GSC Power Division 2003-8 +1mm Oversized Exhaust Valves for 4G63T Evo 1-9/DSM

Vehicle Fitment

• 1992-2006 Mitsubishi Lancer Evolution I-IX (4G63T, AWD)
• 1990-1999 Mitsubishi Eclipse GST / GSX (4G63T, FWD/AWD)
• 1990-1998 Eagle Talon TSi (4G63T, FWD/AWD)
• 1990-1994 Plymouth Laser RS Turbo (4G63T, FWD/AWD)
• 1991-1992 Mitsubishi Galant VR-4 (4G63T, AWD)

Fitment Notes

• Fits all Mitsubishi 4G63T turbocharged applications including DSM 1G (1990-1994) and 2G (1995-1999) platforms, and Evo I through IX (1992-2006)
• +1mm oversized head (31.5mm) - requires valve seats to be machined to match; not a drop-in replacement for standard 30.5mm seat heads without seat cutting
• Does not fit the Evo X (4B11T engine) - see GSC 4B11T-specific exhaust valves for that application
• Does not fit the naturally aspirated 4G63 (non-turbo) found in certain Eclipse base trim levels
• Verify head has been properly machined for +1mm oversized valve seats before ordering

Features

• Modified 23-8N stainless steel alloy construction delivers superior high-heat resistance and oxidation durability for turbocharged, high-EGT exhaust environments
• +1mm oversized 31.5mm head diameter allows use of freshly cut or reconditioned valve seats, restoring a proper sealing surface in worn or damaged cylinder heads
• Micro polished valve stems minimize oil retention and reduce drag between stem and guide, promoting smoother motion and longer guide life
• Swirl polished valve heads improve airflow across the valve face, enhancing combustion efficiency and supporting power gains in modified 4G63T applications
• Back-cut head design reduces overall valve weight, enabling higher RPM operation, increased boost pressure tolerance, and compatibility with aggressive cam profiles without raising spring seat pressures
• Maximized under-head dish removes unnecessary mass while retaining structural integrity under sustained high-temperature and high-pressure conditions
• Precision-adjusted valve tip lengths reduce tipping requirements at installation, maintaining correct valvetrain geometry and minimizing setup time when dialing in valve lash
• Engineered keeper groove geometry balances intake and exhaust spring pressures, reducing dependency on valve spring shims and optimizing performance from GSC spring and retainer kits
• Valve tips are hardened or stellite-tipped to within 5 Rockwell C of the rocker or tappet surface, minimizing wear at the valve tip interface over extended service
• Designed, machined, and validated in-house on CNC machining centers using advanced 3D FEA and flow simulation software with real-world flow testing on actual engine applications

Specifications

• Position: Exhaust (set of 8 - one per cylinder, full engine set)
• Valve Head Diameter: 31.5mm (+1mm oversized)
• Valve Stem Diameter: 6.6mm
• Overall Valve Length: 109.20mm
• Material: Modified 23-8N stainless steel
• Stem Finish: Micro polished
• Head Finish: Swirl polished with back cut
• Application Engine: Mitsubishi 4G63T (2.0L DOHC Turbocharged I4)
• Quantity per Set: 8

Included Components

• 8x GSC Power Division +1mm Oversized Exhaust Valves (31.5mm head, 6.6mm stem, 109.20mm length)

Product Description

The GSC Power Division 2003-8 is a set of eight +1mm oversized (31.5mm) exhaust valves for the Mitsubishi 4G63T turbocharged engine, covering all DSM platforms (1990-1999 Eclipse GST/GSX, Eagle Talon TSi, Plymouth Laser RS Turbo, and Galant VR-4) as well as the full Lancer Evolution I-IX lineup (1992-2006). Built from a modified 23-8N stainless steel alloy, these valves are engineered to withstand the extreme exhaust gas temperatures generated in high-horsepower, high-boost 4G63T applications, where factory valves frequently distort, leak, or fail prematurely under hard use.

The +1mm oversized head is the defining characteristic of this part number and makes the 2003-8 the correct choice when cylinder heads have been machined or reconditioned with oversized valve seats. This commonly occurs during head rebuilds where the OEM seat has been damaged, pitted, or worn beyond serviceable limits - situations where restoring proper sealing geometry requires a larger valve to match the newly cut seat diameter. The GSC 2003-8 ensures that even rebuilt or high-mileage heads can achieve factory-quality valve sealing with a performance-grade component. All other construction details mirror the proven GSC stainless valve line: micro polished stems for reduced guide drag, swirl polished and back-cut heads for improved flow and reduced mass, precision-adjusted tip lengths for easier valve lash setup, and a keeper groove geometry that balances spring pressures across the valvetrain.

These valves are sold as a complete set of 8 and are intended for use in heads that have been properly machined to accept the +1mm oversized seat diameter - they are not interchangeable with standard 30.5mm seats without machine work. For builds requiring oversized valves due to head reconditioning, the GSC 2003-8 pairs well with GSC Manganese Bronze valve guides, Viton valve stem seals, and GSC's 4G63T valve spring kits to complete a full performance valvetrain rebuild. If the cylinder head has not been cut for oversized seats, refer to GSC part number 2001-8 (standard 30.5mm exhaust valve) instead.