If you’re building a Honda K-series engine, a common question pops up: can I use K24 crankshaft in K20? The short answer is yes, you absolutely can, and it’s a popular upgrade for a significant reason. This swap is a foundational part of building a high-displacement K-series hybrid motor, often called a K24/K20 Frankenstein build. It’s a direct bolt-in part that transforms your 2.0-liter engine into a 2.4-liter powerhouse.
This article will walk you through everything you need to know about using a K24 crankshaft in your K20 block. We’ll cover the benefits, the exact parts you need, the step-by-step process, and crucial considerations to ensure your build is successful and reliable.
Can I Use K24 Crankshaft in K20
The core of this modification is simple. The K20 and K24 engine blocks share the same basic architecture. This means the crankshaft from a 2.4L K24 engine will physically fit into the block of a 2.0L K20 engine. The bearing journals are the same size, and the bolt pattern for the flywheel or flexplate is identical. It’s not an adaption; it’s a direct replacement.
However, just dropping in the crankshaft alone won’t give you more displacement. The increased stroke of the K24 crank is what creates the larger engine size. To take full advantage of this, you must pair it with the correct accompanying parts, primarily the K24 connecting rods and K20 or aftermarket pistons.
Why Swap a K24 Crank into a K20?
The primary goal is to increase displacement and torque. Here’s what you gain:
* More Torque: This is the biggest benefit. The longer stroke of the K24 crank (99mm vs. the K20’s 86mm) creates more leverage, resulting in a substantial increase in low-end and mid-range torque. Your car will feel much stronger on the street.
* Increased Displacement: Combining the K24 crank and rods with your K20 block (or a K24 block) and K20 pistons creates a 2.4L engine. More displacement generally means more potential power.
* Cost-Effective Power: Compared to forced induction or high-end internal work, a crankshaft swap as part of a hybrid build is a relatively affordable way to gain significant performance.
* OEM Reliability: Since you’re using factory Honda parts designed to work together, the resulting engine can be very reliable when assembled correctly.
Parts You Will Need for the Swap
You cannot just use the crankshaft by itself. Here is the essential parts list for a complete K24 bottom end in a K20 block:
1. K24 Crankshaft: The centerpiece. Source one from a K24A1, A2, A3, A4, A8, etc. They are all functionally the same for this swap.
2. K24 Connecting Rods: You must use these. They are longer than K20 rods to match the longer stroke of the K24 crank.
3. Pistons: You have two main options:
* K20 Pistons: Using your stock K20 pistons with K24 rods and crank creates a compression ratio around 11.0:1 to 11.5:1, which is excellent for a naturally aspirated build.
* Aftermarket Pistons: For forced induction or specific compression goals, aftermarket pistons designed for the K24 stroke are the best choice.
4. Rings and Bearings: Always use new main bearings and rod bearings during assembly. You will need standard or oversized bearings depending on the condition of your crankshaft journals and block. Always measure.
5. Balance Shaft Removal Kit (Optional but Recommended): The K24 crank is designed to work with a balance shaft system. The K20 block does not have this. You must either:
* Install a K24 oil pump and balance shaft assembly (requires some case milling).
* More commonly: Use a K20 oil pump and install a balance shaft delete kit (plug and gear removal). This is the standard route.
Step-by-Step Process Overview
This is a major engine build. You should have mechanical experience or work with a professional machinist and builder.
1. Preparation and Disassembly
First, remove the K20 engine from the vehicle and disassemble it completely. You need to strip the block down to a bare casting.
* Drain all fluids.
* Remove the cylinder head, oil pan, and timing cover.
* Remove the pistons and connecting rods.
* Unbolt and remove the original K20 crankshaft.
* Thoroughly clean the entire K20 engine block. This is critical.
2. Machining and Inspection
Never assume everything will just fit. Take everything to a reputable machine shop.
* Have the K20 block inspected, hot-tanked, and decked.
* Get the cylinder bores measured and honed. If you are using aftermarket pistons, the shop will need them to size the hone correctly.
* Have the K24 crankshaft inspected, polished, or ground if necessary. Check for straightness and journal wear.
