Tips Building A Racing Motor

Building a reliable racing engine isn’t just about bolting shiny parts together; it’s about control over your clearances. When you’re spinning a small-block Chevy or a massive big-block past 6,500 RPM, standard factory specs won’t cut it.

### 1. Main and Rod Bearing Clearances
Do not rely on Plastigage for a race motor. Use a dial bore gauge and a micrometer to measure your journals.
* **Small-Block:** Aim for .0020 to .0025 inches on the mains and .0022 to .0027 inches on the rods.
* **Big-Block:** Because the crankshaft is heavier and flexes more under load, open it up slightly to .0025 to .0030 inches on the mains. If it’s too tight, you’ll spin a bearing before you finish your first pass.

### 2. Piston-to-Wall Clearance
Forged pistons expand significantly more than factory cast or hypereutectic pistons. If you run a tight factory clearance, the piston will expand, scuff the cylinder wall, and seize. For a dedicated race motor using 2618 forged alloy, set your piston-to-wall clearance between .0045 and .0060 inches, depending on whether you are running a small-block or a big-block, and if you plan to add boost or nitrous.

### 3. Piston Ring Gapping
Proper end gap keeps the ring ends from butting together when they get hot. If they butt, they will break the piston ring lands.
* **Top Ring Gap Formula:** Multiply your cylinder bore by .0045 for a standard high-compression engine, or up to .0065 if you run a supercharger.
* For a typical 4.250-inch bore big-block, your top ring gap should be right around .0024 to .0026 inches. File-fit them carefully and deburr the edges.

### 4. Valve-to-Piston Clearance
When running a high-lift camshaft with aggressive hydraulic roller lifters, you must physically check your valve-to-piston clearance using clay on the piston dome. You need at least .080 inches of clearance on the intake valve and .100 inches on the exhaust valve. Remember, aluminum rods stretch at high RPM, so give yourself a safety cushion.

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