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XPAG Valve Stem Seals

Save The Planet. ( Reduce Oil Consumption.)


When ‘International’ motoring legislation started to target the pollution the car engine produces, it was not all bad news. One thing that very quickly became apparent was that well sealed and properly vented engines did not leak oil. For years the British motor industry used very basic venting systems, and left the poor owner to cope with an engine that was destined to leak and consume oil after a few tens-of-thousands of miles. Today, there are tidy little all-aluminium, multi-valve, over-head-camshaft engines with sealed breathing systems that run over 100,000 miles, without leaking one drop of and hardly consuming any oil. The sealing of the cylinder bores, and the valve guides, makes the oil seem unnaturally clean for considerable periods. Where as a well used XPAG’s dip stick will tell you of the huge amount of carbon the oil is carrying after a couple of thousand miles, by its thick and very black constituency, today’s plastic dip-stick end will appear almost clean. So how is it done, and can we ‘transfer’ any of this modern engineering to our rather elderly engine?


Well, not much can be used, as the tolerances and seals used are for modern metals and materials. Our XPAG is basically a Morris saloon car engine, with the cylinder bores of grey-cast-iron bored direct into the casting. The rope seal at the front can be updated to the TF/Wolseley 4/44 neoprene sprung-lip-seal, and the rear scroll seal fitted with a rather expensive after-market sealing system. The way the modern engines keep their oil in, is to keep the crankcase pressure below ambient, that is, below the normal outside air pressure. This means any leak in theory will be into the engines crankcase, not out. This is done by using the inlet manifold and a controlling system to ‘consume’ any excess gasses. Again this is not practicable on our engines, the venting system is too crude. But, one of the methods of reducing oil consumption, and stopping burnt oil getting into the exhaust system hence reducing the pollution produced., is to improve the valve stem sealing. On modern cars catalytic converters are fitted, either two-way on older carburetter cars, or three-way on fuel injected cars. Burning oil can coat these internally with carbon and so reduce their efficiency.


AseriesstemsealbIt is not only the mechanical engineering that has improved. The oil one buys today is a far cry from that sold in the 1950’s. The straight SAE 20 or SAE 30 oils of those days soon broke down. Multi-grade 20/50 is the best to use in an XPAG today. This oil in itself is a very advanced bit of engineering, but be warned that the thinner semi-synthetic oils meant for modern engines will not suit. Oil is so good today that even the cheap-reclaimed oils from supermarkets is far better quality than that used originally. in our ‘dirty’ engines, changing the oil and its filter is required very regularly, the oil possibly as often as every 3,000 miles., ( unless you have a filter system that uses a modern element.)


Oil also reduces the octane rating of petrol, so if your engine uses and burns oil, you are more likely to suffer ‘pinking’ if you have raised the compression ratio. If it is still the standard 7.3 to 1 ratio, designed for post-WW2 pool-petrol of about 80 Octane, oil burning will not make much difference.


On our engines, the oil is stopped from getting into the inlet manifold by a shroud around the valve stem and a tiny ‘O’ ring seal under the valve cap. The ‘O’ ring seals the cap, the valve stem and the collets as one. This turns the cap into a ‘table’ and no oil can run down the stem and into the guide, instead it runs off the ‘edge of the table’. But as the inlet valves wear their guides, and there is a fair drop in pressure in the inlet manifold, oil inevitably gets sucked down the guides as oil-mist. A bit of crankcase-compression caused by worn rings letting combustion gasses past will assist the escaping oil. The exhaust valves do not suffer this, as there is no where near the same ‘depression’ in that manifold. If anything the exhaust can blow ‘up’ these guides if badly worn. A study of a more modern engine will reveal that inlet valve guides are fitted with seals, not the stem and cap. The seal affixes to the top of the guide, and the stem runs inside it. This is very effective, in fact so effective if fitted to a XPAG exhaust guide the exhaust valve will soon seize-up through lack of oil. So do not fit such seals to the exhaust valve guides. How ever, fitting them to the inlet valve guides is very effective, and will make a noticeable reduction in oil consumption.


So which seals do you need? Those intended for the EARLY version of the 1275cc BMC engine fit nicely. These are simple flexible neoprene caps. Later seals have a steel outer-case and this will not let the seal fit over our guides. Seals will be found in Mini 1275cc, MG Midget 1275cc, Morris Marina 1.3, and Metro 1.3 gasket sets. They can be purchased separately at some motor factors and a picture will be in any workshop manual for these cars.


So we can apply a little of today’s technology to the XPAG engine. It may not be ground-breaking technology, but it will do its bit to reduce pollution, and save your oil.


Neil Cairns.