Off with her head! PBO’s Ben Meakins undertakes the process of removing and replacing the cylinder head of a Yanmar 1GM10 engine, step by step
I’m no mechanic, but even I could tell that when my Hunter Impala Polly’s Yanmar 1GM10 started losing revs and chucking quantities of black oil out of the exhaust, something fairly major was wrong. This brought on one of those periods of deep despair that only a large, looming engine bill can engender in the mind of a cash-strapped boat owner.
However, some research revealed that a common problem on these engines occurs when the inner jacket of the exhaust elbow corrodes through, resulting in hot, salty exhaust water being injected directly into the exhaust port on the cylinder head.
The general consensus was that on an engine this size, removing the cylinder head was a relatively simple job. Doing so would allow me to check for corrosion, clean out any build-up of carbon in the exhaust port and take the head to an expert to inspect it properly for damage. I was lucky in that the installation on Polly allowed full access to the top of the engine – on some boats, the engine would have to be removed.
Doing the fairly straightforward yet time-consuming removal and refitting of the cylinder head myself would save me having to pay a substantial amount for a mechanic’s time, leaving only the parts cost: not insignificant by any means, but a fixed price at least.
A good workshop manual was essential. You can buy them online from Yanmar dealers and eBay. Some are available to download for free from www.motoren.ath.cx.
Step 1: Removing the head
I had prepared the engine beforehand by removing the negative battery cable, draining the cooling system and taking off the alternator.
2. Checking for damage
With the head removed, it was time to clean up and take stock. I took the cylinder head and elbow to Marine Power in Bursledon, the local Yanmar dealer.
Here, Pete checked out the damage with an expert eye. With a small screwdriver, he pointed to a large hole in the exhaust elbow’s inner tube – and, unfortunately, a corresponding hole in the wall of the cylinder head. ‘I’ve not seen many this bad,’ he said. That explained where the oil was coming from – straight from the pushrod space to the exhaust. Luckily, he explained, I’d caught
it in time: hot exhaust water inside the engine block for a prolonged period would have been more terminal.
It is sometimes possible for a hole in a cylinder head to be welded up, but on the Yanmar 1GM10 the cast-iron walls are thin and welding could have an uncertain result. Therefore, following a call to Polly’s co-owners we bit the bullet and, with wallets significantly lighter, left with a new cylinder head, exhaust elbow and head gasket, ready to fit.
A new head for the 1GM10 comes with a set of valves – just as well, as the old exhaust valve seat was looking a bit pitted – but it was possible to reuse some of the parts attached to the old head.
The thermostat housing, temperature sensor and pre-combustion chamber could all be used again, so I set about removing these on the bench at home. The thermostat housing was a matter of removing two bolts. The temperature sensor unscrewed with some persuasion from a spanner and long extension bar, but the pre-combustion chamber, sitting at the base of the injector, needed more force. It eventually came free with the use of a softwood dowel and a large mallet.
Step 3: A thorough clean
If you are reinstalling the old cylinder head it will need a good clean in solvent, and the cooling passageways will require blasting out with compressed air. However, as I was replacing the whole head, cleaning efforts could focus on the engine block itself.
Step 4: Replacing the head
With the mating surfaces scrupulously clean, I could begin to fit the new head. This part of the job is essentially the reverse of the removal process.
What it cost: (2013 prices)
Cylinder head, new: £600
Head gasket £55.02
Exhaust elbow £180.80
Exhaust elbow gasket £6.89
Thermostat housing gasket £2.14
Air filter base gasket £2.14
Was it worth it?
With the batteries reconnected and the engine seacock turned on – and with no engine bits left over! – the motor started first cough and ran faultlessly thereafter. If your boat has a Yanmar 1GM10 – or indeed, any engine – you should remove the exhaust elbow on an annual basis, clean out any carbon deposits and check carefully for corrosion. For the minimal cost of an exhaust elbow gasket each year, this simple maintenance and inspection procedure could save you a large bill!
I consulted with a marine engineer when unsure of any step and, with care, was able to successfully refit the head. If you’re in any doubt, calling in the professionals could save you from
making expensive mistakes.