Buying Guide
How to Size a Standby Generator for Your Business
Most undersized generators were sized off a utility bill. Here is how to work out the load that actually matters, and why oversizing causes its own problems.
A diesel generator engine almost never fails without warning. It tells you for months beforehand, in oil pressure, oil consumption, exhaust colour and the way it starts on a cold morning. The expensive failures are the ones where nobody was reading the signs.
This is a guide to reading them, and to deciding honestly whether an overhaul makes sense or whether the money is better spent elsewhere.
The signs that matter
Falling oil pressure
The single most useful number on the panel. As bearing clearances open up, oil escapes more easily and pressure drops — particularly when hot and at idle, which is why the gauge often looks acceptable on a cold start and poor after an hour on load.
What matters is the trend. A set that showed 4 bar hot two years ago and shows 2.5 bar hot today is telling you something specific, even if 2.5 bar is still inside the manufacturer's limit. Record the figure at every service, hot and at rated speed. Without that history you are guessing.
Before condemning the bottom end, rule out the cheap causes: wrong oil grade, a diluted or over-full sump, a tired pressure sender, a blocked filter or a stuck relief valve. A faulty sender has sold a lot of unnecessary rebuilds.
Rising oil consumption
Track litres per hundred running hours rather than litres per top-up. A gradual climb points to rings and bores; a sharp step change points to something more specific, such as a turbocharger seal or a valve stem seal.
Exhaust colour
- Blue is oil being burnt. At start-up only, suspect valve stem seals. Continuous under load, suspect rings and bores
- Black is unburnt fuel — injection, air supply or overload, rather than wear
- White when warm can be coolant entering a cylinder, which is a head gasket or a cracked head and needs attention now
Blow-by
Crankcase pressure rising past worn rings is one of the clearest wear indicators. With the engine warm and running, a breather that puffs visibly or a dipstick that lifts is telling you the rings are no longer sealing.
Hard starting and falling power
An engine that cranks longer than it used to, or that no longer holds frequency as load comes on, has usually lost compression. Confirm with a compression or leak-down test rather than assuming; fuel system faults produce similar symptoms and cost far less to fix.
Stop-now symptoms
Some findings are not monitoring items. Shut down and investigate before running again:
- Knocking or rumbling that changes with load — this is often a bearing already failing
- Metal particles or glitter in the oil, or on the filter when it is cut open
- Coolant in the oil, or oil in the coolant
- Oil pressure that collapses suddenly rather than drifting down
- Sudden heavy blow-by accompanied by a power loss
Running an engine through a bearing failure turns a rebuild into a crankshaft, a block, or a new engine.
Rebuild or replace?
Once the engine genuinely needs work, the question is scope. Honest answers depend on three things.
What the block and crank are worth
If the block is sound and the crankshaft can be ground within limits, a rebuild is usually the better value. If the crank is beyond its last undersize, or the block has been bored to its limit, the economics change quickly.
How many hours the rest of the set has left
Rebuilding the engine on a set whose alternator, control gear and wiring are equally tired buys you one good component in a worn machine. Price the whole set, not just the engine.
Whether the parts are still available
This is the one people forget. An overhaul is only sensible if bearings, gaskets, rings and valve components can still be obtained for that engine. Check availability before committing, not after the engine is in pieces on a bench.
What a proper overhaul includes
A rebuild worth paying for is not a set of bearings and a gasket. Expect:
- Crankshaft inspected, measured and ground if required
- New main and big end bearings sized to the finished journals
- Bores measured, honed or re-bored, with pistons and rings to suit
- Cylinder head stripped, pressure tested, skimmed, valves and guides attended to
- Oil pump checked and relief valve verified
- A full gasket set, every time — nothing reused
- Cooling system cleaned, because the debris from the old failure is still in it
Cutting any of these is how an engine comes back onto the bench within a year.
Making the next one last longer
Most generator engines that wear out early were not unlucky. The usual causes are:
- Running too lightly. A set at 15% load glazes its bores. Load bank it periodically if the real load is small
- Short test runs. Starting weekly and running five minutes is worse than not running it. Get it to temperature
- Old fuel. Diesel degrades and grows contamination in a standby tank. Filter, polish or turn it over
- Service intervals by date only. Oil ages whether the set runs or not, but hours matter too — use whichever comes first
If you are weighing up an overhaul, send us the engine make, model and serial number along with what you are seeing. We will tell you what is still available for it before you commit to stripping anything.
