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MG MGB Technical - Valve oil seals

When stripping the cylinder head down. It had the cup type of oil seals fitted to the end of the valve guides and valves but only on the inlet valves. I would have thought they should be on all of them. Is this correct. None of my books or manuals show them. It is a later head with single springs of of a 18v engine.
Trevor Harvey

when my friend rebuilt his mini he was told to fit them only on the inlet as this is where the suction takes place and hence more likely to have the oil leak down into the combustion chamber. Unfortunately he still had a smokey exhaust. When i rebuilt my cylinder head on my mgb i put the seals on all the valves. My car doesnt smoke at all, even when left for a long time. I dont think i'd ever fit them to just the inlet.
A P New

I don't think that's unusual, in fact I seem to recall that Peter Burgess doesn't normally fit them on the exhausts - hope I'm right about that, perhaps he will chip in. Maybe its because if there is any blow-by up the valve guides, it will be coming out of the cylinder for the exhaust valves, whereas with the inlets oil could be sucked in. Is that a sensible theory?
Mike Howlett

If the guides are within specs, the seals are only installed on the inlet valves due to suction. RAY
rjm RAY

Seals are normally fitted to the Inlet side only to seal against high vacuum from the inlet manifold and avoid the higher temps of the exhaust valves.

Pete.
Peter Thomas

Neither Peter nor I fit seals to exhausts - oil burning is not an issue for the exhausts and they need the extra lubrication.
Chris at Octarine Services

Trevor-
High gas pressures within the exhaust port momentarily restrict oil from going down the valve stem to both that induced by mechanical transference resulting from valve stem movement and that by capillary action. As the pulse of hot exhaust gases passes out of the port it leaves a partial vacuum in its wake, ambient pressure within the rocker arm cover then forces oil down the valve stem in order to both lubricate the bore of the valve guide and provide a heat-conducting medium. Thus, the absence of a valve stem oil seal on the exhaust valve guide will have no practical effect upon oil consumption. The film of oil on the valve stem is an essential part of the cooling of the exhaust valve as it fills the gap between the stem and the bore of the valve guide, acting as a medium for conducting heat out of the valve. Because bronze valve guides have closer operating clearances, valve stem seals are not only unnecessary on the exhaust valve guides, but are actually undesirable as they reduce lubrication of the valve stem, accelerating wear of not only the valve stem and its valve guide, but also of the valve seating surfaces as a consequence of attendant misalignment.

However, the opposite is the case where the intake valve is concerned. The low atmospheric pressures in the intake ports draw oil down the stem of the intake valve quite readily, leading to high oil consumption and carbon buildup on the head of the valve and inside of the combustion chambers. Additionally, the oil being mixed with the incoming fuel / air mixture consequently interferes with combustion efficiency and actually lowers its octane rating, making preignition a very real risk. Always install the highest quality valve stem seals on intake valve guides.

Deflector type valve stem seals grasp the valve stem, moving up and down with the valve, shielding the valve guide like an umbrella. Positive valve stem seals remain in a fixed position on the valve guide, acting like a squeegee to control lubrication of the valve stem as it slides in the valve guide. An insufficient supply of oil causes premature wear of both the valve stem and valve guide, while too much oil entering the valve guide results in excessive oil consumption, faster buildup of carbon deposits on the piston crowns, valves, and inside of the combustion chambers, and faster spark plug fouling, all of which are sometimes blamed on worn rings or worn valve stems.

Unlike the other seals of the engine where the goal is zero leakage, the valve stem oil seal must produce a controlled flow (regulated) leak. It is much more difficult to achieve controlled flow leakage because the margin for error is so small since it is so important for a thin film of oil to remain between the valve stem and valve guide. However, the amount of oil used to form this film must be strictly controlled. For this reason, do not use the Original Equipment O-ring type valve stem seals (BMC Part # AEK 113) on the intake valves. The design of this type of oil seal only permits it to prevent oil trapped inside of the valve spring retainer cup from draining down the valve stem and puddling atop the valve, a condition that induces smoking upon startup and leads to heavy, flow-restricting carbon deposits atop the heads of the valves, as well as carbon accumulation on the roofs of the combustion chambers and the crowns of the pistons. They actually work by making the oil coming down from the rocker arm flow outward to the edge of the spring cap (cup) and then down the outside the diameter of the spring. It does not seal the valve stem so much as it directs the oil away from it. This archaic design characteristic is based on the theory that whatever oil goes down the valve stem into the valve guide and from there into the combustion chamber occurs by gravity flow only. This theory disregards the tremendous vacuum forces acting upon the lower end of the intake valve guide and the valve stem, as well as the mist and spray effect that the rapidly reciprocating springs, rocker arms, and pushrods have upon the oil inside of the rocker arm cover. Being made of Nitrile, the Original Equipment O-ring type valve stem seals are prone to failure when operating under thermal conditions above 200O Fahrenheit (93.3O Celsius), a temperature commonly attained in even Original Equipment specification engines when working under a heavy load or in high ambient temperatures. This being the case, they should always be checked for signs of deterioration at every periodic valve adjustment.

Instead, install a set of Fel-Pro valve stem seals that require no modification of the valvetrain components (Fel-Pro Valve Stem Seal Part # SS 70373 for Chevrolet Vega 4 cylinder 140, 1986-1992 Ford 351 Windsor; also Advanced Performance Technology Part # 70373) onto the intake valve guides. These seals have to be slipped over the valve stem using a protective cover before being pressed onto the valve guide with a special tool. Being made of Viton, they are not prone to failure until thermal conditions rise above 450O Fahrenheit (232.2O Celsius), which is much higher than that which any properly maintained street engine experiences. In addition, these positive guide design valve stem seals do a far superior job by eliminating vacuum loss. As a side benefit of the elimination of this vacuum interference with flow through the ports, the fuel-air mixture is more stable and can be more accurately metered to a finer degree, thus increasing both power output and fuel economy.

If you fail to use the O-ring type stem seals on the exhaust valves, you may expect to see a puff of smoke upon starting the engine up from cold. If you have no seals of any kind on the exhaust valves, a small amount of oil may be expected to drain out of the valve collars and run down the valve stems while the engine is off. Most of it will puddle atop the closed valves and be burned inside of the exhaust system. However, if an exhaust valve is open, the oil will drip into the combustion chamber and form some carbon on the piston crown when combustion starts. If the engine is permitted to run for a while, the carbon will hopefully be burned away. However, if the engine is used for short trips and not allowed to reach full operating temperature, then you will be faced with carbon buildup and all of its attendant problems. The proper sequence for fitting the O-ring type stem seals is to first fit the valve springs and their spring cap (cup), compress the valve springs by means of pressure on their spring cap (cup), then fit the O-ring onto the shoulder of the valve stem. There is a recess provided in the valve spring cap (cup) to retain the oil seal. Next, fit the collets, then release the valve spring.
Stephen Strange

Hi All

I tend to fit ex stem seals even though they are not needed, I find myself in the odd position that if any engine burns oil after fitting one of my heads I am at fault, if I fit seals it cannot be down the guides so therefore I don't automatically get blamed! For Road heads we use MGC style seals. For Race use we use O rings in the valve cap OE style for inlets only.
Originally Bs were fitted with in and ex O ring seals in the valve caps.

Peter
peter burgess

This thread was discussed between 22/12/2009 and 27/12/2009

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