Triumph · 1970–1977The Stag Archive

How-To & Repairs / Cooling

Coolant flush and corrosion protection

The annual service job that keeps a Stag alive: draining tired coolant, flushing the system and refilling with fresh inhibited coolant. Do this every year, without exception. Fresh inhibitors are what stop the iron block corroding the alloy heads through galvanic action.

Tools needed

  • Basic spanner set
  • Clean funnel and long-nosed filler
  • Coolant catch tray and sealable disposal container
  • Hose for flushing

Parts needed

  • Fresh good-quality inhibited coolant suitable for mixed-metal engines
  • Deionised or distilled water for the mix
  • Optional: cooling-system flushing agent recommended for classic alloy engines

Warnings

  • Coolant is toxic to pets and wildlife — never leave it in an open tray and always dispose of it responsibly.
  • Do not top a Stag up with plain water long-term. Diluting the inhibitors is what starts the corrosion cycle.

Steps

  1. 01. Warm the engine briefly, then let it cool fully

    A short warm-up helps the old coolant mix and drain cleanly. Then leave the car until the block is stone cold before opening anything.

    Warning:Never open the radiator, expansion tank or block drain while the system is hot.

  2. 02. Drain radiator and block

    Open the radiator drain and the block drain(s) into a clean tray. Note the volume and appearance for your records.

  3. Owner-uploaded photo slot
    Old coolant appearance. The colour and clarity of the old coolant in the catch tray.

  4. 03. Flush with clean water

    Refill with clean water, run the engine to temperature with the heater on, then drain again. Repeat until the water runs clear. If using a flushing agent, follow its instructions and flush thoroughly afterwards.

  5. 04. Refill with correct coolant mix

    Refill slowly with the correct mix of coolant and deionised water, bleeding at the highest points as you go. Run the engine to full temperature and top up as the system settles.

  6. Owner-uploaded photo slot
    Fresh coolant at the correct level. The expansion tank at the correct cold level after refilling.

  7. 05. Log the service

    Record the date, mileage and coolant used in the car's history file. Make the next flush a diary entry — annual is the target, not 'when it looks tired'.

Related faults

Further reading

Figures and procedures should be confirmed against the official Triumph workshop manual; for safety-critical or specialist work, consult a specialist.

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