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MG Midget and Sprite Technical - Oil seal on exhaust valve

Apologies for raising this topic (OK it's in the Archive but I like to get to the point). I have just got my cylinderhead back after having new valve guides fitted (1330cc MG Metro). I notice that the engine builder has fitted valve seals on all 8 valves. My recollection is that seals on the exhaust are not only unnecessary but also undesirable. Am I correct - should I remove them from the exhaust valves?
Chris Hasluck

Chris, - good can of worms question!

The theory is that the exhaust guides are only open to the combustion chamber when the valves themselves are open. And this only occurs on the exhaust stroke, so the valve chamber into which the guide opens is at that moment being force fed with exhaust gases and is therefore at a positive pressure. And any leakage in the guide is upwards, preventing oil from leaking down the guide. So no additional seal is needed.

Conversely, the inlet valve is open when the piston is on its suction stroke and this suction is transmitted to the valve chamber and therefore drags oil down the inlet valve guide. So it needs an oil seal.

So that's the theory. However life isn't that simple, especially at higher revs and also if you have a non-standard cam shaft. These can mean that either valve overlap, or the suction caused as a pulse of exhaust travels down the exhaust manifold result in suction applied to the exhaust valve chamber, drawing oil down the guide. So some therefore advise adding seals to all the guides, not just the inlets. I do so, as my engine smoked badly after being rebuilt with new lead-free guides and new valves.

The argument against, is that the guides do need some lubrication, and adding seals to the exhausts restricts this too much.
Guy Weller

here are links to three seal makers advocating the use of their product on exhaust valves, and their reasons why (besides the doubling of their product sales):


http://aftermarket.federalmogul.com/en-US/Technical/Documents/Valve%20Stem%20Seal%20Facts.pdf

http://www.hastingsmfg.com/ServiceTips/valve_seals.htm

http://www.autozone.com/autozone/repairinfo/common/repairInfoMain.jsp?targetPage=productsHowToInspect&leftNavPage=productsHowTo&pageId=%090996b43f80a013bf&subtitle=inspect

By they way, they all say their "seals" do not seal, but meter the flow of oil, since all valves need some amount of oil for lubrication.

IIRC, there was some argument against using exhaust valve seals with cast iron valve guides, but most rebuilders only use silicon bronze guides these days.


http://www.minispares.com/article/Technical~Information/Brakes/Valve~guides~-~Materials~and~usage.aspx?22f56378-e9db-4f9e-824e-9ac72ea62b002e546e09-de73-403e-8c84-7e916e232b755753d7f4-beae-43e1-9aab-60272626c930

http://www.swiftune.com/Product/349/swiftune-bronze-valve-guide-set.aspx


Norm "lots-o-links" Kerr

Norm Kerr

Norm, thanks for the links. Unfortunately I could not get all of them to work (namely the aftermarket.federalmogol site; the autozone site and the minispares site). Anyway, where I did get through the message is clear enough - seals on exhaust! The guides just fitted are manganese bronze guides from Minispares.
Chris Hasluck

3 links wouldn't work for me either, Chris. But the "Hastings" one confirms my explanation of why the exhaust valves need seals as well. At least as regards "the suction caused as a pulse of exhaust gas" causing a valve chamber vacuum.
Guy Weller

The links work if you copy and paste them into the browser URL edit field. Not sure where the problem lies but the problem is caused by the embedded spaces (%20) and other embedded characters.
David Billington

This thread was discussed on 05/12/2013

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