blow-by.
• Use a proper ring compressor sized for an 86 mm bore and a light coating of assembly lube on all rings before seating them in the cylinder.
• Clean the piston lands, grooves, and cylinder walls; remove all debris to prevent scoring and false seating of the rings.
• Inspect the piston, ring lands, and cylinder for scoring or damage; replace any damaged components to ensure proper seal.
• After assembly, perform a careful break-in at light load for the first 15 minutes and avoid high RPM until rings seat and compression stabilizes.

Why Upgrade?

Running a worn piston-ring set on a 400cc Foreman can lead to persistent compression loss and increasing oil consumption, which accelerates wear and reduces performance. Replacing with the OS 0.75 oversize kit restores sealing efficiency and helps return reliable compression, throttle response, and overall engine longevity. This rebuild-friendly kit is built to endure the demands of trail riding and farm work, reducing downtime and the risk of costly top-end failures.