O-ring valve seals have been used in MG series engines providing many years of service. Introduction of new materials and designs for valve components since the cars have left the assembly line offer an opportunity to upgrade the cylinder head.

The original designed use of the O-ring was as seal filling the inside bottom of the spring retainer cap. The early engines up through approximately the first four thousand MGB engines also used a oil deflector under the spring retainer cap. Oil is required to lubricate the valve stems and original equipment cast iron valve guides. The oil circulating in the engine cylinder head pumps through the rocker shaft. Oil splashed on top of the spring retainer cap was blocked by the O-ring from running down the valve stem into the valve guide. The original O-ring and oil deflector design allows a small amount of oil splashed through the valve springs to coat the valve stems and penetrate the clearance with the valve guides.

After many miles, the valve stems and guides wear and the clearance tolerance between them increases. The increase in tolerance allows more oil to seep down the intake valve stems inside the valve guides. The intake valves are most prone to oil leakage because suction or vacuum is created on the intake cycle of the piston stroke with the intake valve open drawing oil into the combustion chamber. On the exhaust stroke of the piston cycle gasses are pushed out of the engine. The oil burns in the combustion chamber and is visible exiting the tail pipe as blue grey smoke. Carbon deposits develop in the combustion chamber, building up on the valve heads, stems, and guides. The worn valve stems and guides result in increased engine oil usage, loss of power, and increase hydrocarbon emissions in the exhaust which could cause an emissions test failure.

77 - oil deflector used in MGA and a few early MGB engines.
78 - spring cap retainer
79 - o-ring

MGB Original Equipment
valve stem
intake .3422 to .3427
exhaust .3417 to .3422
valve guide
outer diameter .5635 to.5640
inner diameter .3442 to .3447
stem to guide clearance
intake .0015 to .0025
exhaust .002 to .003
spring inner diameter
dual inner 18G-18GK .720 
single 18V 1.00 

Alternatives to using the original O-ring seal. The two basic types of valve stem oil seals are umbrella seals and positive seals. Umbrella or defector type seals including O-rings are used on older push rod engines. Umbrella seals are installed on the valve stems. They move up and down the valve stem as it opens and closes deflecting oil preventing it from flowing down the valve stem into the guide. Positive seals are pressed in place on the end of the valve guides and wipes the oil off the valve stems as the stems move up and down. A thin layer of oil lubricates the valve stem and moves past the seal to the guide.

The valve seals are made of nitrile, polyacrylate, fluoroelastomer (Viton), silicone, nylon or Teflon. Nitrile is an inexpensive material and is used in umbrella type seals with an operating temperature rang of -40 to 250 degrees Fahrenheit. The temperature range is good for intake valves seals and too low for exhaust valve seals. Next is polyacrylate, about twice as expensive as nitrile and has a higher maximum temperature of 350 degrees. Polyacrylate is used for umbrella seals and some positive seals. Positive seals made from nylon resist oil and can withstand temperatures between -40 to 300 degrees. Silicon is a high grade seal material and can tolerate temperature of -60 to 400 degrees. Positive valve stem seals made of fluoroelastomer (Viton) have a temperature range of -5 to 450 degrees. Viton is flexible, more durable than silicone, and resists wear making it very durable over other materials. The best positive seal and most expensive is made of Teflon. Teflon is hard like nylon but it has a temperature range of -5 to 600 degrees Fahrenheit.

Positive seals made from Viton, part number 70373, distributed by Fel-Pro fit MGB valve guides without any modification. John Twist of University Motors recommended using Fel-Pro SS 70373. SS indicating a set of eight. Moss Motors USA offers an up rated Viton positive valve seal in its MGB catalog. The Moss seal part number #290-890, fits any MGA, MGB, dual or single valve spring arrangement without any modification to the valve guide. Both positive seals fit 11/32 inch 0.343 valve stems and stock size valve guides 9/16 inch 0.562 outer diameter.

The professional engine builders like Hap Waldrop of Acme Speed Shop in Greenville, South Carolina are using bronze valve guides with positive seals on the valves. Some recommend only using positive seals on the intake valves if the exhaust valves have close clearance with the bronze exhaust valve guides. The advantage in using a bronze valve guide is because bronze wears better than iron and needs no lubrication. Machinist's need to be aware that bronze guides used in MG cylinder heads take more clearance than with cast iron at non operating temperature. A bronze guide needs a good .003-.004 inch clearance when the engine is cold, but will tighten up at operating temperature.

Re-building cylinder head iron guides and O-rings or improved valve seals bronze guides and improved valve seals offer professional and hobbyist engine builders many options. Installing improved valve seals on worn valve stems and iron guides provides a temporary oil consumption improvement. Eventually the cylinder head will need to be rebuilt with new valve stems and guides. The first option for a cylinder head rebuild is to use umbrella seals or O-rings with iron valve guides. The age old tested original cylinder head combination of components. The other possibility is use positive seals with the iron guides with the option of intake valves only or all valves. Another combination is using umbrella seals on bronze valve guides. This pairing is the least likely to reduce oil consumption at engine start because of the properties of the components. The bronze guides require more cold clearance and would permit increased oil flow. Umbrella type seals would not prevent the oil from entering the valve guides. The best option is to use positive seals on bronze valve guides with the option of intake valves only or all valves.

Positive Seal
70373
material nitrile or polyacrylic
stem diameter .343
guide diameter .485
inside depth .330
height .470
seal diameter .680

References
- Acme Speed Shop, Greenville, SC, USA, Hap Waldrop.
- MGB Workshop Manual.
- Moss Motors, Galeta, CA, USA, Kelvin Dodd.
- The MGA With An Attitude, MGAguru.com, Barney Gaylord.
- Supplemental Information for 297-108 VALVE STEM SEAL SET (8) MG TC, TD, TF, MGA, MGB, Michael Grant, Moss Motors.
- University Motors, Ada, Michigan, USA, John Twist.
- Valve Stem Seals Materials and Designs, By Larry Carley http://www.enginebuildermag.com/Article/2522/valve_stem_seals_materialsdesigns.aspx 7/1/1998