Stiga ST Series
The engine inside almost every Mountfield
The story of this engine
Stiga's in-house engine division builds the engines fitted to nearly every Mountfield, Stiga and selected Atco walk-behind petrol mower sold in Britain. The ST120 and ST160 are the workhorses. Reliability is a notch below a Honda GCV but considerably above the Loncin-class budget engines. Parts are universally available at any Stiga or Mountfield dealer.
Why owners like it
- Massive UK installed base — the engine in roughly half of British petrol mowers
- Cheapest dealer-network parts in the category
- Acceptable noise — about the same as a Briggs 500E
- Mountfield/Stiga dealer network is one of the densest in the UK
Where it falls short
- Not as durable as a Honda GCV — typical 800-hour service life vs 1,500
- ST120 carb is a recurring service item — varnishes after winter storage
- Drive cable on Mountfield self-propelled chassis fails before the engine
Service intervals & costs
What this engine actually needs every year, and what it costs to keep alive.
| Interval | Task | Cost |
|---|---|---|
| Every season (or 25 hours) | Oil change — SAE 30 | £8 DIY / £35 dealer |
| Every 2 seasons | Foam air filter — wash and re-oil | £0 DIY / £10 replacement |
| Every 3 seasons | Spark plug | £4 DIY / £8 dealer |
| Each season start | Drain stale fuel or stabilise | £6 stabiliser |
| When deck looks worn | Replace blade and re-balance | £18 generic / £28 OEM |
What goes wrong, and when
Every engine family has predictable failures by age. Knowing what's coming saves money on used buys and makes negotiation easier.
Parts cost reference
UK 2026 prices — OEM (genuine) vs aftermarket. Generic parts work fine on consumables; stick to OEM on anything that affects timing or tolerance.
| Part | OEM | Aftermarket |
|---|---|---|
| Foam air filter | £10 | £3 |
| Spark plug | £8 | £3 |
| Carburettor service kit | £25 | £10 |
| Drive cable (Mountfield SP) | £18 | £9 |
| Replacement engine (used) | — | £80–£120 |
| Generic 46cm steel blade | £26 | £8 |
The Stiga ST series is the engine for buyers who want a known quantity at the lowest price. It's not the longest-lived engine you can buy, but it's the easiest to keep running, the cheapest to service, and the engine inside the mower most British gardens already own.
Related reading →Mowers in our catalogue with this engine (12)
See every engine family
Side-by-side hub of all the engines we cover — Briggs, Honda, Kawasaki, Stiga, Loncin and more.