Topic: How do valve seals seal?

[spammasher]

Hi i am in the process of replacing the valve stem seals ( i think they are called ) so i removed the springs and removed the old valve seal to put the new one in and thought to myself what is this thing sealing. By looking at it , the seal just slides over the valve and it doesn't go all the way to the bottom so i thought what is it sealing and how does it seal. I hope this makes sense. I am hoping this will help fix a blue smoke issue on start up, i have done a leak down test on each cylinder at TDC of course and most exhaust and intake valves are letting the pressurized air escape more than is acceptable.

[calj737]

they're designed to "scrape" the oil off the stem as it moves up and down through the seal. This prevents oil from being dragged down and through the guide and into the combustion chamber. You may also have worn guides thus allowing more movement of the stem within the guide, and increasing the movement of the stem through the seal. Its all connected and dependent...

[MCRider]

they're designed to "scrape" the oil off the stem as it moves up and down through the seal. This prevents oil from being dragged down and through the guide and into the combustion chamber. You may also have worn guides thus allowing more movement of the stem within the guide, and increasing the movement of the stem through the seal. Its all connected and dependent...

Right.

And what the original poster is describing, a failed leak down test, is an issue with the valve sealing against the head in the combustion chamber. As you Cal, describe, the valve stem seal does seal against oil being drawn into the combustion chamber and causing smoke.

The worn guides issue could also be causing the valve head to rattle on its seat, causing the failed leak down test.

[madmtnmotors]

Be sure to push the seal down so the lower part of the seal fits over the top of the valve guide. As mentioned, the seal prevents oil from leaking down between the valve stem and the inside of the guide. The guides are press fit into the head so there is little to no oil that would leak down between the outside of the guide body and the head.