Generator Engines for Sale
Replacement diesel engines for standby and prime power gensets. Send us the engine number and the set's kVA rating, and we'll confirm the right genset build, the price and when it can be with you.
When a generator engine fails, the alternator, canopy, control panel and fuel tank are usually fine. It's the diesel driving them that has let go: a seized bottom end, a cracked head, or an old engine using oil faster than you can top it up. Replacing that one engine is often quicker and cheaper than buying a new set, and the panel, wiring and changeover switch stay as they are.
Commercial Engines supplies reconditioned, used and new generator engines across the UK, mainly Perkins, Cummins, Lister and Kubota genset builds. We supply service engineers looking after standby sets on farms, pumping stations and telecoms sites, and hire fleets running site generators. We work from the engine number so the replacement drops onto the same mounts and couples to the same alternator.
Why a Genset Engine Isn't Just Any Diesel
A genset engine runs at one fixed speed and has to hold it as the load comes on and off. It needs a governor set up for constant speed, a flywheel and housing that couple straight to the alternator, and a power rating for generator duty. A tractor or plant engine of the same family is built for a throttle and a gearbox, so even when the block matches, the fuel pump, governor, flywheel and sump often don't.
That's why we ask for the engine number first. It tells us the exact factory build, including whether it's a genset version and what speed it was set for. Add the kVA on the set's rating plate and a photo of the engine and alternator together, and we can confirm what bolts straight in before you spend anything.
- Engine number and build checked against your set
- Speed rating and governor confirmed for 50Hz
- Flywheel housing and drive coupling matched to the alternator
- Ancillaries agreed up front: what moves over, what's included
Perkins Generator Engines: 400, 1100 and 2000 Series
Perkins is the most common engine in UK standby gensets, from small 400 Series sets on lighting towers and welders to 1100 and 2000 Series engines in standby sets for farms, factories and pumping stations. Genset builds usually carry a G in the model code, as in the 1104A-44TG2, and that matters: a Perkins engine for a generator has a different governor and fuel setting from the same 1104 in a telehandler. Send the full model code and the engine number from the plate on the block, and we'll match the replacement to your Perkins generator, whether that's a reconditioned engine, a used one or new. For the farm, plant and marine side of the range, see Perkins engines under Related Services below.
- 400 Series (403, 404) for compact and portable gensets
- 1100 Series (1103, 1104, 1106) for mid-size standby and prime power sets
- 2000 Series (2206, 2506) for larger standby installations
- Older 1000 Series genset engines, by rebuild or a suitable replacement
Cummins Generator Engines
The Cummins genset engines most often found in UK sets are the mechanical B and C Series G-Drive builds, such as the 4BTA3.9-G, 6BTA5.9-G and 6CTA8.3-G, along with the later electronic QSB and QSL. A Cummins generator engine is built for fixed-speed running, so an engine pulled from a truck or bus, such as the ISBe, isn't a straight swap even when the block looks the same. The engine data plate (on B and C Series engines it's usually at the front, on the gear housing) gives the engine serial number and the CPL (control parts list) number. Send both and we'll confirm the build for your Cummins genset and tell you what's available.
Lister, Kubota and Small Genset Engines
Smaller sets for farms, workshops, boats and remote sites often run a Lister or a Kubota. A Lister generator, including the older Startomatic standby units, often has years left in the alternator and frame, and many of the engines are still worth keeping going. Kubota three and four cylinder engines such as the D1105, V1505, V2203 and V3300 power a lot of small and mid-size Kubota gensets and silent site sets. For these engines, rebuilding the existing unit is often the better spend, especially where the whole set is in good order. Tell us the engine model and the set it's in, and we'll quote both routes where both make sense.
1500rpm or 3000rpm: Getting the Speed Rating Right
In the UK, mains power is 50Hz. A four-pole alternator gives 50Hz at 1500rpm, which is why most diesel standby and prime power sets run at that speed. Small portable sets with two-pole alternators run at 3000rpm. American sets run at 1800rpm for 60Hz, so an engine from a US-spec generator needs resetting and checking before it can drive a UK set at 1500rpm. Get it wrong and the set runs off-frequency, or the engine can't make its rated output. If you run a 1500rpm diesel generator, we check the speed setting on every engine we quote for it.
Getting the Set Running Again
A standby set with a dead engine is a site with no backup power, so the first thing you need is a straight answer. If the engine is on the shelf, we'll tell you and give you the despatch day. If it has to be sourced or rebuilt, you get the lead time and the price before you commit.
