Replace or repair the automatic transmission cooler lines that route transmission fluid from the transmission to the radiator-mounted cooler and back, preventing leaks and maintaining proper transmission temperatures.

Warnings

Tools required

Parts

• Transmission cooler line set (steel or braided stainless) × 1 — Use OEM specification or aftermarket equivalent for 1966 Corvette Powerglide/M20/M21
• Transmission cooler line fitting O-rings (if applicable) × 4 — Use OEM specification

Fluids

• Dexron III ATF — 2 qt

Preparation

1. Park vehicle on level ground and set parking brake firmly.
2. Place wheel chocks behind rear wheels.
3. Allow engine and transmission to cool completely (minimum 30 minutes after operation).
4. Gather all tools, parts, and fluids before beginning work.
5. Position drain pan under transmission area to catch fluid spillage.
6. Raise front of vehicle using floor jack at front crossmember jack point.
7. Secure vehicle on jack stands placed under frame rails behind front wheels.
8. Verify vehicle stability by gently pushing before working underneath.

Procedure

1. 1

   Inspect existing cooler lines and identify routing
   From underneath the vehicle, locate the two transmission cooler lines running from the right side of the transmission (passenger side) forward to the radiator. The 1966 Corvette routes these lines along the right frame rail. Note the routing, bracket locations, and any clips securing the lines. The upper line typically runs from the transmission to the radiator cooler (supply), and the lower line returns fluid from the radiator to the transmission. Take photos if available to document original routing.
2. 2

   Drain fluid from cooler lines at radiator
   Position drain pan under the radiator area where the cooler lines connect. Using a flare nut wrench or line wrench, slowly loosen the cooler line fittings at the radiator. The radiator-side fittings are inverted flare fittings; verify the exact line and fitting size against vehicle documentation before sourcing parts. Allow transmission fluid to drain from the lines into the pan. The lines may contain 1-2 quarts of fluid. Once drained, remove the fittings completely from the radiator. Expect residual fluid to continue dripping.
3. 3

   Disconnect cooler lines at transmission
   Move drain pan under the transmission. Locate the two cooler line fittings on the right side of the transmission case. Apply penetrating oil to the fittings if corrosion is present. Using the appropriate flare nut wrench, carefully loosen and remove both cooler line fittings from the transmission. The fittings thread directly into the transmission case. Turn counterclockwise to remove. Plug the transmission ports with clean shop rags to prevent contamination and further fluid loss.
4. 4

   Remove cooler line mounting brackets and clips
   Trace the cooler lines along the frame rail and identify all mounting brackets and clips. The 1966 Corvette typically uses 2-3 metal clips that secure the lines to the frame. Remove the fasteners securing these brackets (typically 1/4" or 5/16" bolts or sheet metal screws). Keep all hardware organized for reinstallation. Carefully work the old cooler lines free from any rubber isolators or grommets.
5. 5

   Remove old cooler lines from vehicle
   Once all connections and brackets are removed, carefully extract the cooler lines from the vehicle. They may be routed through tight spaces near the exhaust crossover pipe and frame rails. Maneuver them downward and out from under the vehicle. Inspect the old lines for the failure point (corrosion, impact damage, or fitting failure) to prevent recurrence.
6. 6

   Prepare and inspect new cooler lines
   Compare new cooler lines to the old lines to verify correct length and fitting configuration. The 1966 Corvette uses inverted flare fittings at both the transmission and radiator ends. Inspect all flare seats for damage or debris. If using steel lines, ensure flares are properly formed with no cracks. If using braided stainless lines, verify AN fittings are properly assembled and oriented. Clean all fitting threads with a wire brush.
7. 7

   Route new upper cooler line
   Starting at the transmission end, route the new upper cooler line (supply line) from the upper transmission fitting port forward along the right frame rail toward the radiator. Follow the original routing path documented in step 1. Ensure the line clears all exhaust components, suspension parts, and sharp edges. The line should have gentle bends with no kinks or sharp angles that could restrict flow. Leave fittings finger-tight at both ends until routing is confirmed.
8. 8

   Route new lower cooler line
   Route the new lower cooler line (return line) from the lower transmission fitting port forward to the radiator following the same path as the upper line. Maintain separation between the two lines using the factory bracket positions. Ensure both lines run parallel and do not contact each other or any moving components. Verify adequate clearance from the steering linkage and exhaust system throughout the full range of suspension travel.
9. 9

