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MG MGB Technical - valve stem

my 74 b chrome bumper has a slow oil leak down the valve from the head. I was told that some the the American cars have some seal on the top of the valve. Does the MG have some sort of valve stem seal to prevent the oil from getting down. If so what is the Moss Part Number.

Thanks

Rick
rick baranyi

Hi Rick,

There is an O ring, as shown in attached picture, #14.
This O ring comes with the head gasket kit, however, I have measured a new one I have and it is 8.8 mm OD and 3.8mm Id. It is black in colour so I presume it is a standard nitrile one and not viton, which in my experience are brown.
Some say the O ring is only necessary on the inlet valve (which has suction) but my car had them on all valves, and 8 O rings come in the kit.

Herb Adler

There is a school of thought that you need the oil down the valve guides to lubricate them, I got really tired of the puff of blue smoke on start up and fitted them (I got a set from Summit Motors) and so far so good,
Stan Best

Are these valve stem oil seals? It seems to me that they would be pushed up the stem the first time is depressed, then just sit there. But as described they are a 'packing ring' which the workshop manual shows sitting at the bottom of the groove the collets sit in and as such won't have any effect on sealing the guides. All engines are shown as having those, no additional seal on any. I have seen talk of seals that sit on top of inlet *guides*, but the implication is these are aftermarket and as such getting round wear or some other problem.
PaulH Solihull

Rick-
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. 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 4500 Fahrenheit (232.2 degrees 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. Use them on the intake valves only.

Do not use a valve guide mounted oil seal on the exhaust valve guide in a mistaken attempt to reduce oil consumption. 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. However, the inertia of the hot exhaust gases traveling nearly perpendicular to the nose of the valve guide prevents these gases from traveling up the valve stem and consequently reducing lubrication. 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 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.

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 vale spring retainer caps (cups), compress the valve springs by means of pressure on their valve spring retainer caps (cups), then fit the O-ring onto the shoulder of the valve stem. There is a recess provided in the valve spring retainer cap (cup) for retention of the oil seal.
Stephen Strange

Rick-
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. 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 450 degrees Fahrenheit (232.2 degrees 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. Use them on the intake valves only.

Do not use a valve guide mounted oil seal on the exhaust valve guide in a mistaken attempt to reduce oil consumption. 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. However, the inertia of the hot exhaust gases traveling nearly perpendicular to the nose of the valve guide prevents these gases from traveling up the valve stem and consequently reducing lubrication. 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 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.

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 vale spring retainer caps (cups), compress the valve springs by means of pressure on their valve spring retainer caps (cups), then fit the O-ring onto the shoulder of the valve stem. There is a recess provided in the valve spring retainer cap (cup) for retention of the oil seal.
Stephen Strange

This is a common fix on the B series head. The original "O" ring type seal worked when everything was new and there was minimal clearance between the valve stem and the valve guide. The Vega "umbrella" type seals slip over the valve guide, intake only needed, and minimize the amount of oil that can be drawn down the valve stem into the combustion chamber. They can be installed with the head still on the car and only 4 are needed. They're also quite inexpensive. RAY
rjm RAY

The O rings are fitted inside the cap - not on the stem below the cap and stop oil running down the stem itself - the oil is shed over the cap and runs down the spring instead.

The O rings are fitted to all valves.

The "improved" stem seal fitted to MGC engines can be fitted to MGB engines as well - these sit on the top of the guide and "wipe" the stem, they do not eliminate oil reaching the guide entirely.

Normally I would only fit the improved seals to the inlet valves as this combats the oil being drawn in to the engine and being burnt.

Exhaust valves only need the O rings if any at all, as little oil reaches the valve head and I doubt you will see any oil smoke as a result.
Chris at Octarine Services

This thread was discussed between 27/11/2010 and 29/11/2010

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