GM 3804816 Cast Crankshaft Undersize 402 3.75 30R/20M

( Brand: Gm ), ( Manufacturer Part Number: 3804816 ), ( Part Type: Crankshaft ), ( Material: Cast Iron ), ( Vintage Part: Yes ), ( Finish: Polished ), ( Country Of Origin: United States )

Review GM Chevy 402 Cast Crankshaft 3.75 Inch Stroke Undersize 30r/20m fits 396 427

The **GM 3804816 Cast Crankshaft** is a meticulously engineered, high-performance component designed to deliver exceptional durability and reliability in high-stress applications, particularly within Chevrolet s legendary big-block V8 engines, including the iconic 396, 402, and 427 cubic inch displacements. Crafted from premium cast iron, this crankshaft features a **3.75-inch journal diameter**, a precise undersize specification that ensures optimal clearance for modern or rebuilt engines while maintaining structural integrity under extreme loads. The **30/20 main bearing journal configuration** (30-degree offset on the main caps and 20-degree offset on the crankshaft itself) is a hallmark of GM s big-block architecture, allowing for a compact yet robust design that balances power delivery with smooth operation. This crankshaft is engineered to withstand the rigors of forced induction, high RPM, and aggressive camshaft profiles, making it an ideal choice for both street and competition applications. Its balanced weight distribution and precision-machined surfaces contribute to reduced vibration, enhanced longevity, and improved overall engine performance. Whether installed in a classic muscle car restoration, a high-performance street rod, or a drag racing engine, this crankshaft provides the foundational strength needed to handle the demands of modern fueling systems and high-output components. Built to exacting specifications, it ensures seamless integration with GM s big-block engine family, offering a blend of heritage reliability and contemporary performance for enthusiasts seeking both power and durability.

### **Pros and Cons of Buying a GM 3804816 Chevy 396/402/427 Cast Crankshaft (3.75" Undersize, 30R/20M)**

#### **Pros**

1. **Compatibility with Big-Block Chevy Engines**

- This crankshaft is designed for GM s legendary big-block engines (396, 402, 427, 454, etc.), making it a reliable choice for restorations, swaps, or performance builds. It fits stock and aftermarket blocks without major modifications.

2. **Undersized for Rod Journal Clearance**

- A **3.75" undersize** provides additional rod journal clearance, reducing stress on connecting rods and bearings. This is particularly useful for high-RPM applications, forced induction, or engines with longer-than-stock rods (e.g., 6.0" or 6.125" rods).

3. **30R/20M Main Journal Design**

- The **30R (30-degree radius)** main journals are optimized for high-stress applications, offering better load distribution than standard 20-degree radius (20M) journals. This design reduces stress concentrations, improving durability in performance engines.

- The **20M main journals** (on the same crank) are still viable for stock or mild applications, providing versatility.

4. **Cast Construction for Cost-Effectiveness**

- Cast cranks are more affordable than forged cranks, making them a practical choice for budget-conscious builders or those restoring classic engines. They are widely available from aftermarket suppliers.

5. **Widely Available and Easy to Source**

- This part is stocked by multiple suppliers (e.g., Crane, Eagle, JE, etc.), ensuring availability for quick replacement. It s also compatible with aftermarket rod sets and bearing kits.

6. **Good for Stock or Mildly Modified Engines**

- If used in a stock or mildly tuned engine (e.g., carbureted, naturally aspirated), this crank can provide reliable service with proper maintenance.

---

#### **Cons**

1. **Limited High-Stress Performance Capability**

- While the **30R journals** improve strength, a **cast crank** is inherently weaker than a **forged crank** under extreme loads (e.g., nitrous, extreme forced induction, or high-RPM drag racing). Forced aluminum or nitrous applications may require a forged crank for safety.

2. **Potential for Cracking Under Extreme Stress**

- Cast cranks are prone to **fatigue failure** if subjected to excessive torque, misfires, or poor lubrication. They are not recommended for **high-horsepower builds (500 HP)** without reinforcement (e.g., rod bolts, bearings, and oil pressure).

3. **No Balancing Included (May Require Dynamic Balancing)**

- New cast cranks often need **dynamic balancing** to prevent vibration, especially when paired with aftermarket rods or pistons. This adds to the cost and time required for assembly.

4. **Limited Longevity in Aggressive Applications**

- Unlike forged cranks, cast cranks have a **shorter lifespan** under heavy duty. They may require replacement sooner if used in **high-mileage or performance-driven engines**.

5. **Potential for Warping or Misalignment**

- Cast cranks can warp over time, especially if not properly supported during machining. This may require **resurfacing** or **regrinding** to maintain proper crankshaft alignment.

6. **Not Ideal for Extreme Builds (e.g., Supercharged, Turbocharged, or Nitrous)**

- For **forced induction or nitrous applications**, a **forged crank** (e.g., Eagle, JE, or Crane forged) is strongly recommended due to superior strength and durability.

---

### **Ending Conclusion**

This **GM 3804816 cast crankshaft (3.75" undersize, 30R/20M)** is a **solid choice for stock, mild performance, or restoration projects** where budget and compatibility are priorities. Its **undersized rod journals** and **30R main journals** provide an upgrade over stock cranks, making it suitable for **naturally aspirated or carbureted engines** running up to ** 400-450 HP** with proper supporting components (e.g., aftermarket rods, bearings, and oil system).

However, for **high-performance applications (500 HP), forced induction, or nitrous**, a **forged crankshaft** is the **superior and safer option**. If you plan to run this crank in a **stock or mildly modified engine**, it will serve well with regular maintenance. If you intend to push the engine hard, consider upgrading to a **forged crank** for long-term reliability.

---

### **Recommendation**

- You are restoring a **stock or mildly modified big-block Chevy engine**.

- Your engine will run **naturally aspirated or with a carburetor**.

- You are on a **budget** and need a reliable, widely available part.

- You plan to **dynamically balance** the crank with your rod set.

- You are building a **high-performance engine (500 HP)** with **forced induction or nitrous**.

- You need **maximum durability** for **drag racing or extreme applications**.

- You want the **longest possible lifespan** without potential failure risks.

- If you plan to **supercharge, turbocharge, or run nitrous**, invest in a **forged crankshaft** (e.g., Eagle 30R, JE 30R, or Crane forged) for **superior strength and reliability**.

For most **stock-to-mildly-modified builds**, this crank is a **good value**, but for **serious performance**, a forged crank is the **smart long-term choice**.

Details:

GM Original Cast Crankshaft. I encourage this.4. Read the completely before buying. Crankshaft has been shot blasted and reground. 030 under on the rods.

I'm not a business or company. 020 under on the mains, then micro polished.

specifications cranshafengin:

• for models: 396, 427
• for makes: Chevy
• brand: Gm
• mpn: 3804816
• part type: Crankshaft
• material: Cast Iron
• vintage part: Yes
• finish: Polished
• country of origin: United States

seller cranshafengin:

• city: Wichita, Kansas
• # reviews: 1451
• rating: 100.0%

payments cranshafengin:

• other
• credit card: Discover
• wallet: Paypal

general cranshafengin:

• condition: Remanufactured
• Car Truck Parts Accessories > Engines Engine Parts > Crankshafts

shipping cranshafengin:

• class: Standard
• transit: 4-11 Days
• carrier: UPS
• service: UPS Ground
• type: Fixed
• cost: $0.00

offer cranshafengin:

• started: February 7, 2026
• options: Ship-to-home
• availability: In Stock
• quantity: 1
• sold: 0

returns cranshafengin:

• policy: No Returns

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