   Install cooler line mounting brackets
   Reinstall all mounting brackets and clips along the frame rail in their original positions. Secure with original hardware or new fasteners of equivalent size and grade. The brackets should hold the lines firmly but not crush them. Ensure rubber isolators are in place where the lines pass through metal clips to prevent vibration and chafing. Verify lines cannot move excessively or contact the frame rail directly.
10. 10

    Tighten radiator-side fittings
    At the radiator, remove the shop rags or plugs from the cooler ports. Thread the new cooler line fittings into the radiator by hand, ensuring proper engagement without cross-threading. The upper line connects to the upper radiator cooler port, and the lower line to the lower port. Once hand-tight, use a flare nut wrench to snug the fittings. Tighten firmly but do not overtighten inverted flare fittings, as this can damage the flare seat. Tighten the inverted flare fittings firmly by feel until snug, then approximately one quarter turn more; do not overtighten as this can damage the flare seat. Verify the factory torque specification before applying a fixed value.
11. 11

    Tighten transmission-side fittings
    Remove plugs from the transmission cooler ports. Inspect ports for debris or damage. Thread the cooler line fittings into the transmission case by hand, ensuring proper thread engagement. Once hand-tight, use a flare nut wrench to snug the fittings. Apply the same care as at the radiator end to avoid overtightening. Tighten firmly by feel until snug, applying the same care as at the radiator end to avoid overtightening. Verify the factory torque specification before applying a fixed value.
12. 12

    Add transmission fluid
    Lower the vehicle to the ground safely. Locate the transmission dipstick on the right rear side of the engine compartment. Remove the dipstick and insert a long-neck funnel into the dipstick tube. Add 2 quarts of Type A (Suffix A) automatic transmission fluid—or a modern Dexron-compatible ATF as a backward-compatible substitute—slowly to replace the fluid lost during line replacement. Reinsert the dipstick but do not check the level yet, as the transmission must be run to circulate fluid through the new lines.
13. 13

    Circulate fluid and check for leaks
    Start the engine and allow it to idle in Park. With your foot on the brake, shift through all gear positions (Park, Reverse, Neutral, Drive, Low) per the Powerglide 2-speed shift pattern and hold each position for 3-5 seconds. Return to Park. This circulates fluid through the new cooler lines and purges air from the system. Immediately inspect all four fitting connections (two at radiator, two at transmission) for signs of leakage. Shut off engine if any leaks are detected and re-tighten fittings as necessary.
14. 14

    Check and adjust transmission fluid level
    With the engine at normal idle in Park and the parking brake set, remove the transmission dipstick and wipe it clean. Reinsert fully, then remove and check the fluid level. The level should be between the ADD and FULL marks on the dipstick when hot. Add additional Type A or Dexron-compatible ATF in small increments (1/4 quart at a time) as needed to bring level to the FULL mark. Do not overfill. Recheck the level after each addition.
15. 15

    Final inspection and road test
    Raise the vehicle again and perform a final visual inspection of all cooler line connections, routing, and bracket security. Verify no contact points with exhaust, suspension, or frame edges. Lower the vehicle and take a short test drive (5-10 minutes) at varied speeds to bring the transmission to full operating temperature. After the test drive, immediately recheck all fittings for leaks while the system is at full pressure. Recheck fluid level once hot and top off if necessary.

Reassembly

1. Ensure all cooler line fittings are tightened securely but not overtightened to avoid damaging flare seats.
2. Verify all mounting brackets are reinstalled in original positions with proper hardware.
3. Confirm cooler lines have adequate clearance from all moving parts, exhaust components, and sharp edges.
4. Double-check transmission fluid level is at FULL mark on dipstick when hot and engine is idling in Park.

Verification

• Start engine and verify no fluid leaks at any of the four cooler line fittings while system is under pressure.
• Confirm transmission fluid level is correct on dipstick when transmission is at operating temperature.
• Verify smooth transmission operation with no slipping or delayed engagement in all gears.
• Inspect cooler lines after 50-100 miles of driving for any signs of leakage, chafing, or movement at mounting points.
• Monitor transmission operating temperature to ensure cooler system is functioning properly (normal range 175-200°F).

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