How-To & Repairs / Cooling
Cooling system overhaul and upgrade
A full refresh of the Stag V8's cooling system — radiator, water pump, fan, hoses, thermostat and a second temperature sender — carried out as a single package. This is the single most important job on the car and the foundation of long-term reliability for the alloy heads and iron block.
Tools needed
- Standard metric and imperial spanners and sockets
- Torque wrench (see workshop manual for values)
- Coolant catch tray and sealable disposal container
- Soft mallet and small pry tools for hose removal
- Multimeter for testing fan and sender circuits
Parts needed
- New radiator (uprated alloy core is the common choice)
- New or overhauled water pump and gasket
- Electric cooling fan and thermostatic switch
- Second temperature sender for the opposite cylinder head
- Full set of hoses and stainless clips
- New thermostat and housing gasket
- Fresh, good-quality inhibited coolant
Warnings
- Never open the system when hot — pressurised coolant will scald.
- Do not run the engine after refilling until the system is fully bled; airlocks cause immediate localised overheating on the Stag.
Steps
01. Drain the system and inspect what comes out
With the engine cold, drain the radiator and the block into a clean tray. The colour and content of what comes out is the first piece of diagnostic evidence — sludge, rust or oily film all tell a story about how the car has been kept.
- Owner-uploaded photo slot
Coolant condition on draining. What the coolant looked like as it drained — colour, sludge and any debris in the catch tray. 02. Remove the radiator, hoses and fan
Label hose routing before disturbing anything. Lift the radiator carefully to avoid damaging the core against the fan or crank pulley.
03. Flush the block thoroughly
Reverse-flush the block with clean water until it runs clear. This step is worth taking time over — old casting sand and electrolytic residue are the underlying cause of many Stag cooling problems.
Warning:Do not use aggressive acids without specialist advice — they can attack the alloy heads.
- Owner-uploaded photo slot
Block after flushing. The block interior after reverse-flushing, showing water running clear. 04. Fit the new radiator, water pump and hoses
Install the new components dry-fit first to check clearances, then bed them in with fresh gaskets. Route hoses with generous clearance from the exhaust and moving parts.
05. Wire in the electric fan and second temperature sender
Run the fan on a fused feed via a thermostatic switch. Add the second temperature sender in the opposite cylinder head so both banks are monitored, not just the factory-sensed one.
- Owner-uploaded photo slot
Second sender installed. The added temperature sender in the opposite cylinder head, wired to gauge or warning light. 06. Refill, bleed and pressure-test
Refill slowly with the correct inhibited coolant mix, bleed at the highest points and pressure-test the system before starting the engine. Run to full temperature with the heater on and re-check every joint for weeping.
Figures and procedures should be confirmed against the official Triumph workshop manual; for safety-critical or specialist work, consult a specialist.
More guides
- Coolant flush and corrosion protection (Cooling)
- Replacing the timing chains (overview) (Engine)
- Setting the ignition timing (Ignition)
- Balancing and tuning the twin Strombergs (Fuel/Carburettors)
- Rust inspection walkthrough (Bodywork)
- Curing the "Triumph twitch" — rear driveshafts, UJs and the CV upgrade (Driveline)
- Overdrive won't engage — Laycock A/J-type troubleshooting (Transmission)
- Hot-starting and vapour lock (Fuel/Carburettors)
- Head gasket failure and freeing seized head studs (overview) (Engine)
- Freeing seized brakes and brake overhaul (Brakes)
- Lucas electrics — earths, connectors, charging and gauges (Electrical)
- Converting a Stag from automatic to manual (Transmission)