Engines and Components› Pistons and Piston Rings

CP Pistons

Forged Piston and Ring Set, 100mm Bore, 8.2:1 Compression, EJ257

$885.15

SKU: SC7421

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Overview

The CP Pistons SC7421-4 is a forged piston and ring set designed for Subaru EJ257 engines found in WRX and STI models. These high-strength aluminum pistons feature a 100mm bore, 8.2:1 compression ratio, and deep valve relief depths for high-lift camshaft compatibility.
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The CP Pistons SC7421-4 is a high-performance forged piston and ring set engineered to withstand the extreme demands of high-horsepower and high-boost Subaru EJ257 engine builds. Constructed from high-strength aluminum forgings, these pistons are fully CNC machined and balanced to within +/- 1 gram to ensure precision and reliability in competitive or modified street applications. Technically advanced, this set includes a custom skirt cam and barrel design, accumulator grooves, and double pin oilers with a force-fed oil ring. The pistons feature deep valve reliefs to accommodate high-lift camshafts and are designed specifically for +1mm valve sizes. The kit includes wrist pins and performance rings, with pin fitting included to streamline the assembly process. Installation requires professional machining, and users should note that this product is not CARB legal and is not for sale in California. Fitment is specific to Subaru EJ257 engines, including 2004-2021 STI models and 2006-2014 WRX turbo models.

Product descriptions are provided by our supplier partners and may not always be perfectly accurate. Not sure this fits? or call (760) 282-4645 and we’ll verify before you order.

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Specifications

BrandCP Pistons
Part NumberSC7421
CategoryEngines and Components
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Built for enthusiasts. Every order is backed by our 30 day return window on eligible items. Unused and uninstalled products may be returned with approved RMA and no restocking fees when received in new condition. Orders placed through the MOTORMIA app with a complete build qualify for our Fitment Guarantee — free return shipping if the part doesn’t fit your exact setup. The guarantee covers fit only; installation must follow manufacturer instructions, and we recommend a qualified shop for parts marked Difficulty 4–5.

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Shipping

Orders are processed in 1–3 business days. In stock parts ship promptly from our partner network. Special-order items either aren’t currently stocked or are built to your spec — expect 4–6 weeks. If a part goes on backorder while your order is in the queue, we’ll reach out and work through discounted alternatives together.

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