What these engines are
The K21 and K25 are four-cylinder industrial engines built around a common architecture: an overhead-valve cylinder head with a bathtub combustion chamber, a camshaft in the block, and a chain-driven timing arrangement. They are supplied for gasoline, for LPG, and in a combined form that runs either.
Two engine control arrangements exist. One is carbureted, with a distributor and high-tension leads, a mechanical fuel pump and a governor. The other is electronically controlled, with an ignition coil unit, low-voltage distribution leads, an electronically controlled throttle and an engine control system that sets idle and high idle. Specifications and procedures diverge between the two, so establishing which arrangement an engine has comes first.
The two engines side by side
They share a bore. The stroke is what separates them, and everything set by stroke follows from that.
| K21 | K25 | |
|---|---|---|
| Displacement | 2,065 cc | 2,488 cc |
| Bore x stroke | 89 x 83 mm | 89 x 100 mm |
| Compression ratio, gasoline | 8.7 | 8.7 |
| Compression ratio, LPG | 9.3 | 9.2 |
| Maximum output, gasoline | 39.6 kW (53.8 PS) at 2,700 rpm | 45.8 kW (62.2 PS) at 2,700 rpm |
| Maximum output, LPG | 41 kW (55.7 PS) at 2,700 rpm | 46.9 kW (63.7 PS) at 2,700 rpm |
| Maximum torque, gasoline | 149 N·m at 2,000 rpm | 175 N·m at 1,600 rpm |
| Maximum torque, LPG | 151 N·m at 1,600 rpm | 188 N·m at 1,600 rpm |
| Connecting rod | Longer | Shorter |
| Valve mechanism | Overhead valve, pushrod | |
| Combustion chamber | Bathtub | |
| Firing order | 1-3-4-2 | |
| Idle / high idle | 700 rpm / 2,700 rpm | |
| Maximum speed | 3,600 rpm no load, 3,000 rpm under continuous load | |
| Engine oil | 10W-30, class SJ, 3.7 liters (3.9 US qt) with filter | |
| Power take-off | Silent chain-driven side PTO | |
The rod length runs against expectation: the K21 rod is the longer of the two, not the shorter.
How the valvetrain works
The camshaft runs in the block, in three bushings of stepped size, front to rear. Valve lifters sit on the lobes, push rods carry the motion up through the head, and a rocker shaft assembly at the top converts it to valve lift. There is no overhead camshaft, and the camshaft is chain driven.
The rockers are handed. Left and right rockers are marked, and the shaft itself carries a mark at the upper end, toward the crankshaft pulley, so the assembly goes back the correct way round. Coil springs space the rockers along the shaft. They are compressed and tied with wire before the retaining bolts are loosened, which is what makes reassembly manageable; the stated alternative is to loosen the four bolts evenly.
Each rocker carries an adjusting screw and locknut, so valve clearance is set mechanically rather than by selecting shims. Both intake and exhaust take the same clearance, and it is set with the engine hot.
Valve timing is identical on both engines and on every cylinder. The intake opens shortly after top dead center and closes well after bottom dead center; the exhaust opens before bottom dead center and closes at top dead center. Each valve is open for 216 degrees. Where one valve is measurably out against the others, the lobe is the thing to suspect, and a worn or damaged lobe means the camshaft is replaced rather than corrected.
The timing drive and the side power take-off
There are two chains inside the front of this engine, and they do different jobs.
The timing chain runs between the crankshaft and camshaft sprockets. Both sprockets carry matching marks, and the chain carries marking plates in a contrasting color, with a fixed link count between them, so the two shafts can only be timed one way if the marks are used. A chain tensioner presses on the slack side through a synthetic rubber shoe; the tensioner body has oil holes that must line up with the block, and the shoe must sit parallel to the chain. Fitted the other way round, the shoe prevents the tensioner bolt from being tightened, and the tensioner is damaged. Shims behind the crankshaft gear set the height of the two gear end faces relative to each other.
The second chain is the side power take-off. A drive sprocket at the crankshaft turns a driven sprocket carried on ball bearings, both enclosed by the front cover, with a splined output. The spline gets molybdenum grease on assembly. A chain here that has stretched announces itself as an unusual slapping noise when the take-off is loaded.
The front cover also carries the front oil seal, and the bushing under the crankshaft pulley is the signal plate bushing the crankshaft position sensor reads.
The bottom end
The crankshaft runs in five main bearings. Four of them are plain; the center one carries a thrust bushing, and that bushing is what sets crankshaft end play, so end play out of range is a center bearing job rather than a crankshaft job. Bearing shells have to stand proud of the cap parting face when the cap is off: crush height must be present. Caps carry embossed arrows and numbers marking both position and direction, and they are pulled down working outward from the center, with the crankshaft turned by hand after each bolt to confirm it still turns under light force.
