Installing an oil cooler thermostat is one of the most effective ways to manage oil temperatures in a performance car. By regulating when oil bypasses the cooler, a thermostat helps the engine reach its ideal operating temperature faster and maintains consistent cooling under load. Yet many enthusiasts rush the installation, making simple errors that lead to poor performance, leaks, or even engine failure. Understanding these common pitfalls is critical whether you are building a track car, a turbocharged street machine, or a high-horsepower drag racer. This guide covers the top mistakes to avoid and provides detailed best practices to ensure your oil cooler thermostat system works reliably for years.

Mistake 1: Ignoring Manufacturer Guidelines

Every oil cooler thermostat comes with specific instructions from the manufacturer. Similar to how a Mocal thermostat requires a certain orientation and port assignment, skipping those directions can cause the thermostat to stick open or closed, resulting in poor oil flow or no cooling at all. Many enthusiasts assume that all thermostats are universal, but the reality is that each brand designs its valve for a specific flow direction, spring rate, and bypass configuration.

What usually goes wrong: Installers ignore port labeling or mount the thermostat upside down. This can prevent the wax element from sensing oil temperature correctly, leading to delayed opening or constant bypass. Always read the data sheet that comes with the unit. Some thermostats require a specific “engine side” and “cooler side” — mixing those up sends hot oil straight to the cooler even when cold, defeating the purpose of a thermostat altogether.

Recommended action: Before assembling, lay out all components and mark each port with tape. Check the vehicle’s service manual for recommended oil cooler locations and any factory thermostatic bypass provisions. If you are retrofitting a thermostat on a factory oil-to-water cooler system, consult the vehicle’s technical service bulletins to avoid interfering with existing flow paths.

Mistake 2: Incorrect Placement of the Thermostat

Where you install the thermostat in the oil line matters immensely. A common error is mounting it directly on the engine block or too close to the oil pan, where heat from the engine can skew the thermostat’s sensing. Another is placing it near the cooler inlet without considering the pressure drop through the lines.

Optimal Thermostat Position

The ideal location is between the engine oil source (such as a sandwich plate or filter adapter) and the oil cooler. The thermostat should be positioned so that it receives oil that has already been heated by the engine, but not so close that radiated heat from the block fools it into opening early. A distance of 12–18 inches from the engine is typical for most inline thermostats. Some sandwich plate units are designed to mount directly between the oil filter and the engine block — these are inherently well placed, but you must ensure the plate isn’t contacting the exhaust manifold or other heat sources.

Bypass vs. Inline Types

Two common designs exist: bypass (three-port) thermostats and inline (two-port) thermostats. A bypass thermostat routes oil back to the engine when cold, allowing it to warm up faster. An inline thermostat simply stops flow to the cooler. Using the wrong type for your setup can lead to unexpected behavior. For example, if you use a bypass thermostat but plumb it without a return line to the engine, oil will dead-head and cause pressure spikes.

Always match the thermostat type to your system layout.

Pro tip: Mount the thermostat in a location that allows easy access for servicing. If you ever need to replace the element or clean the valve, you do not want it buried under intake plumbing or inside a frame rail.

Mistake 3: Using the Wrong Parts or Materials

An oil cooler thermostat installation is only as strong as its fittings and hose. One of the most frequent failures is using mismatched thread types — for example, NPT (National Pipe Thread) vs. BSP (British Standard Pipe) or JIC (Joint Industry Council) flare. These threads look similar but are not interchangeable. Forcing them together can crack the thermostat body or strip threads, causing catastrophic oil leaks.

Thread Compatibility and Sealing

Always confirm the thread pitch and sealing method. Most performance cooling systems use -10 AN or -12 AN fittings. If your thermostat has NPT ports, use proper NPT-to-AN adapters with a qualified sealant. Avoid using Teflon tape on AN flare fittings — the tape can shred and clog the thermostat valve. Instead, use a small amount of special sealant on the threads of NPT adapters and rely on the flare for sealing on AN connections.

Hose material: Use only hose rated for oil service and high temperature (typically -40°F to 300°F or higher). Silicone-lined hose with aramid braiding (like Earl’s Super Stock) is a safe choice. Avoid standard rubber heater hose — it degrades quickly in oil and causes leaks.

Thermostat Temperature Rating

Selecting the wrong thermostat opening temperature is another materials mistake. Common ratings are 180°F (82°C), 195°F (91°C), and 212°F (100°C). For most performance street cars, a 195°F thermostat works well, allowing the engine to reach normal operating temperature before the cooler opens. For race-only cars, a lower 180°F unit may be used to keep oil cooler in extreme conditions. Installing a 160°F thermostat on a street car will prevent the oil from reaching optimal viscosity, increasing engine wear over time.

Mistake 4: Not Bleeding the System Properly

Air trapped inside the oil cooler circuit is a silent killer. When you fill a dry system, the oil must displace all air from the lines and cooler. If air remains, it can cause erratic thermostat operation, pressure fluctuations, and even frothing of the oil in the sump. Frothing leads to loss of lubrication and increased engine temperatures.

