If you have diagnosed loss of compression, in past days, and with a common automobile type of engine, now would be the time to remove the cylinder head to look inside.
In view of the extortionate cost of cylinder head gaskets for small specialist marine diesels it is questionable whether this can be justified. If there is a strong suspicion that problems lie within the cylinder head, or that bore wear has reached unacceptable levels, then removal of the head to establish the cause of the problem prior to commencing more major work is a sensible move.
Exploratory removal of the head may still be justifiable and worth exploring with automotive-derived engines such as Thorneycroft, for which gaskets are available for a much more reasonable cost.
If the test findings are that the valves are the problem, one lifeline may be that the tappets are too tight, holding the valves open a little when nominally closed. It is worthwhile checking that there is some clearance on every one.
The trick is, for a 4-cylinder, 8-valve engine, to check the clearance of No.1 with No.8 fully down, No.2 with No.7 fully down, etc. The same applies no matter the number of cylinders, always add up to one more than the total number of valves. (Owners of 4-valve per cylinder V-8s check your manual!)
The problem is that valves that have been in this tight clearance condition for an appreciable time will have burnt and no amount of adjustment will restore them to good sealing. If this check brings no further relief it may at this stage be worth investing in a “proper” compression test, before deciding whether to fund a fairly major overhaul or even a new engine.
Running tests
An engine that starts within a few seconds of cranking, from cold, almost certainly has no compression problem. If it fires up, but runs erratically until warm, it probably has a compression problem on just one or two cylinders. If it does not start easily it may be due to compression but it could equally be a problem with a variety of other things. A poor-starting engine that looks OK on compression testing could have glow-plug, fuel or air problems.
Oil pressure is the best guide to oil system condition, including pump and all bearings. Few small boat engines are equipped with an oil pressure gauge. Fortunately, the oil pressure switch that is screwed into the block has a thread that is common to the ones used in many automotive applications. This means that you can buy a cheap pressure gauge from your car parts shop that will substitute for the switch. If you want to fit the gauge permanently you can buy a T-piece that will take the gauge fitting and the pressure switch.
An engine in good condition started from cold will have an oil pressure of maybe 45-65 psi, as determined by the setting of the oil pressure relief valve, both at tick-over and higher revs. As the engine approaches normal operating temperature the oil viscosity reduces, increasing leakage through the various small clearances in the engine.
Engines in good condition will retain a pressure at speed that is similar to the original value, whilst at tick-over it will reduce somewhat, perhaps to somewhere around 30 psi. A worn engine may have a running pressure of 30 - 35 psi and a tick-over pressure as low as15 - 20 psi. At this level the pressure switch light may be flickering or permanently on.
Levels of expenditure
Work on diesel engines can be roughly classified into high, medium and low cost, although everything is relative and even “low cost” can lead to sharp intake of breath.
Work that needs engineering skills and equipment to be applied away from the boat, and is therefore high cost, would include rebores, crankshaft regrinds and cylinder head rebuilds with valves.
In many cases the cost of the replacement parts needed for this type of work is now so high that a new engine is the only economic choice, unless the owner can carry out the majority of the disassembly and rebuild himself.
Medium cost work will be anything that requires the purchase of specialised parts such as bearings, oil pump, valves and full engine gasket sets. In work within this category carried out by a yard, the cost of the parts is a small proportion of the total bill and it will repay the owner to carry out most of it himself. As ever, having any specialist work such as valve guide replacement, valve seat cutting and the like done by an automotive shop will be a far cheaper route.
DIY gaskets
A cylinder head gasket for one of the popular small engines like Volvo, Yanmar or Bukh will cost about £100. A full engine set will cost not much short of £200. Few people could make a cylinder head gasket, but almost anyone could make the others. Since the age of 10, my daughter has been cutting gaskets for me, using only a pair of scissors.
The raw materials, Klingerit, cork and heavy paper, are available in sheet form from any automotive supplier at very low cost. There is no magic involved, just patience and care. It helps to have a few extra tools like punches and a small ballpein hammer but these are not essential. Any metal seals and O-rings that are needed can usually be purchased separately.
Incidentally, old marine charts are an excellent material for making your own gaskets!
I have left low cost work until the end, as I struggled to think of very much. Routine jobs like filters and anodes not included, the only checks and adjustments normally needed are to tappets. Front and rear oil seals are cheap but changing the rear one will require separation of the gearbox from the engine and this may only be possible after lifting the whole assembly out.
Replacement of collapsed or broken engine mounts barely sneaks into the low-cost bracket, at anything up to £100 each! Having injectors professionally cleaned and spray patterns tested is well worth the cost about every five years.
Maximum revs
Perhaps the most effective low-cost job is to motor the boat with the engine at maximum revs for about half an hour, then gradually reduce speed over five to ten minutes to allow temperatures to decline gently. This treatment is routinely carried out by many yards, for which they charge extortionately, and it markedly improves the performance of engines that normally never use more than 70% of their rev range.
Vyv Cox is a Chartered Engineer, holding qualifications in both metallurgy and mechanical engineering. Now retired, he worked for 30 years in the petroleum industry, responsible for the diagnosis of engineering failures. Originally based in Anglesey, with wife Jill he cruised extensively between Tobermory and the Scillies. Relocating to Holland they cruised most regions from the Baltic to Biscay in their Sadler 34. They are now meandering eastwards across the Mediterranean and have currently reached Greece.
