Lapping valves?

[RichardGoller]

So one of the valves is not sealing properly, 50psi lower compression than the rest of cylinders and like 40% leakdown. If I'm there I thought I might as well replace the valve, but my question is what would be the best way to do this? Obviously the very best would be to take it to a machine shop, fit new seat and cut it to angle but the all the head needs to be stripped I guess, something I don't want to do. If the seat seems alright is it ok to just do a light lapping on it with a fine lapping compound? Also will it have a better chance of sealing with the old or the new valve? Thanks

[DukeofAlexandria]

Originally Posted by RichardGoller

So one of the valves is not sealing properly, 50psi lower compression than the rest of cylinders and like 40% leakdown. If I'm there I thought I might as well replace the valve, but my question is what would be the best way to do this? Obviously the very best would be to take it to a machine shop, fit new seat and cut it to angle but the all the head needs to be stripped I guess, something I don't want to do. If the seat seems alright is it ok to just do a light lapping on it with a fine lapping compound? Also will it have a better chance of sealing with the old or the new valve? Thanks

I mean….you won't know which is best until you actually see it and look at the valve. If it's just some debris/carbon trash, lap it and see if that works.

If it's marred and something serious got into it, then that's a whole other issue and you'll need to not only machine shop/replace the valve, but also try and determine what got caught and where it came from.

Hopefully just a fluke and it's trash/carbon. How many miles?

-Duke

[tbuck]

First thing would seem to be to find out why your current valve failed (if it turns out to be a valve, as it might be other things). There are a number of things that can cause valve damage and, whatever that issue is, that needs to be resolved beforehand. The cause of the damage will also dictate the best course to take regarding your other questions. If the valve was damaged due to debris, for example, the seat will likely need to be replaced as well. Until you get the head off and find out what happened the remedy will not be certain.

[amrazM]

You have to ask yourself why the valve isnt sealing, if the valve is damaged chances are the seat is too.

[RichardGoller]

The head is off, I will start disassembling it in the following days, but what I do know for now is that oil seals it up enough to bring the low compression reading back, so in my head, it shouldn't be something dramatic