It was a limestone quarry, and the 400 kVA set had started missing under load — a valve-train kind of miss, the uneven lope you feel in your chest before you hear it. The operator swore the filters were fresh. The oil looked fine. The controller showed no alarms. I pulled the valve cover anyway, because at 3,100 hours with no valve clearance check on record, there was exactly one place left to look.
Every exhaust valve was tight. Three were being held open by a hair — enough that one seat had started to burn, that blue-black ring you never want to see. The engine wasn’t starving for fuel. Four valves simply couldn’t close.
Valve lash is the tiny gap between the rocker arm and the tip of the valve stem. On the diesel engines you’ll find in most generator sets — overhead-valve designs, the usual arrangement — that gap is how the valvetrain deals with heat. Metal grows when it gets hot. The lash gives everything room to expand and still let the valve slam shut. When the gap is right, you never think about it. When it drifts, the engine starts lying to you.
And it always drifts. Valve seats pound themselves deeper into the head over thousands of hours, and the gap gets smaller. Valves get tight. Not loose — tight. That’s the direction that kills.
Here’s the thing nobody tells new techs: a loose valve is the friendly failure. It clatters. You hear it at idle, that unmistakable tap-tap from under the cover, and you know exactly where to look. The valve opens late, closes early, lifts a fraction less — the engine loses a little power and complains loudly about it. Annoying. Fixable. Cheap, if you catch it.
A tight valve is the opposite. It makes no noise at all. The valve just doesn’t quite close. Combustion gas leaks past the seat on every power stroke — superheated gas, thousands of times a minute, aimed at one thin ring of metal. The seat burns. The valve face burns. Compression drops on that cylinder. And the first symptom you notice is the one I found at the quarry: the engine starts missing under load, and every easy answer checks out fine.
First, the engine gets harder to start than it used to be — compression leaking past a tight exhaust valve shows up at cranking speed first, which is why a valve problem so often gets misread as a starting problem. Second, it misses or runs rough once it’s warm and loaded, but behaves at idle. (Heat expands the valve stem, which eats whatever lash was left.) Third, the exhaust runs hotter than you remember, and the set starts working harder for the same kilowatts. One of these is a coincidence. Two is a pattern. Three means open the valve cover.
The procedure is simple. The discipline is the hard part:
A four-stroke hits top dead center twice per cycle: once on compression, once on exhaust. The gauge only tells the truth on the compression one. On the exhaust stroke the exhaust valve is supposed to be open — measure there and you’ll “find” a huge gap, adjust it “correct,” and make everything worse.
My rule: watch the companion cylinder. On most inline sixes, when cylinder one is at TDC on compression, cylinder six’s valves are rocking — one closing, one just cracking open. Or simpler: nudge the crank, and if either rocker on your cylinder moves, you’re on the wrong stroke. Roll it one full turn and try again.
I once adjusted a set after it had sat “a couple hours” because the client was in a hurry. Came back the next morning, rechecked, and half of them were wrong. Iron and aluminum shrink at different rates, and the numbers on the data plate assume everything is at ambient temperature. Overnight. Park the truck and come back tomorrow. The engine will wait — waiting is a generator’s whole job.
The engine makers I deal with generally call for the first lash check early — a few hundred hours, after break-in — and then on a regular interval after that. The exact number is on your data plate. Use it.
But here’s the standby problem. A backup set that exercises twenty minutes a month piles up maybe thirty hours a year. At that rate the manual’s hour-based interval arrives sometime around year sixty. Nobody waits sixty years. In practice the cover never comes off until something breaks — I opened one at a small food warehouse last year that hadn’t been touched in six years. Forty minutes with a feeler gauge, and the hard starting the owner had “gotten used to” was just gone.
Put it on the calendar instead of the hour meter: once a year, folded into the annual service. And check whether your service contract even includes it — most of the ones I’ve read don’t. The contract covers oil, filters, and a guy with a clipboard. Valve lash is line-item work.
You need a feeler gauge set, basic wrenches, and patience. It’s not a hard job; it’s a careful one. The part everyone gets wrong is the last step: after you tighten the locknut, the setting moves. Recheck every valve after locking. If it moved, do it again. The second pass takes half as long.
Call a tech — or talk to the shop that services your set — if the lash won’t hold between checks. A rocker or pushrod wearing that fast is telling you something bigger is wrong. Same if you find a valve at zero clearance that’s been run that way for a while. That one’s already hurt. Don’t adjust it and hope.
The quarry set got a new exhaust valve seat, a fresh gasket, and a line in its logbook: valves checked at 3,100 hours, next due at 5,000 — the kind of note that quietly adds years to an engine’s working life. Forty minutes of work. A ten-dollar tool. The operator still tells the story as the day the engine “had a cold.” Fine by me. I’ll take the boring explanation that turned out to be right.
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