A leak-down test measures cylinder sealing by introducing compressed air into each cylinder at top dead center and measuring pressure loss, helping diagnose worn rings, valves, or head gasket issues.

Warnings

Tools required

Parts

• Spark plug anti-seize compound × 1 — Use OEM specification
• Shop rags × 1 — Generic

Preparation

1. Warm the engine to normal operating temperature, then shut off and allow to cool until safe to touch but still warm (approximately 30-45 minutes).
2. Disconnect the negative battery cable to prevent accidental starting.
3. Remove the air cleaner assembly and note any vacuum line connections.
4. Label all eight spark plug wires with cylinder numbers using tape or wire markers (driver side front to rear: 1-3-5-7, passenger side front to rear: 2-4-6-8).
5. Disconnect all eight spark plug wires from the spark plugs.
6. Set up the air compressor in a location where the hose can reach all cylinders, and verify output pressure is regulated to 90-100 PSI.
7. Prepare a chart or notebook to record leak-down percentages and observations for each cylinder.

Procedure

1. 1

   Remove all spark plugs
   Using the appropriate spark plug socket (13/16" for most 1966 327 applications), remove all eight spark plugs. Inspect each plug for condition and note any unusual fouling, oil deposits, or erosion that may correlate with leak-down results. Set plugs aside in order by cylinder number.
2. 2

   Identify cylinder 1 TDC compression stroke
   Locate cylinder 1 (driver side front). Rotate the engine by hand using a breaker bar on the harmonic balancer bolt (clockwise from the front) while watching the timing marks. Bring cylinder 1 to top dead center (TDC) on the compression stroke. Verify this is the compression stroke by confirming both intake and exhaust valves are closed (rocker arms should have clearance, not holding valves open). The timing mark on the harmonic balancer should align with the 0-degree mark on the timing tab.
3. 3

   Install leak-down adapter in cylinder 1
   Thread the leak-down tester adapter fitting into the cylinder 1 spark plug hole. Hand-tighten firmly, ensuring the threads engage properly. Do not over-tighten as this can damage the spark plug threads in the cast iron heads.
4. 4

   Connect air supply and perform leak-down test on cylinder 1
   Connect the leak-down tester to the adapter. Slowly apply compressed air (90-100 PSI) while monitoring both gauges on the leak-down tester. The input gauge shows supply pressure, the output gauge shows cylinder pressure. Calculate leak-down percentage: ((input PSI - output PSI) / input PSI) × 100. Record the percentage. A healthy cylinder shows 5-10% leak-down, acceptable is 10-20%, concerning is 20-30%, and above 30% indicates significant wear or damage.
5. 5

   Diagnose leak source for cylinder 1
   While air pressure is applied, listen and feel for air escaping to diagnose the leak source. Air hissing from the throttle body or carburetor indicates intake valve leakage. Air from the tailpipe indicates exhaust valve leakage. Air bubbling in the radiator (with cap removed) indicates head gasket or cracked head/block. Air from the oil filler cap or breather indicates piston ring leakage (blow-by). Note findings for cylinder 1.
6. 6

   Remove tester and rotate to cylinder 2
   Release air pressure and disconnect the leak-down tester. Remove the adapter from cylinder 1. Rotate the crankshaft clockwise approximately 90 degrees to bring cylinder 8 to TDC compression stroke (firing order is 1-8-4-3-6-5-7-2). Install the adapter in cylinder 8 spark plug hole.
7. 7

   Test cylinder 8 and continue firing order sequence
   Repeat the leak-down test procedure for cylinder 8 (steps 4-5). Record results. Continue through the complete firing order: 1-8-4-3-6-5-7-2, rotating the crankshaft 90 degrees between each cylinder to maintain TDC compression stroke positioning. For each cylinder, verify both valves are closed before applying pressure, perform the test, diagnose leak sources, and record all findings.
8. 8

   Test remaining cylinders
   Complete leak-down testing on cylinders 4, 3, 6, 5, 7, and 2 in sequence, following the same procedure. For each: rotate crankshaft 90 degrees clockwise, confirm TDC compression stroke with both valves closed, install adapter, apply air pressure, measure leak-down percentage, diagnose leak location, and record results.
9. 9

   Analyze results and compare cylinders
   Review all eight cylinder results. Compare leak-down percentages across cylinders. Variation of more than 10% between cylinders suggests uneven wear or specific component failure. Consistent high leak-down across all cylinders may indicate general wear. Pattern analysis: adjacent cylinders with high leak-down and radiator bubbling suggest head gasket failure between those cylinders; one cylinder significantly worse suggests valve or ring issue specific to that cylinder.
10. 10

    Reinstall spark plugs
    Apply a small amount of anti-seize compound to the spark plug threads (avoid getting any on the electrode or insulator). Install all eight spark plugs hand-tight, then snug with the spark plug socket. Do not over-torque as the cast iron head threads are vulnerable to damage.
11. 11

    Reconnect spark plug wires
    Reconnect all spark plug wires to their correct cylinders following your earlier labeling. Verify firing order: 1-8-4-3-6-5-7-2. Incorrect wire routing will cause misfire or engine damage. Ensure each wire seats fully onto the spark plug terminal with an audible click.

Reassembly

1. Reinstall the air cleaner assembly and reconnect any vacuum lines that were disconnected.
2. Reconnect the negative battery cable.
3. Start the engine and verify normal operation with no misfires.

Verification

• Review recorded leak-down percentages: all cylinders should ideally be below 15% with less than 10% variation between cylinders.
• Verify engine starts and runs smoothly with no misfires after reassembly.
• Compare leak-down results with other symptoms (compression test results, oil consumption, coolant loss) to diagnose root cause of any issues found.
• Document results for maintenance records and future comparison if engine wear is being monitored over time.

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