COMPRESSION
In order to deliver its full power a gas engine must have good compression, and compression should frequently be tested by cranking the engine slowly and steadily with the ignition switched off. If compression is good, there will be a springy, elastic resistance that becomes greater as a piston approaches the end of a compression stroke, and that throws the piston outward as dead center is passed. Compression should be the same for all cylinders.
If there is a leakage of compression, the only resistance will be from the bearings, and it will be the same for all parts of the stroke.
A compression leak often makes a hissing noise that can be distinctly heard, and by which it can be located, but more often it makes no sound, and its location must be found by testing. The leak may be at any of the openings into the combustion space; at the valves, around the spark plugs or piston rings, or at the cylinder head gasket.
To discover whether the gasket leaks, run gasoline along the line of the gasket joint with a squirt can while the engine is being cranked briskly; at a leaky place it will be sucked in or blown out. The same test should be made around the spark plug.
The remedy is to reset the cylinder head, using a new gasket, and being sure that the surfaces are clean and free from grit.
Piston ring leaks are usually caused by the rings sticking in their grooves through the formation of carbon. To test for piston ring leaks, pour a half pint of cylinder oil into each cylinder, and crank the engine slowly. The oil will form a seal around the pistons, and if compression is then improved, the rings are shown to be at fault.
To free the rings, pour a few tablespoonfuls of kerosene into each cylinder, and spread it by giving the engine a few turns; after standing for an hour or so the carbon should be sufficiently softened to free the rings.
If the leakage of compression is due to the rings being worn and loose in their grooves, they must be replaced.
The most usual cause of compression loss is leaking valves. With its continual pounding against its seat, and the heat to which it is exposed, a valve and its seat will become rough and pitted, and will leak; when in this condition the valve must be ground.
A valve is ground by spreading grinding compound on the seat, and turning the valve against it. This requires the valve spring to be taken off; the exact method of doing this depends on how these parts are made.
If the valves are in a removable cylinder head, valve grinding is most easily done by taking the cylinder head to a bench. In many designs the valve seats are part of the cylinder casting, and the job is done on the tractor.
In grinding a valve the valve is not turned around in one direction only, for this would cut grooves in the valve and seat. To obtain smooth surfaces the valve should be given part of a turn in one direction, and then turned equally in the other direction; after every few turns the valve should be lifted and dropped to another position on the seat. In this way the grinding is made even all around.
Fig. 89.—Grinding Valve in Engine with Fixed Head
The best tool for valve grinding is a carpenter’s brace with a screw driver blade fitting the slot in the valve, as shown in [Figure 89]. This drawing illustrates a cylinder with a fixed head; the valve is reached by unscrewing the plug from the opening directly above it. When grinding valves in an engine of this design the opening between the valve pocket and the combustion space should be plugged with a rag or waste to prevent the grinding compound from getting into the cylinder.
With the valve grinding tool in position, swing the handle back and forth ten or twelve times; then lift the valve, place it in a new position, and repeat. The valve is lifted most easily by a light spring placed under the valve disk, as shown in [Figure 89].
From time to time the valve disk and seat should be cleaned off and examined to see whether they are smooth and free from pits and scores. If they appear to be, make marks around the valve disk with a lead pencil, replace the valve, and give it a complete turn. If this wipes off the pencil marks all around the valve, the grinding is complete, and the valve may be replaced with its spring and spring retainer. It is not necessary to grind until the entire thickness of the valve disk and seat are smooth; a narrow band all around will make the valve tight.
After grinding, and before replacing the valve, all traces of the grinding compound should be wiped off, and great care taken that none of it gets into the cylinder, valve stem guide, or other working part.
Fig. 90.—Grinding Valve in Detachable Head
On an engine with a removable head containing the valves, the head may be taken to a work bench, which makes grinding easier. This is illustrated in [Figure 90]. On an engine in which the valve and its seat may be taken out, the seat may be clamped in a vise, as shown in [Figure 91]. With valves of either of these types, the grinding may be tested by turning the head or the seat so that the disk is down, and pouring in gasoline. If the valve is not tight, the gasoline will leak through, and grinding must be continued.
Fig. 91.—Grinding Valve in Detachable Seat
When a valve seat is very badly worn it must be redressed, which is done with a cutting tool to be obtained from the maker of the tractor, and illustrated in [Figure 92]. This has a stem fitting the valve stem guide which centers the tool and assures a true cut. If a seat is so worn as to need redressing, the valve will be in such bad condition that it must be discarded and a new one used. This must be ground in before the engine is run.
Grinding a valve lowers it in its seat, and usually makes it necessary to readjust the push rod. When an engine is cold there is a space of about ¹/₃₂ inch somewhere between the cam and the valve stem; in [Figure 93], this space is shown to be between the valve stem and the rocker arm. As the engine heats up the valve stem lengthens, and this space permits it to do so.
Fig. 92.—Valve Seat Cutter
If the space is too small, the stem will come against the rocker arm or the push rod, and the valve will be held off its seat, causing a compression leak. If the space is too great, the valve will open too late and close too early. The space must therefore be carefully adjusted, and this is arranged for on practically all makes of tractor engines.
Fig. 93.—“Holt” Valve Arrangement
One-thirty-second of an inch is the thickness of a 10-cent piece; it should just be possible to slip a slightly worn dime into the space when the engine is cold.