BURNED VALVE
Given that leaded petrol was phased out a quarter of a century ago, I am surprised how many petrol Land Rovers I come across that have still not been converted to run on unleaded. The procedure is simple although not especially cheap: the cylinder head is removed, exhaust valve seats machined away and hardened inserts fitted, then the whole lot put back together. It is the sort of job that only makes economic sense (as compared to using a lead replacement additive in the fuel) if the head has to be removed for some other reason, such as head gasket failure. It says a lot for the inherent toughness of the old two and a quarter that so many of these engines have just carried on running without any top-end attention since around the time John Major became prime minister.
What is even more surprising is that many of these engines have kept plodding along without the use of lead replacement additive. Lead in petrol served two functions: it boosted the octane rating and also lubricated the thin sealing ring where the exhaust valves met their seats. Manufacturers quickly discovered that this allowed cast iron cylinder heads to be made with the valve seats directly machined into the relatively soft iron head material. When engines without hardened valve seats are run on unleaded, the exhaust valve seats will gradually become pitted and eroded, leading to a progressive loss of compression. Once the gas leakage across a valve seat passes a certain point the valve itself begins to erode, a process which will rapidly accelerate until the point where there is no compression and hence no combustion.
The film of lead deposited on the seats continued to provide protection for some time after lead was removed from petrol, but this ‘lead memory’ effect will in most cases have dissipated long ago. Basically the harder an engine is worked, the faster the valve seats will erode, and an old Land Rover that is just used for pottering around will run for many years on straight unleaded without any ill effects. Regular motorway use of the same vehicle is likely to kill it rather more quickly.
A mid-1970s Series III truck cab arrived at the workshop on a transporter a couple of days ago. It had started running on three cylinders: the owner had provisionally diagnosed a burnt valve. The first job was to carry out a compression test, as the last vehicle I had in with an owner-diagnosed compression failure turned out to just need a new set of plugs. I removed the plugs, screwed the compression tester into number one plug hole, spun the engine on the starter for a couple of seconds, and got a reading of 10 psi as opposed to the normal 140 or so. I tried number two cylinder just to make sure my gauge wasn’t faulty, got a reading of 145 psi on that one, and then set about removing the cylinder head.
Lifting the head on a two and a quarter is not technically difficult, and I reckon it would make a fine project for a DIY enthusiast looking to move on from basic servicing to more complicated jobs. The only real problem is that the head is cast iron, very large and very heavy. Don’t try and lift it on your own, either get a friend to help you or use some kind of hoist.
With the head off the damaged valve was immediately obvious. The cylinder bores have a wear ridge around the top but not enough to worry me – “they all do that, sir” – and I have seen engines which ran fine, used very little oil and turned out to have much worse bore wear than this one. So, the head will go off for pressure testing (I have no reason to suspect internal cracks, but better safe than sorry) and provided it passes that test it will be fitted with new valves and hardened seats for unleaded petrol, only 25 years late. Not bad.