How to Bleed an Oil Cooler System

  1. Pre-fill the cooler and lines: Before connecting the thermostat, pour clean engine oil into the cooler inlet and tilt it to let air escape. You can also use a vacuum fill tool if available.
  2. Install with the thermostat temporarily wide open: Some thermostats have a small bleed screw or can be held open manually during initial startup. If not, you can leave the thermostat out and install a bypass loop for the first minute of running.
  3. Run the engine at idle with the oil fill cap off: Watch for oil returning to the fill neck. If bubbles appear, air is still in the system. Gently rev the engine to 1500–2000 RPM while monitoring oil pressure. Do not exceed 2500 RPM until you are sure no large air pockets exist.
  4. Check the dipstick: After bleeding, top off oil level. The cooler will hold extra oil; you may need to add more than the normal capacity.

Symptoms of incomplete bleeding: Low oil pressure at idle, fluctuating oil temperature gauge, unusual ticking sounds from lifters, or the thermostat never fully opening. If you notice these, stop the engine and inspect for air leaks at fittings.

Mistake 5: Selecting an Incorrect Temperature Rating

This ties back to materials but deserves its own section because it’s so common. Many tuners choose a low-temperature thermostat (e.g., 160°F) thinking it will keep the engine cooler. In reality, oil that never reaches 212°F can accumulate moisture and sludge, particularly in street-driven cars that see short trips. The oil fails to boil off water condensation, leading to acidic oil and premature wear.

Choose based on application:

  • Street / daily driver: 195°F – 212°F (similar to engine coolant thermostat)
  • Street / track day: 180°F – 195°F (compromise between warm-up and high-load cooling)
  • Dedicated race car: 160°F – 180°F (oil will stay in optimal range during continuous high RPM)

Always verify that your oil cooler and lines have enough capacity to handle the heat when the thermostat opens. A 160°F thermostat combined with an oversized cooler can overcool the oil in cold weather, again causing the thermostat to cycle open/closed and never stabilize.

Mistake 6: Poor Hose Routing and Line Protection

Oil cooler lines that are too long, too short, or chafing against other components can cause leaks or flow restrictions. A common routing mistake is making sharp 90-degree bends near the thermostat ports, which add backpressure and reduce flow. Another is running lines near the exhaust without heat shielding. Even with modern synthetic oil, oil inside a line subjected to 400°F+ exhaust heat will degrade and create varnish.

Best Practices for Hose Routing

  • Use the shortest, most direct path between thermostat and cooler. Avoid loops that collect air.
  • Support lines every 12–18 inches with P-clamps or brackets to prevent vibration fatigue.
  • Keep lines away from exhaust, sharp edges, and moving suspension parts. Use nylon sleeving or silicone heat protection if routing near heat sources.
  • Avoid routing lines below the oil pan. That area is vulnerable to road debris and impact.

Consequences of poor routing: Abraded hose can rupture suddenly. A single pinhole leak under pressure can empty your engine of oil in seconds. Always inspect hoses after installation and periodically thereafter.

Mistake 7: Failing to Secure the Thermostat and Lines

Even if everything is plumbed correctly, a loose thermostat bracket can cause the unit to vibrate and eventually crack the housing. Many thermostats include mounting tabs, but if you are using an inline unit without tabs, you must create a custom bracket. Securing lines with proper clamps is equally important — loose lines can twist and loosen fittings at the thermostat ports.

Vibration kills: In a high-performance car, engine vibration and road shock are amplified. A thermostat that is not rigidly mounted will experience cyclic stress on its ports. Over time, the aluminum or brass body can develop hairline cracks. Use rubber-lined mounting brackets to isolate some vibration, but ensure the thermostat itself is fixed relative to the engine or chassis.

Checklist for secure installation:

  • Thermostat body clamped or bolted to a solid structure (engine block, strut tower, aftermarket brace)
  • Hoses secured at both ends (thermostat and cooler) with proper length – not too much slack, not taut
  • AN fittings tightened with the correct wrench, not over-tightened (use torque specifications if available)
  • All fasteners lock-wired or coated with thread locker if subjected to extreme vibration

General Installation Tips for Success

Beyond avoiding mistakes, following a thorough installation process will save you headaches later.

Pre-installation Checks

  • Inspect the thermostat visually: ensure the valve moves freely and the o-rings are not dry or cracked.
  • Clean all threads with a wire brush and apply appropriate sealant only where needed (no sealant on AN flare seats).
  • Pre-assemble the system off the car to check fitment — you may need adapters.

First Start Procedure

  1. Fill the engine with oil and prime the system (crank with fuel/spark disabled or use a pre-oiler).
  2. Start the engine and let it idle for 30 seconds. Check for leaks at every fitting.
  3. Raise RPM to 2000 and hold for 10 seconds — watch the oil pressure gauge. If pressure drops sharply, you might still have air trapped.
  4. Let the engine cool for 15 minutes, then recheck oil level and top off.
  5. Test drive gently for 10 miles, monitoring oil temperature. The thermostat should begin to open when oil reaches its rated temperature.

Using a temperature gun: After the test drive, stop and check the surface temperature of the thermostat housing. It should be hot. Check the line going to the cooler — it should be cooler than the housing if the thermostat is still closed, or similar temperature if it has opened.

Conclusion

An oil cooler thermostat is a valuable addition to any performance car’s lubrication system, but only if installed correctly. The most common mistakes — ignoring instructions, poor placement, wrong parts, air in the system, incorrect temperature rating, bad routing, and insecure mounting — are all avoidable with careful planning and attention to detail. By following the guidelines outlined here and consulting reputable resources like the Mocal thermostat technical data or Setrab product support, you can ensure your oil cooling system works as intended. A properly installed thermostat keeps oil at the ideal viscosity, protects your engine from thermal shock, and delivers consistent performance lap after lap or mile after mile.