How much does a Subaru piston weigh?
In Subaru engines, piston weight varies by model and specification, but most piston units weigh roughly a few hundred grams each (typically around 350–430 g per piston with rings and wrist pin). There is no single universal weight for all Subaru pistons.
The weight depends on engine family (EJ, FA/FB), material (cast vs forged), and whether you're counting just the piston or the assembled unit. This article breaks down typical ranges and shows how to determine the exact figure for your engine.
Understanding piston weight in Subaru engines
Piston mass is part of the engine's reciprocating weight, affecting balance, inertia, and valve timing. Subaru uses horizontally opposed "boxer" configurations that place weight distribution considerations into engine tuning and balancing. The actual mass is a function of bore size, crown design, pin diameter, and ring pack. To answer the question precisely for a given car, you must know the engine code and piston design.
Estimated ranges for common Subaru piston configurations
These ranges reflect typical factory or aftermarket piston configurations used in Subaru engines. Numbers are averages and can vary with exact part numbers and measuring conventions (piston alone vs assembled with wrist pin and rings).
- Stock cast aluminum pistons (common in EJ, FA, and FB families). Roughly 350–420 g per piston when measured as an assembled unit (piston, rings, and wrist pin).
- Forged aluminum pistons used in performance or high-mileage builds. Roughly 380–460 g per piston (assembled), depending on crown geometry and pin size.
- Lightweight or high-strength aftermarket options. Some designs target 320–400 g per piston (assembled), though exact weights vary by manufacturer.
In practice, piston weights vary widely even within the same engine family, so for precise tuning or balancing you should verify the exact part numbers from the manufacturer or Subaru's official catalogs.
Notes on measurement and selection
When selecting pistons, consider not only the per-piston weight but also how the weight compares to other pistons in the set, the wrist pin weight, and the overall balance of the rotating assembly. Even small differences can affect engine dynamics, especially in high-rev or high-performance builds.
How to find the exact piston weight for your engine
To identify the precise weight for your engine, use these steps:
- Identify your engine code (for example, EJ25, FA20, or FB25) from the vehicle VIN or engine stamping.
- Check the official Subaru parts catalog or service manual for the exact piston part number and its weight, or contact the piston manufacturer (e.g., Wiseco, JE Pistons) with the exact bore and piston type.
- If you have access to the actual piston, weigh it on a precise scale (including piston, wrist pin, and rings if you want the assembled weight) and compare to catalog specs.
- When balancing a rebuilt engine, consider the total reciprocating mass of all pistons, pins, rings, and connecting rods to achieve proper dynamic balance.
Having the precise weight helps with engine balancing, particularly in high-performance builds or when replacing pistons with different materials or weights.
Summary
The weight of a Subaru piston is not fixed; it depends on the engine family, piston design, and whether you count the assembly. Stock pistons tend to fall in the 350–420 g range per unit (assembled), while forged or aftermarket pistons can range higher or lower depending on design. For exact figures, consult the engine code–specific catalogs or the piston manufacturer.
Why are Subaru pistons different to other cars?
Subaru firmly believes that the Horizontally-Opposed Engine is the optimum design for driving enjoyment. The pistons face away from each other in a 180º symmetrical layout around the crankshaft and work to balance out each other's vibrations, delivering a smooth, shudder-free feel.
How much does a Subaru engine weigh?
A bare Subaru EJ251 engine weighs approximately 262 lbs (119 kg), while more complete versions with accessories can weigh around 300 lbs (136 kg). A complete EJ207 engine without accessories is about 220 lbs (100 kg). The exact weight varies significantly depending on the specific engine model and what parts (like accessories, fluids, or turbo) are included.
Engine weight by model
- EJ251 (bare): About 262 lbs (119 kg) without oil, coolant, or accessories like the alternator or A/C pump.
- EJ251 (more complete): Around 300 lbs (136 kg) or more, depending on the accessories, intake, and exhaust manifolds included.
- EJ207 (without accessories): Approximately 220 lbs (100 kg), including the heads and oil sump, but no accessories.
- EJ207 (complete with turbo and intercooler): Around 345 lbs (156 kg).
Factors affecting engine weight
- Accessories: Adding components like the alternator, power steering pump, A/C pump, and turbocharger significantly increases the weight.
- Fluids: The engine's "wet" weight, including oil and coolant, is heavier than its "dry" weight.
- Intake and exhaust: Manifolds and other intake/exhaust components add to the total weight.
How much does an engine piston weigh?
Oem piston weighs around 430 grams.
How big is a 350 piston?
A standard "350" Chevy engine piston bore size is 4.000 inches, but this can change with aftermarket parts, with common options being 4.010 inches and 4.040 inches. The piston size is often listed with its stroke (3.480 or 3.750 inches) and compression height, which can vary depending on the specific piston type, such as flat-top or dome top.
Common 350 piston sizes
- Standard bore: A stock 350 Chevy engine has a bore size of 4.000 inches.
- Oversize/aftermarket bore: Aftermarket pistons are available in various oversizes, such as 4.010 inches and 4.040 inches, to increase displacement or repair worn engines.
Example piston specifications
| Feature | Specification (Example) |
|---|---|
| Bore | 4.000 in. to 4.040 in. |
| Stroke | 3.480 in. or 3.750 in. |
| Compression Height | 1.125 in. or 1.545 in. |
| Piston Type | Flat top or Dome top |
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