Pistons are aluminum, of a thermal flow type, and a round dent on the crown marks the front. The piston pin is a press fit in the connecting rod but is meant to turn freely in the piston, so the two ends of the same pin behave differently: removing it means heating the piston and pressing the pin out, while a pin that will not rotate by hand in the piston bore condemns the piston and pin together.
Ring detail matters on assembly. The top ring is chrome plated on its face. The oil ring is a combined type with an upper rail, a lower rail and an expander; it is removed by taking the two rails off first and then the expander. Rings are fitted oil, second, then top, with the maker's mark upward and the gaps distributed around the piston rather than stacked in line. The front of the connecting rod is found by looking at the assembled rod and piston from the front: the projection on the rod lies to the right.
For rebuild, pistons come in standard and two oversizes, and main and rod bearings in a range of undersizes matched to a reground journal or pin. Cylinders are bored as a set — if one bore needs it, all of them get done — and pistons are a selective fit against the finished bore rather than a drop-in part.
Cylinder head, gasket and head bolts
The head gasket locates on two dowels driven into head bolt holes at each end of the block deck, and it goes down with the copper side facing up. Head, block face and gasket all have to be clean and dry of oil and dust before the head goes on, while the bolt threads and the underside of the bolt heads get antirust or engine oil.
Head bolts are angle tightened. The sequence runs a first pass to a low figure, a second pass to a higher one, a full release back to zero, a return to the first figure, and then a specified angle. That angle is measured with an angle gauge or a protractor, not estimated by eye. A substitution torque figure is published for general service in place of the angle step.
Fuel systems and engine control
On carbureted engines the fuel side is conventional: a mechanical pump, a fuel filter, a carburetor with a water-heated choke, an accelerator pump, a fuel cut solenoid, a fast idle control device and a mechanical governor that limits maximum speed. Ignition comes from a distributor with a pickup coil and high-tension leads.
On electronically controlled engines the throttle is electronic, idle and high idle are set by the engine control system rather than by a screw, and ignition comes from a coil unit with low-voltage distribution leads. Fuel pressure is held in the system and has to be released before any fuel line is opened.
LPG runs its own hardware: an air horn, an LPG holder carrying a filter and injector, and gaskets and an O-ring at each joint, feeding the throttle chamber and the intake manifold. LPG also runs the higher compression ratio of the two fuels. Gasoline calls for unleaded regular at 91 octane or better. LPG calls for a 30P or higher grade in cold seasons, and a 100P grade at ambient temperatures at or below -5°C (23°F).
What carries across and what does not
Shared between the two engines: bore, piston diameter and piston pin, crankshaft journal and rod pin diameters, the five-bearing main arrangement, valve clearance, valve timing, firing order, spark plug type, oil grade and capacity, idle and maximum speeds, the oversize piston and undersize bearing ranges, and the side power take-off arrangement.
Different between them: stroke, displacement, connecting rod length, output, peak torque and the speed it arrives at, the LPG compression ratio, and — on carbureted engines only — ignition timing at idle. Everything else that differs on a given engine tends to come from the fuel system and engine control arrangement rather than from which of the two engines it is.
Frequently asked questions
Are the K21 and K25 the same engine?
They share an 89 mm bore, the same valvetrain layout and valve dimensions, the same timing and power take-off arrangement, the same firing order and the same valve clearance. What separates them is stroke, and therefore displacement, crankshaft, connecting rod length, output and peak torque.
Is this an overhead cam engine?
No. The camshaft runs in the block, in three bushings, and drives the valves through lifters, push rods and a rocker shaft assembly. Clearance is set at an adjusting screw on each rocker rather than by shim selection.
Timing belt or timing chain?
Chain. The camshaft is driven by a timing chain with a spring-loaded tensioner acting through a synthetic rubber shoe, and both sprockets carry timing marks that pair with marking plates on the chain. There is a second, separate chain in the front of the engine driving the side power take-off.
Does running LPG change the engine mechanically?
The compression ratio is higher on LPG than on gasoline, and output and peak torque differ. The fuel hardware is entirely different — an air horn, holder, filter and injector in place of the gasoline arrangement. The core engine is the same.
How many main bearings does it have?
Five. Numbers one, two, four and five are common parts; the center bearing carries a thrust bushing and sets crankshaft end play. If end play is out of range, the center bearing is what gets replaced.
What oversizes and undersizes are available?
Pistons are listed at standard plus two oversizes. Main and connecting rod bearings are listed in a series of undersizes, each matched to a journal or pin reground to a stated diameter. Boring is done to all cylinders at once, and the piston is a selective fit to the finished bore.
Why does it matter whether the engine is carbureted or electronically controlled?
Because ignition, fuel delivery and throttle control are different systems. One has a distributor, a mechanical fuel pump and a governor; the other has a coil unit, an electronic throttle and an engine control system. Diagnostic routines, adjustment procedures and a long list of parts diverge on that one distinction.