Caterpillar/Towmotor GP28NM Seating

The K21 and K25 are four-cylinder industrial engines sharing an 89 mm bore and differing in stroke, at 2,065 cc and 2,488 cc. Both use an overhead-valve pushrod valvetrain with a rocker shaft and screw adjusters, a bathtub combustion chamber, a timing chain, five main bearings and a chain-driven side power take-off. Both run gasoline or LPG.

Routine service on a K21 or K25 covers valve clearance set hot at 0.38 mm on every valve, a cold retighten of the head bolts and manifold nuts, belt deflection, oil and filter, plugs, air and fuel filters and coolant. Most running complaints trace back to compression, ignition or fuel, and a compression test separates them quickly.