Transmission Oil Cooler | Sierra 18-2588

Sierra 18-2588 Marine Transmission Oil Cooler | 3/8" NPT Oil | 1-1/4" Water | 6 GPM Oil / 25 GPM Water | Replaces Chrysler 3527779, Mercury 63832A1

The Sierra #18-2588 is a raw water-cooled marine transmission oil cooler. Cools automatic transmission fluid using raw (sea) water flow through a tube-and-shell heat exchanger. 3/8" NPT oil inlet/outlet fittings. 1-1/4" water hose connections. Oil flow rating 6 GPM max; water flow rating 25 GPM max. 10" tube bundle, 12" overall length, 9" x 2" core. Shipped bare metal, ready for primer and paint. Replaces Chrysler 3527779, Mercury 63832A1, and Marine Power 1440-000.

Product Details:

• Part Number: 18-2588 (Sierra)
• SKU: SIERRA-18-2588
• Manufacturer: Sierra
• Type: Marine Transmission Oil Cooler — Raw Water Cooled
• Oil Fittings: 3/8 in. NPT inlet and outlet
• Water Connections: 1-1/4 in. hose (inlet and outlet)
• Max Oil Flow: 6 GPM
• Max Water Flow: 25 GPM
• Core Dimensions: 9 in. x 2 in.
• Tube Bundle Length: 10 in.
• Overall Length: 12 in.
• Finish: Bare metal — ready for primer and factory paint
• Replaces: Chrysler 3527779; Mercury 63832A1; Marine Power 1440-000
• Condition: New

Cross-Reference — Replaces:

• Chrysler: 3527779
• Mercury: 63832A1
• Marine Power: 1440-000

Additional Information

Marine Transmission Oil Cooler — Function:

The transmission oil cooler uses raw water flow to remove heat from automatic transmission fluid (ATF) circulating through the marine gear. Inboard marine transmissions (such as Chrysler TorqueFlite, Borg-Warner, Hurth, and similar units) generate significant heat during operation — particularly during maneuvering, docking, and sustained slow-speed operation where the transmission is held in gear against load. Overheated ATF breaks down rapidly, causing clutch pack and internal transmission component failure. The oil cooler maintains ATF temperature within the operating range required for reliable transmission function and service life.

Tube-and-Shell Heat Exchanger Design:

The 18-2588 is a tube-and-shell (tube bundle) heat exchanger. Raw water flows through the outer shell; transmission oil flows through the inner tube bundle. The tube bundle provides a large surface area for heat transfer between the oil and water circuits without mixing the two fluids. The 10" tube bundle inside a 12" overall length shell is a compact design suited to typical inboard engine compartment installations.

Connection Specifications — Confirm Before Ordering:

Before ordering, confirm that your installation can accommodate the following connections:

• Oil side: 3/8 in. NPT threaded fittings (inlet and outlet) — connect to transmission ATF cooler lines
• Water side: 1-1/4 in. hose connections (inlet and outlet) — connect to the raw water circuit, typically teed off the raw water pump discharge or routed through the raw water cooling circuit

Confirm existing transmission cooler line fittings and raw water hose sizes before ordering to ensure the 18-2588 is a direct fit for your installation.

Flow Rate Ratings — 6 GPM Oil / 25 GPM Water:

The 18-2588 is rated for a maximum of 6 GPM oil flow and 25 GPM raw water flow. These ratings define the maximum flow capacity the cooler can handle without exceeding its design limits. Confirm your transmission's ATF circulation rate and raw water pump output are within these limits for your specific installation. Exceeding the oil flow rating reduces cooler efficiency; exceeding the water flow rating risks cooler housing pressure limits.

Bare Metal Finish — Primer and Paint Required:

The 18-2588 is shipped bare metal and requires primer and paint before installation if corrosion protection is desired for your engine compartment environment. Apply a marine-grade primer compatible with aluminum or the cooler's base material, followed by paint to match factory finish or engine compartment specification. Bare metal can be installed as-is in non-corrosive environments, but priming and painting is recommended for long-term corrosion resistance in the marine bilge environment.

Installation Notes:

• Confirm oil fitting size (3/8 in. NPT) and water hose size (1-1/4 in.) match your installation before ordering
• Confirm ATF flow rate and raw water flow rate are within 6 GPM oil / 25 GPM water maximums
• Apply marine-grade primer and paint before installation for corrosion protection
• Install with oil flow direction per the cooler inlet/outlet marking — do not reverse oil flow
• Ensure raw water circuit provides adequate flow through cooler for effective heat transfer
• Check all ATF and water connections for leaks after installation before returning to service

Frequently Asked Questions

1. What transmission oil lines connect to this cooler?
The oil side uses 3/8 in. NPT threaded fittings. Use appropriately rated ATF transmission cooler lines and fittings rated for the operating pressure of your marine transmission's ATF circuit.

2. What connects to the water side?
1-1/4 in. hose connections on both the inlet and outlet. The water side connects to the raw water cooling circuit — confirm raw water circuit routing and hose size in your engine's service manual before installation.

3. Does this cooler need to be painted before installation?
It is shipped bare metal and ready for primer and paint. Painting is recommended for long-term corrosion resistance in the marine bilge environment. Apply marine-grade primer and compatible paint before installation.

4. What is the oil flow maximum?
6 GPM maximum oil flow; 25 GPM maximum raw water flow. Confirm your transmission's ATF circulation rate and raw water pump output are within these limits.

5. What OEM part numbers does this replace?
Chrysler 3527779, Mercury 63832A1, and Marine Power 1440-000.