14 replies to this topic

[Bounce]

Ive just picked up my head gasket set from my local motor factors. Im a bit confused as to why there are only 4 stem seals?


I asked the guy in the shop and he said that the exhaust valves have O-rings instead of stem seals. But there are no O-rings in the kit??????

[dklawson]

The o-rings were used on the very early small bore engine valve stems. Later and large bore engines use the valve stem seals. They are only used on the intakes, that's why you've got 4 instead of 8.

The argument is made that they prevent the vacuum in the intake manifold from sucking oil down the stem and burning it. The exhaust valves also appear to benefit from the little bit of oil that runs down their stems.

[Bounce]

Ok. Ive just seen this on minispares

And this is what i have. I did ask for an SPI one.

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[miniboo]

normal practice is to use only 4 on th eintakes but putting them on all 8 is thought to be better

[Guess-Works.com]

I wouldn't basically for the reason DK has says, when put on the exhaust they corrode with the heat, and then you get bits of valve seal floating around in your oil.

[dklawson]

Putting seals on all eight may not be advisable if the head has just been rebuilt with new guides. Give it a little while to break in before putting seals on the exhaust valves.

BMC/BL/Rover (like other companies) did things the most cost effective way... but they didn't leave the 4 seals off the exhaust by accident or to save about $1. If it meant risking a warranty engine rebuild/replacement the exhaust seals would be there from the factory.

[Bounce]

okie dokie. cheers guys.

[Bass Man]

Guessworks is spot on here (again) rather a contradiction in his handle I think!!

rubber seals on the exhausts valves would degrade very rapidly, this would probably cause the valves to get sticky and slow to return which will in turn cause loss of compression and eventually failure to fire.

Guess what the previous owner had done to the cylinder head on my car!!!!!

[Jake Blues]

This raises another question then.....what is the temperature difference between the valve stem of an exhaust valve compared to an inlet at full working temperature? Surely it can't be that much?

[dklawson]

The difference in valve stem temperatures is significant. The inlet valves are cooled by the incoming, unburned charge. The exhaust valves open while the charge is ending its burn so they are exposed to very hot gasses. The valve head transfers this heat up the stem. Years ago I remember there was a fair amount of research being done with sodium filled valves to improve the heat transfer away from the valve head and up the stems with the intent of dumping the heat to the guides and cylinder head. By the nature of the material, stainless steel alloys conduct heat "poorer" than regular grades of steel, but even stainless exhaust valves will run hotter than similar intakes.

[Guess-Works.com]

Time to get one of them infra red thermometer things on the head after a little run in the motor....

Anyone got one ?

[Jake Blues]

The difference in valve stem temperatures is significant. The inlet valves are cooled by the incoming, unburned charge. The exhaust valves open while the charge is ending its burn so they are exposed to very hot gasses. The valve head transfers this heat up the stem.

Yeah, I know all that but this has got me thinking...given the speed in which things are happening in the head and the heat being spread all around etc what would the difference be? I mean, you wouldn't be able to touch one would you? So, in relation to the question of why will the stem seals melt on the exhaust and not the inlet, why aren't they made to withstand the extra heat on the exhaust?

[dklawson]

Sorry in not replying sooner.

Keep in mind the purpose of the seals is to keep oil from being drawn off the head. Intake valves seals prevent manifold (port) vacuum from drawing oil down along the relatively cool exhaust valves, into the chambers, and burning. By contrast, the exhaust valve are in an environment where the ports are pressurized (generally). You aren't likely to draw oil down along them and if you did, it would be oil blown out partially burnt in the exhaust. As I mentioned earlier, the exhaust valves have a more demanding environment and need a little bit of lube anyway. So, particularly when the guides are new and relatively tight, you don't necessarily want the seals. They are preventing a limited amount of beneficial oil from oozing down the guides.

If you insist on going with exhaust valve stem seals, there are certain seals for other cars that are made of Viton instead of the nitrile rubber used on most intake seals. The Viton seals can handle the higher temperatures. Sorry, I can't tell you which cars use these seals. There's a guy in the U.S. (Todd Miller, who is a guru in the MG-1100/Austin America world) who has found these Viton seals and was selling them to anyone who was interested in them. Google "Todd Miller" and "ADO16" and you'll find his web site. He can either sell them to you or probably direct you to an alternate source.

[Bass Man]

(Todd Miller, who is a guru in the MG-1100/Austin America world) who has found these Viton seals and was selling them to anyone who was interested in them. Google "Todd Miller" and "ADO16" and you'll find his web site. He can either sell them to you or probably direct you to an alternate source.

WOW!! Is this place a mine of information or what, you kids don't know you're born, when I had my first car Haynes was all you got, if you were lucky!!

Keep up the good work!!

Edited by Bass Man, 12 February 2007 - 03:10 PM.

[dklawson]

Rather than let this thread die a peaceful death....

Please see this related thread which surfaced just yesterday:
http://www.theminifo...x...f=6&t=41442