Wiseco K598M995AP Forged 99.50 mm Dish Pistons for Subaru EJ25 / EJ257 (9.0:1)

Vehicle Fitment

• 2006-2007 Subaru Impreza WRX (EJ255 2.5L Turbo)
• 2008-2014 Subaru Impreza WRX (EJ255 2.5L Turbo)
• 2004-2007 Subaru Impreza WRX STI (EJ257 2.5L Turbo)
• 2008-2014 Subaru Impreza WRX STI (EJ257 2.5L Turbo)
• 2015-2021 Subaru WRX STI (EJ257 2.5L Turbo)
• 2004-2008 Subaru Forester XT (EJ255 2.5L Turbo)
• 2009-2013 Subaru Forester XT (EJ255 2.5L Turbo)

Fitment Notes

• Fits EJ255 and EJ257 turbocharged engines only
• 99.5mm bore is stock bore size; no overbore required
• Yields ~8.9:1 compression with 51cc cylinder head
• Piston-to-valve clearance must be verified during assembly
• Recommended piston-to-wall clearance: 0.003" (street) / 0.004" (race)

Features

• Forged 2618 aluminum alloy for high-boost and nitrous applications
• Armor Plating on ring grooves and wrist pin bore for durability
• ArmorGlide skirt coating reduces friction and noise
• -19cc dish with four valve reliefs for oversized valves and high-lift cams
• Offset wrist pins reduce piston slap and friction
• Complex skirt profile optimizes ring seal and bore stability
• Crown design promotes cross-flow scavenging during overlap
• Designed to resist detonation and pre-ignition

Specifications

• Bore: 99.5mm (3.917 in.) -- Stock size
• Stroke: 3.110 in. (79mm)
• Rod Length: 5.137 in. (130.5mm)
• Compression Height: 1.209 in. (30.7mm)
• Dish Volume: -19cc
• Wrist Pin Diameter: 0.905 in. (23mm)
• Wrist Pin Style: Press-fit or full-floating
• Top Ring: 1.25mm steel, gas nitrided
• Second Ring: 1.5mm cast iron, phosphate coated (Napier style)
• Oil Ring: 2.0mm chrome-plated carbon steel
• Piston Weight: ~371 grams
• Quantity: Set of 4

Included Components

• 4x Forged pistons with Armor Plating and ArmorGlide coating
• 4x Steel nitrided/Napier ring sets (1.25mm / 1.5mm / 2.0mm)
• 4x Wrist pins (0.905" diameter)
• 8x Wrist pin retainer clips (Spirolox)

Product Description

The Wiseco K598M995AP Sport Compact Piston Kit delivers race-proven strength for high-performance EJ25 turbo builds in the Subaru WRX, WRX STI, and Forester XT. Forged in-house from 2618 aluminum alloy using Wiseco's advanced grain-flow technology, these pistons provide superior strength for high-boost, nitrous, and aggressive street/track applications where cast or hypereutectic pistons fail. The -19cc dished crown with four generous valve pockets accommodates oversized valves and high-lift camshafts while the squish area design resists detonation and pre-ignition for extended engine life.

Current production pistons feature Wiseco's Armor Plating--an abrasion-resistant coating on ring grooves and wrist pin bores that increases aluminum hardness, prevents ring micro-welding, reduces carbon buildup, and combats detonation-induced corrosion. The ArmorGlide skirt coating further reduces friction for improved power output and quieter operation. Complete with premium steel nitrided top rings, Napier-style second rings, and chrome oil rings, this drop-in kit requires no overbore at the stock 99.5mm bore size. Offset wrist pins minimize piston rock and friction losses while the precision-machined ring grooves ensure optimal seal to minimize blow-by and oil consumption.