There isn't much an amateur mechanic with a limited tool kit can (or should) do to the major components inside marine diesels apart from making sure that it has a good supply of fuel and air and clean lubricating oil.
You can, however, check and adjust the gap between the rocker and the valve. There has to be a gap - usually about the thickness of a fingernail - to allow for the different rates at which the various components expand and contract as they warm up. Without it, there's a very real risk that the valves won't shut completely: they may even come into catastrophic contact with the pistons. If the gap is too large, the valves may not open as far as they should, and the engine will certainly be noisier than it should be.
1 Read the engine manual to find out what the valve/rocker clearances should be, and whether they should be adjusted with the engine cold or at normal running temperature. Note that the clearances for inlet valves may be different for those for exhaust valves, because exhaust valves get hotter.
2 Remove the rocker cover - a relatively thin metal box on top of the engine, usually with the oil filler cap in the middle. Some engines have a separate rocker cover for each cylinder, or for each of two or three groups of cylinders.
3 Check the gap on each valve in turn, when the valve is completely closed and the gap is
its widest. There are two ways of finding out when this happens. On a multi-cylinder engine, the best way is to find the 'magic number' for your engine by adding one to the number of cylinders. For a four-cylinder engine, for instance, the magic number is five.
4 Turn the engine slowly by hand , if necessary using a spanner on the crankshaft (big nut on the lowest of the pulleys at the front of the engine). Watch the rockers moving as you do so, until the two rockers for one cylinder are 'on the rock' - that is, when one is rising and the other falling - signifying that this particular cylinder is at the end of its exhaust stroke and just beginning its induction stroke. Subtract the number of this cylinder from the 'magic number' to find the number of the cylinder that is ready to have its valve clearances checked. If, for instance, you have a four-cylinder engine and number 2 cylinder's valves are on the rock, number 3 cylinder is ready, because 5 - 2 = 3.
5 On a single-cylinder engine, the clearance for one valve should be checked when the other valve is fully depressed. You can use this approach for a multi-cylinder engine, but it will take longer!
6 Slacken the lock-nut on the rocker whose clearance you are about to adjust, and then unscrew the threaded adjuster about one or two turns.
7 Set a feeler gauge to the clearance specified in the engine manual, and slip it between the valve stem and the rocker. Gently wiggle the feeler gauge whilst tightening the adjusting screw, until you can feel the feeler gauge being nipped between the valve stem and the rocker.
8 Leave the feeler gauge in place, and hold the adjusting screw with a screwdriver while you tighten the lock-nut. When it's tight, wiggle the feeler gauge again to check that you haven't upset the adjustment: you should feel a slight resistance, but it shouldn't be jammed tight.
9 Repeat the process for each valve in turn, then replace the rocker cover, making sure that the cork or rubber sealing gasket is smooth, undamaged and properly seated.
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