We deliver to sites, yards and workshops anywhere in the UK. Lines are open Monday to Friday 8am to 5:30pm and Saturday 9am to 1pm. Ring with the engine number and the kVA, and we'll check stock and price while you're on the phone where we can.
- Despatch timing confirmed when you order
- Delivery to site anywhere in the UK
- Price and lead time agreed before you order
- Fitting advice for your engineer or service company
Standby or Prime Power: Rating and Running Hours
The same engine can carry two ratings. The standby rating is higher and assumes the set only runs when the mains fails. The prime rating is lower because the set is the main power source and runs for long periods. If your set has moved from standby to daily running, for example on a site with no grid connection, an engine sized for standby may be working harder than it was built for. That's worth knowing before you buy a like-for-like replacement.
- Standby: runs during mains failures and test runs, higher rating, low hours
- Prime: main power source, lower rating, long running hours
- Tell us how the set is used and roughly how many hours a year it runs
Why Standby Engines Fail
Standby engines rarely wear out from hard work. More often they fail from the opposite. A set that is only ever run off-load on a weekly test, or run on a very light load, doesn't get hot enough to burn its fuel cleanly. Unburnt fuel and soot build up in the exhaust and on the injectors, the bores glaze, and oil consumption climbs. This is called wet stacking. Other common causes are old fuel left in the tank for years, coolant that hasn't been changed, and batteries that won't turn the engine over when it matters. If your engine has failed this way, a replacement will go the same way unless the test routine changes. We'll tell you if we think that's what happened.
Re-Engine the Set or Replace the Whole Generator?
If the alternator tests well and the control panel, canopy and fuel tank are sound, replacing the engine usually costs much less than a new set and keeps the existing installation, wiring and changeover switch as they are. A new generator makes more sense when the alternator is also failing, when the set is too small for what the site now needs, or when the existing engine is so old that a like-for-like replacement can't be found. Larger sets can also bring emissions and permitting questions when they are replaced, so check those with your service company before you decide. We'll quote the engine options and tell you when we think the set isn't worth saving.
Generator Engine Price: What Changes the Cost
There isn't one price, because a small Kubota for a 10kVA set and a six cylinder Perkins for a standby set at a pumping station are very different jobs. We'll quote every option that fits so you can compare. The things that move the price most:
- Engine size and build: six cylinder and electronic engines cost more
- Condition: used, reconditioned or new
- Exchange: returning your old engine as a core usually brings the price down
- Ancillaries: whether the radiator, starter, charging alternator, governor and fuel pump are included
- Availability: common builds are easier to supply than obsolete engines
Reconditioned, Used or New?
The right choice depends on how the set is used, how many hours it runs and how long you plan to keep it.
| Option | Relative Cost | Condition Known? | Best For |
|---|---|---|---|
| Reconditioned | Mid | Yes, rebuilt and run before despatch | Standby and prime sets with a sound alternator and installation |
| Used | Lowest | Partly: inspected and run-tested, not rebuilt | Low-hours standby sets and older site generators |
| New | Highest | Yes | Critical sites, sets under warranty, or where no good core exists |
*We'll quote the options that suit your set and say where the extra money is or isn't worth it.
Generator Engines by Make
Send the engine number, the model code and the kVA from the set's rating plate, and we'll confirm the build, stock and price.
Perkins
Perkins genset engines in the 400, 1100 and 2000 Series, including 1103, 1104 and 1106 engines for standby and prime sets.
Cummins
4BTA3.9-G, 6BTA5.9-G and 6CTA8.3-G G-Drive engines, plus QSB and QSL generator builds.
Lister
Lister and Lister Petter engines in older standby sets, by rebuild or a suitable replacement.
Kubota
D1105, V1505, V2203 and V3300 engines for small Kubota sets and silent site generators.
Caterpillar
CAT genset engines matched to the set's build and speed rating.
Not Sure?
Send a photo of the engine plate and the set's rating plate, and we'll identify the engine for you.
Applications & Use Cases
Wherever a diesel engine is critical to operations — on the road, on the water, in the field, or in a plant room — we have the right engine for the job.
Standby Power
Replacement engines for backup sets at offices, farms, care homes and commercial sites.
Pumping & Utilities
Engines for pumping stations and treatment sites where the set has to start first time.
Telecoms & Remote Sites
Engines for mast and remote-site sets that run without a grid connection or as backup.
Hire Fleets
Site generators back on hire with a reconditioned or used engine instead of a new set.
Farms
Standby sets for dairies, poultry units and grain stores, where a power cut costs stock.
Service Companies
Generator engineers buying engines for their own customers' sets.