Goal: Add (install) a transmission oil cooler for a Jeep NSG370 6‑speed gearbox and understand every component, why it’s done, how it works, and what can go wrong. Written for a beginner mechanic. No questions.

Quick overview (one‑line): The NSG370 is a manual gearbox that can run hot under towing, high RPM, or heavy use. An auxiliary transmission oil cooler moves some gearbox oil through a small heat exchanger in front of the radiator/AC condenser so heat is shed to the air; cooler oil preserves lubricants, synchros, bearings and shift quality.

Why this repair/upgrade is needed (theory)
- Heat source: gears, bearings and synchros generate heat from friction when the gearbox is used hard (towing, long climbs, repeated spirited shifts). Heat degrades lubricant viscosity and additive chemistry and accelerates wear.
- Consequences of overheating: oil thins or chemically breaks down → reduced film strength → increased gear wear, burnt synchros (hard shifting), bearing damage, leaks, shortened gearbox life.
- How a cooler helps: it draws hot oil out of the case, exposes it to airflow across a finned core (or plate exchanger) and returns cooler oil to the case. Lower oil temperatures maintain proper viscosity and protect components. Think of the cooler like a person sweating when running: you move blood (oil) to the skin (cooler core) to dump heat into the air.

Main components (detailed descriptions)
1. Cooler core (heat exchanger)
- Tube‑and‑fin radiator style or stacked-plate type.
- Function: provides large surface area for oil to give up heat to air. Mount in front of the vehicle’s radiator or A/C condenser where airflow is highest.
- Materials: aluminum cores are common (lightweight, good conductivity). Plate coolers are compact but need more careful plumbing.

2. Thermostatic bypass valve (optional but recommended)
- A valve that bypasses the cooler until oil reaches a set temperature (e.g., 80–90°C / 175–195°F).
- Function: prevents overcooling when oil is cold (improves shift feel in cold weather) and reduces unnecessary flow through the cooler at low temp.
- Types: inline thermostat or sandwich‑type thermostatic assembly.

3. Transmission hose and fittings
- Hose: high‑temperature oil hose (rubber or stainless‑braided synthetic hose rated for automatic transmission fluid/gear oil). Choose hose rated for the lubricant you will use.
- Fittings: hose ends and adapters (barbed hose fittings with worm‑clamps, or reusable AN fittings with ferrules). Use fittings sized to the cooler ports (commonly 1/2" ID hose or AN-8/AN-6 depending on cooler).
- Function: convey oil to and from the cooler. Must resist heat, oil degradation and abrasion.

4. Case adapter / port or fill plug adapter
- Many gearboxes have no dedicated cooler ports. You can:
- Use factory ports if present (some transmissions have threaded cooler ports).
- Replace/change the fill or drain plug with an adapter that provides a coolant line.
- Install a case-mounted adapter plate with o‑rings to allow hose connection.
- Function: provides a sealed connection point to access gearbox oil.

5. Mounting hardware and brackets
- Brackets, rubber isolators, zip ties, hose clips to secure the cooler to the radiator support and route hoses cleanly.
- Function: prevent vibration and rubbing; keep core away from stones and road debris.

6. Worm clamps / crimp clamps / AN clamps
- Secure hoses to fittings. Use proper clamps sized for the hose type.

7. Transmission fluid
- NSG370 is a manual gearbox; it uses a manual transmission gear oil specified by Jeep (GL‑4 75W‑90 or Mopar NSG370 spec—check manufacturer spec for exact). Using the wrong fluid (ATF vs GL‑4 vs GL‑5) can damage synchros. Use the correct fluid and quantity.
- Function: lubricates and cools internals; the cooler circulates this fluid.

8. Tools & consumables
- Jack and stands or lift, catch pans, socket set, torque wrench, screwdrivers, pliers, line wrenches, rags, degreaser, thread sealant if needed, replacement fill/drain plugs and crush washers or O‑rings.

How the system works (step sequence)
1. Heat generation: oil in the gearbox is heated by friction.
2. Oil exits the case through the adapter/port (often driven by thermal convection and gear splash — there’s no pump in most manuals). If you add hoses and a cooler, some oil will flow out via gravity/thermal siphon as oil warms and becomes less dense; gear motion also sloshes oil and encourages flow.
3. If a thermostatic valve is present, it stays closed until oil reaches operating temp → then opens to send oil through the cooler.
4. Oil flows through the cooler core; air moving across the core (vehicle motion or fan) removes heat.
5. Cooled oil returns to the gearbox, lowering overall oil temp.

Analogy: The gearbox is a crowded kitchen where cooks (gears) generate heat. The oil is the delivery person that carries hot dishes out to a cooling patio (cooler) and brings back fresh coolers so the kitchen doesn’t overheat. The thermostat is like the patio door that stays closed until it’s hot enough to go outside.

Step‑by‑step installation (practical how‑to)
Note: This is a general guide. NSG370 cases vary by vehicle. Read your vehicle service manual for transmission plug locations, torque specs and manufacturer fluid spec before starting.

Preparation and safety
- Park on a flat surface. Block wheels. Use jack stands — never rely on a jack alone.
- Work with a cool engine and gearbox. Wear gloves and eye protection.
- Gather parts and the manufacturer-specified fluid.

1. Locate your transmission fill and drain plugs
- Find the drain (lowest) plug and the fill/level plug (mid‑case). The fill plug is how you’ll refill to the correct level; it’s also commonly where you can install the adapter if you don’t have factory ports.
- Clean area before removing plugs to avoid contamination.