* The shop should “blueprint” the assembly. This means measuring the deck height, piston-to-valve clearance (if using a different cylinder head), and calculating the exact compression ratio.
3. Balancing the Assembly
This is a critical step that is often overlooked. The K24 crankshaft is balanced from the factory for its own pistons and rods.
* When you pair it with K20 pistons (which are lighter than K24 pistons) and K24 rods, the rotating assembly will be out of balance.
* You must have the entire rotating assembly (crank, rods, pistons, rings, and bearings) balanced as a set by the machine shop. This ensures smooth operation at high RPM and prevents premature bearing failure.
4. Engine Reassembly
With all parts machined, cleaned, and balanced, you can begin reassembly.
* Install the main bearings into the block and carefully place the K24 crankshaft.
* Torque the main bearing caps to factory specification in the correct sequence.
* Install the piston rings on the pistons.
* Connect the pistons to the K24 connecting rods (ensure orientation is correct!).
* Install the piston/rod assemblies into the cylinders, using a ring compressor.
* Torque the rod bolts to spec.
* Install the balance shaft delete kit and K20 oil pump.
* Install the timing chain and guides.
* Reinstall the cylinder head with a new head gasket.
Important Considerations and Potential Issues
While the swap is straightforward for experienced builders, there are pitfalls to avoid.
* Oil Pump Compatibility: As mentioned, the balance shaft issue is the main hurdle. The K20 oil pump is the simpler solution with a delete kit.
* Piston-to-Valve Clearance: If you are using a high-lift camshaft or a different cylinder head (like a K20Z3 head on a K24 block), you must check piston-to-valve clearance. The longer stroke can bring the piston closer to the valves.
* ECU Tuning: Your engine will now have 20% more displacement. The stock ECU tune will not be correct. You must get a custom tune via Hondata, AEM, or similar to adjust for fuel, ignition, and VTEC. Running without a tune can cause engine damage.
* Which K20 Block? Most K20 blocks (K20A2, K20Z1, Z3) work well. The rare K20A (JDM Type R) block has a different oil pump setup and is less common for this swap.
FAQ Section
Here are answers to some frequently asked questions.
Q: Can I use a K24 crankshaft in any K20?
A: Yes, it will physically fit any K20A-series block (USDM or JDM). The process is the same for all.
Q: Do I need K24 rods with the K24 crank?
A: Absolutely. K20 rods are too short. You must use the longer K24 connecting rods.
Q: What is the final displacement with a K24 crank in a K20?
A: Using the K20 block’s 86mm bore and the K24 crank’s 99mm stroke, the displacement is 2,354 cc – essentially 2.4 liters.
Q: Can I use my K20 oil pump?
A: Yes, but you must use a balance shaft delete plug kit to block the oil gallery for the balance shaft that the K24 crank expects.
Q: Is balancing the rotating assembly really necessary?
A: For a reliable, high-RPM engine, yes. It’s not worth skipping. An unbalanced assembly causes vibrations that stress bearings and can lead to failure.
Q: What’s the difference between a K20 and K24 crankshaft?
A: The K24 crankshaft has a longer stroke (99mm vs. 86mm). It’s also designed to work with a balance shaft, which the K20 crank is not.
Q: Do I need a new tune after the swap?
A: Yes, 100%. The increased displacement changes all the engine’s airflow characteristics. A custom tune is mandatory for performance and safety.
Final Thoughts
Using a K24 crankshaft in a K20 engine is a proven and effective method to build a potent, torque-rich Honda motor. It’s the heart of the popular K24/K20 hybrid build. The process requires attention to detail, proper machining, and balancing, but the result is an engine that combines the high-RPM capability of the K20 head with the muscular torque of a 2.4L bottom end.
Remember to source quality parts, invest in good machine work, and never skip the final tuning step. With careful planning and execution, this swap can give your car a whole new personality on the road or track. It’s one of the best ways to extract serious naturally aspirated performance from the K-series platform. Just make sure you have all the tools and knowledge before you begin, or partner with a shop that has experience with these builds.