MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Best Choice in Valve Stem Seals?

Is it the general consensus that the Fel Pro brand umbrella type valve stem seals (P/N SS 70373)fitted on the intake valves and the standard OE "O"-ring type seals fitted on the exhaust valves is the best possible combination for an 3-main MGB engine? Would it not be better to use the umbrella seals on all valves?
Frank Grimaldi

Frank-
Do not use an 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. 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, Moss Motors Part # 297-108) 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 200 degrees Fahrenheit (93.3 degrees 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.
Stephen Strange

Frank, I have fitted the FelPro seals to the inlets only. Have not run the engine yet. I turned a slight V notch in the top of the guide (about 20 thou deep)at the level of the retaining ring. Gives me a warm feeling about not losing the seal.
Art Pearse

Stephen,

Are you suggesting that I not use either the original "O" ring seal or the unbrella seal on the exhaust valves? Do other BBS members agree? Comments please.

Frank
Frank Grimaldi

Frank-
The O-rings do in fact prevent oil in the spring caps (cups) from puddling atop closed valves and in the combustion chamber when the engine is at rest, so go ahead and install them. Do not install the Fel-Pro seals on the exhaust valve.
Stephen Strange

Steve,

"Being made of Nitrile, the Original Equipment O-ring type valve stem seals are prone to failure when operating under thermal conditions above 200 degrees Fahrenheit (93.3 degrees 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. "

I don't know where this statement comes from but since the O rings fit INSIDE the valve cap and are squeezed tight by the retained cotters I would find checking them every time you adjust the valves somewhat difficult!

They certainly go hard after time but they retain their oil sealing efficiency even so.

I frequently see the O rings fitted to the valve stem below the cap - this is totally useless.

Chris at Octarine Services

I have worked as a mechanic on these engines since the T series was new and I can assure you that the OE O-ring seals as good as it gets. Providing they are fitted correctly as Chris described they will control excess oil running down the guide and dont prevent the oil mist, which is sufficient to do a good job of lubrication, particularly with the greater use of bronze guides which can be more prone to nipping up. Safe to fit to inlet and exhaust.
My own B has done over 90,000 miles, had the head freshened up and a new set of OE seals when a supercharger was fitted at 60,000 It has no visible oil use on 3000 mile changes. Whenever you pull a head with umbrella seals they are usually hard and useless when the OE seals may be hard but still doing the job.
There are a lot fitted wrong as Chris said.
Denis
Denis4

This thread was discussed between 14/03/2010 and 15/03/2010

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now