2. Drain gearbox oil
- Place a catch pan under the drain plug, remove the drain plug, allow oil to drain fully. Re‑install drain plug and torque to spec (or leave out if you plan to reuse that opening for adapter — some people use the drain for adapter but then must re-fill by fill hole; choose one port and use proper adapter).

3. Decide how to tap the gearbox
- Preferred: use existing cooler ports if present.
- Common: use an adapter that replaces the fill/level plug — this is convenient because the fill port is mid‑case and provides a return point at level.
- Thread size varies; use an adapter that matches your case threads. Use new O‑rings or thread sealant where needed.
- Install adapter/ports to provide two connections: supply (hot out) and return (cool in). Typically, the supply is the higher location so gravity helps return oil.

4. Install hoses and route to cooler
- Measure and cut hose to length, leave slack for movement but avoid loops that will catch debris.
- Route hoses away from hot exhaust parts, moving suspension components and sharp edges. Use grommets if passing through metal.
- Secure hoses with clamps and zip ties. Keep the cooler behind the bumper/radiator support where airflow is strong.

5. Mount the cooler
- Attach cooler to the radiator support or a bracket. Use rubber isolators to reduce vibration.
- If mounting in front of the AC condenser, ensure it won’t significantly restrict airflow; keep it low and protected.

6. Connect hoses, clamps, and thermostat (if used)
- Connect supply and return to correct cooler ports. If using an inline thermostat, fit it close to the gearbox so it senses oil temperature effectively.
- Tighten clamps to a snug torque — avoid over‑squeezing rubber hose clamps which can cut the hose.

7. Refill and bleed
- Refill gearbox with the correct fluid through the fill/level port until the fluid reaches the bottom of the fill hole (standard manual gearbox fill procedure).
- If you used the fill hole for an adapter, refill until it starts to drip from the fill hole and reinstall the fill fitting and torque. If you used separate ports, follow manual for level procedure.
- Cycle the gearbox through gears with the engine off and then start engine and cycle again to circulate oil.
- Check oil level again and top off as needed.

8. Leak check and test
- Inspect all connections for leaks while engine is at idle and after a short drive.
- Monitor gearbox temperature on a test drive. Use an IR thermometer on the case or a temp gauge if you installed one.
- Recheck fittings after the first heat cycle.

What can go wrong (common failure modes and how to avoid them)
1. Leaks at fittings or case adapter
- Cause: incorrect adapter, bad O‑ring, cross‑threading, under/over tightened fittings.
- Prevention: use correct adapters, new O‑rings/gaskets, proper torque, thread sealant where recommended. Inspect after first run.

2. Wrong fluid
- Cause: using ATF or GL‑5 gear oil when the transmission needs GL‑4 or a specific Mopar spec.
- Effect: synchro wear, poor shift quality, premature failure.
- Prevention: use exact manufacturer fluid spec.

3. Overcooling
- Cause: no thermostatic bypass in cold climates and cooler is too effective.
- Effect: thick oil at startup, poor shift feel, increased wear at cold temps.
- Prevention: use a thermostatic bypass valve or smaller cooler core.

4. Hose chafe or heat damage
- Cause: poor routing, contact with exhaust or moving parts.
- Prevention: route away from exhaust, use heat shields, secure hoses with clamps and grommets.

5. Air pockets / incorrect level
- Cause: filling incorrectly after installing cooler lines, leaving air trapped.
- Effect: low oil level in gearbox, inconsistent lubrication, noise.
- Prevention: fill via correct level port, cycle through gears, recheck level after running.

6. Incorrect cooler placement
- Cause: mounting behind stone‑trap areas or too low exposing it to road damage.
- Effect: reduced cooling (less airflow) or physical damage.
- Prevention: mount where airflow is good but protected, consider a protective screen.

7. Adapter damage to case threads
- Cause: cross-threading or over‑torquing adapter.
- Prevention: carefully thread in by hand first, use proper tools and torque.

8. Contamination introduced
- Cause: dirty environment when opening the case or not cleaning hoses/components.
- Prevention: clean area before opening, cover openings, use clean fluid and tools.

Maintenance tips after installation
- Inspect hoses, clamps and core for leaks every oil change interval and after any off‑road event.
- Check and change gearbox oil on the manufacturer schedule or more frequently if used for towing/heavy duty.
- Monitor temperature occasionally. Normal operating temps will be lower post‑cooler; aim to keep temps within safe range per manufacturer guidance.
- Replace any damaged hoses immediately; keep spare clamps and O‑rings in your toolbox.

Practical notes specific to NSG370 (general cautions)
- NSG370 sometimes has specific fill level procedures — always use the factory fill port/level method to avoid overfill.
- NSG370 uses manual‑transmission synchronizers; avoid GL‑5 sulfured gear oils that can harm yellow metals if the manufacturer specifies GL‑4.
- Many manual transmissions do not have a pressurized pump; flow through an auxiliary cooler relies on thermal movement and sloshing. Expect modest flow; a thermostatic valve is very helpful to avoid overcooling.

Summary
- Adding an auxiliary transmission cooler to an NSG370 pulls hot gearbox oil through a small radiator, lowers operating oil temperature and prolongs life by preserving lubricant and reducing wear.
- The system consists of a cooler core, hoses and fittings, a case adapter or factory ports, an optional thermostatic bypass, mounting hardware and the correct transmission fluid.
- Install carefully: use correct adapters and fluid, route hoses safely, secure the cooler, and check for leaks and correct level after filling.
- Watch for common problems: leaks, wrong fluid, overcooling, hose damage and air in the system. Regular inspection and fluid changes will maximize gearbox life.

Done.
rteeqp73

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