Fitting an LS into a European or Japanese chassis is one thing. Keeping it at the right temperature is another problem entirely. The factory LS cooling system was designed around American-market cars with specific radiator dimensions, fan shrouds, and coolant routing that bear little resemblance to what's sitting in your engine bay. Get this wrong and you'll spend the summer watching the temperature gauge climb while you sit in a queue on the M25.

This guide covers radiator sizing, electric fan selection, coolant plumbing, and the small details that catch people out. None of it is particularly exotic, but it does need to be done in the right order.

Why the Stock Setup Does Not Transfer

The LS family produces a lot of heat relative to its displacement. A Gen III LS1 in a Camaro or Corvette relied on a large cross-flow radiator, a factory fan setup pulling significant airflow, and coolant passages sized to move a high volume of fluid. Most donor cars will have a much smaller frontal area and a radiator tunnel that was never intended for this engine.

The coolant outlet on an LS sits at the front of the intake manifold. On some swaps, particularly into narrower bays, this creates a routing conflict with the top radiator hose. On others, the thermostat housing faces the wrong direction entirely. Neither issue is insurmountable, but both need to be resolved at the planning stage, not when the engine is already in. [VERIFY: specific thermostat housing orientation varies by LS generation and vehicle application]

The other factor is the steam vents. LS engines have two small bleed ports at the back of the cylinder heads, one on each side, that need to route to the top of the radiator or to a separate header tank. If you block these off or leave them unplumbed, air pockets form and you will get localised overheating at the rear of the heads, which is difficult to diagnose and unpleasant when it causes head gasket failures. This is probably the most commonly missed detail in LS swap cooling installs.

Radiator Sizing and Core Specification

For a naturally aspirated LS running on the street, a two-row aluminium core with at least 600 mm of fin height is a workable starting point for most mid-size chassis. Boosted applications or track use should go to a three-row or thicker core. The goal is to maximise core area within whatever space the chassis allows.

Aluminium cores transfer heat more efficiently than copper-brass units of the same physical size and are significantly lighter. The fitting side matters too: make sure the inlet and outlet are on the correct sides for your routing, and that the filler neck or header tank provision is in a position where you can actually reach it.

For many UK-based LS swaps, a custom radiator is the practical choice rather than trying to adapt a universal unit. Several fabrication suppliers will build to dimension with the correct inlet and outlet positions for a reasonable lead time. It is worth getting this made before the engine is in so you can trial-fit and adjust the core support if needed.

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Electric Fan Selection

Puller fans are almost always the better option on an LS swap where the engine-driven fan and viscous coupling are being deleted. The fan mounts to the back face of the radiator and pulls air through the core. Pusher fans (mounted to the front) are a compromise used when there is no room for a puller, and they are noticeably less effective.

For a single large-diameter puller fan, 3,000 CFM is a commonly cited minimum for a naturally aspirated LS on road use. Dual slimline fans are useful on wide radiators where a single fan cannot cover the full core width, but you need to verify the combined CFM is adequate rather than assuming two fans are always better than one.

The fan controller matters as much as the fan itself. A simple two-step relay setup with an on/off threshold works, but a PWM controller with a temperature-proportional output is quieter, easier on the fan motor, and reduces the abrupt thermal cycling that comes from bang-bang control. Most standalone ECU setups can drive a PWM fan output directly. If you are running a factory LS PCM or a Holley EFI unit, check whether the fan output is already handled by the ECU before wiring a separate controller.

Coolant Routing and the Steam Vent Problem

The standard LS coolant circuit runs: engine outlet at the front of the intake, top hose to the radiator, bottom hose back to the water pump, with the heater circuit tapped off the water pump inlet and the heater core outlet returning to the thermostat housing or the lower intake.

For the steam vents, the cleanest solution on a swap is a small aluminium header tank plumbed to accept the two bleed lines from the rear of the heads and with a central filler and pressure cap. The tank sits at the highest point in the system, typically somewhere on the bulkhead, and has a small return line back to the top radiator hose or the inlet neck on the radiator. This keeps the circuit permanently bled and avoids the hot-start air-lock that plagues swaps where the steam ports are simply teed into the top hose and the geometry does not work.

Choose coolant hose sizes that match your fittings and avoid tight bends near hot exhaust components. Silicone hose handles the heat and the repeated thermal cycling better than rubber, and it does not degrade internally in the same way. Straight and 45-degree sections joined with aluminium joiners are easier to route neatly than a single moulded hose that may not fit the clearances you are working with.

Thermostat Choice and Fill Procedure

For street-driven LS swaps in the UK climate, a 180°F (82°C) thermostat is a reasonable choice. The factory LS thermostat is 195°F, which is fine for a tightly controlled stock cooling system but can leave less margin when the system is running a smaller-than-factory radiator in a car it was never designed for. Some builders go lower, to 160°F, for track applications, though this can cause issues with fuel trim on factory ECU calibrations that expect operating temperature. [VERIFY: specific calibration impact of thermostat temperature on LS PCM fuel trims]

Filling procedure on an LS swap is not the same as filling a stock car. Air pockets in the rear of the heads are the primary issue. The correct approach is:

1. Fill slowly with the heater control on maximum.
2. Squeeze the top hose repeatedly while filling to purge air from the radiator side.
3. Allow the engine to reach thermostat-open temperature, then return to idle.
4. Watch the coolant level drop as the thermostat opens and the system fills fully.
5. Top up and check again after the first heat cycle.

If the steam vent lines are correctly plumbed to the header tank, this process is considerably more straightforward. If they are not, you will be guessing at whether the air has purged correctly.

What to Order

For most LS swap cooling projects you will be sourcing a combination of custom-fabricated and off-the-shelf parts. The fittings, joiners, and ancillary components are well-covered in the exhaust and cooling range at Billys Speed Shop, which carries Earl's and Kooks hardware alongside more specialist swap-specific items.

If your build also involves sorting the EFI side, the fuel injection and EFI collection has the Holley units and ancillaries to handle the management side, which you will need to address before the cooling system can be properly dialled in on the thermostat and fan switching.

For previous context on choosing the right LS generation for your swap, the LS engine swap guide is worth reading first if you have not been through it.

The Short Version

LS swap cooling fails for three reasons: undersized radiators, missing steam vent plumbing, and inadequate fan airflow. Get those three things right and the engine will run cool in almost any chassis. A decent aluminium core sized to your bay, a properly rated puller fan on a proportional controller, and a header tank that keeps the rear head steam ports bleed-free covers the vast majority of problems. The rest is routing and fill procedure.