Responding to a post on a sailing forum recently, I had a chance to sum up my thoughts, such as they are, on the question of what spares, tools and strategies to bring on passage, by which I mean oceanic sailing beyond easy or timely reach of mechanics, service depots, parts vendors or English-speakers.
What to bring to keep the diesel engine in good order? I would encourage the following ideas: Six months before you go on passage install a new starter and a new alternator. Seal the old, working ones in bags with dessicant and store in a dry place. Six months with a new alt and starter should reveal any issues, like whether you need to upgrade your belt to something of better quality or heftier construction. As for the old, if working, bits, I am a big fan of the "drop in/bolt on" solution: Even in my Lake Ontario Viking 33, I preferred to swap out the water pump entirely than to try to remove an impeller with the aid of dental mirrors and contortion. It is far simpler to carry an entire spare and gasket, ready to go , than to try to do some jobs at sea. This is just my opinion and rule of thumb, however; were I a diesel mechanic, I might have a different viewpoint.
If your diesel is old and has been run seldom, consider a "prophylactic overhaul" whereby the cranks, rods and journals are inspected, the bores observed for scoring, and so on. Boat diesels can and do die from underuse, not overuse, usually...something common on Lake Ontario where the time spent "winterized" greatly exceeds runtimes during the sailing months.
Replace (or have replaced) anything that looks off-spec, and sail away secure knowing that the odds of throwing a rod, etc. are very much lessened. This is also an opportunity to service engine mounts, paint the bilges and to derust and paint the engine a light colour that will show oil leaks.
Impellers can and do fail. Bring several. Consider switching your raw water pump cover to "Speedseal" or something similar that will allow you to avoid bending into a pretzel during a gale. Consider also a high-temperature alarm and a low-oil pressure alarm to warn you if you aren't in the habit of staring at the gauges. I am, and I keep an hourly log of my readings. You might not have an hour, however, if either the cooling or the oil failed.
Bring engine zincs, another "consumable" that can be hard to source.
Bring at least enough hose for a complete replacement, and bring enough hose clamps (aka "jubilee clips") for two complete replacements.
Consider a beefed-up fuel filtering system, as in one with its own pump to pressurize the fuel side without the engine running or one which can polish fuel BEFORE it goes to the daytank.Invest in a Baja-type filter for your deck fill to get dodgy particulates out before it reaches your tanks.
Consider a daytank, perhaps gravity fed. Rig your fuel and return lines so that the daytank contains x litres of "certified clean" fuel, even if your main tanks are filled with dirty goo.
Consider rerouting the fuel and water vent lines away from the topsides/under the gunnels to higher and drier in the boat. The first three causes of diesel death are water in the fuel, from what I've gleamed. Hence, its primacy over some other concerns, like "bring spare injectors". Sure, bring one, but injectors are not a job I would handle myself...aligning them and calibrating them strikes me as a specialist job.
Consider getting rid of the spring-loaded plunger in the exhaust loop and simply run an open hose high into the cockpit or someplace where the occasional spurt of water will PROVE the pump is working and which cannot fail, sucking water back into the manifold.
Bring spare gaskets, gasket goo and gasket paper of different thicknesses. Frequently, a gasket will fail slowly, leading to a weeping pump and partial loss of pressure. To you, it looks like your diesel's running 10 degrees hotter. Look in your bilge and/or keep oil pads beneath your sump. The location of water or oil will tell you where to look for problems.
Figure if there is any method that will start your diesel with zero battery volts. Practise this method, if applicable, a few times.
Go over the entire engine in situ and figure out how you would remove parts for a regasketing, say, when the engine was cold, hot or when you were in heavy weather. Picture the tools you would need and the steps it would take to do these things. Type up the procedure and put them in laminated pages in your maintenance log. You will find a reason to buy fairly esoteric tools, like right-angle screwdrivers, crow's foot wrenches or socket ends and other single-use, frustration relieving devices.
Start a maintenance log!
Get a single-volume book, like Calder's Mechanical and Electrical, that covers basic to intermediate maintenance of diesels and electrical topics.
Take a basic diesel course. Read and bring this book:
Do all these things, particularly the bits about keeping your fuel free from dirt and water, and you will avoid 90% of the common dead diesel issues. I'm newer to this aspect of cruising than I would prefer to be, but these are my strong impressions having already had to solve a few problems to my satisfaction.