Across the UK, thousands of backup generators installed in the 60s, 70s and 80s still sit quietly in plant rooms, rarely noticed but always expected to work the moment they’re needed.
One recent callout reminded us just how important these machines are.
We were asked to investigate a Paxman 8RPH, which was built in 1972, over half a century old.
But at that moment, it wasn’t very happy – the engine would start, run briefly, and then shut down on overspeed.
Despite its age, the engine had only 275 running hours, a true testament to how many backup sets spend most of their lives waiting for the moment they’re called upon.
And when they are called upon, they need to work.
Old Engine, New Problem
After running the engine, the overspeed shutdown triggered immediately. Because these engines pre‑date modern electronics, overspeed protection is fully mechanical. Robust, but reliant on small components that must move in perfect synchrony.
Accessing the speed switch required us to removing the fan cowling and inspection plate.
And when we did so, we discovered the problem:
- The small angled drive coupling pin linking the engine to the speed switch had sheared clean through.
- Without that pin, the speed switch received erratic or no signal.
- To the engine’s protection system, this looked like a dangerous overspeed condition, so it shut itself down exactly as designed.
This fault could easily be mistaken for a governor issue or an electronic failure. But on older engines like this, the simplest mechanical components are often the key.
Back in Business — After 54 Years
With the broken pin removed, cleaned and replaced with a new 4 mm nut‑and‑bolt assembly, the drive coupling was restored and the system reassembled.
The engine started, settled at a steady 1500 RPM, and ran perfectly.
For a machine built when colour TVs were a luxury and analogue gauges ruled the world, its performance was as solid as ever.
Why These Older Sets Still Matter
What makes this case special is not the fault. Mechanical failures happen on engines of any age.
What’s special is the engine’s story:
- Installed over half a century ago, still expected to protect critical power today
- Minimal running hours but maximum importance
- Fully mechanical systems that still outperform expectations
The UK still has thousands of similar legacy generator sets in hospitals, water treatment sites, data centres, manufacturing plants and government facilities.
Many were installed in the 60s–80s, remain mission‑critical, and they work reliably, as long as they are properly inspected, serviced and understood.
If you’ve got old engines, it’s worth remembering:
- Older engines often rely on small mechanical linkages that can create big symptoms when they fail
- Electronic control failures aren’t always electronic — sometimes a £2 component is behind the entire shutdown sequence
- These engines can run beautifully for 50+ years with the right care
- Methodical fault‑finding saves time, cost and unnecessary part replacement
These machines were built in an era of engineering simplicity and durability and many will continue to safeguard the UK’s power resilience for decades to come.
If you’re responsible for legacy diesel engines or backup power assets, especially older Paxman, Ruston, Mirrlees, Blackstone or similar sets, we’re here to keep them reliable, predictable and ready for the one moment they matter most.
Need some help with one? Drop us a message and we’ll be happy to help.
Paul
