Valve sealing surface: Machined vs. lapped
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I'm installing some used LS6 lightweight valves in my heads. They have had the stock 45 degree sealing surfaces re-cut. The heads have fairly low mileage on them. I checked the sealing surface on one of the valves/seats with Prussian blue dye, and the sealing surface was concentric. In this case, is there any reason to lap the valves? I lapped one already, and the finish of the valve sealing surface is rougher than that of the smooth newly machined valves. I checked the sealing surface with prussian blue once again after lapping, and the footprint of the seal looked the same as before lapping. Does the smooth fresh cut valve seal better or worse than the rougher lapped surface?
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Sometimes a hand lapped valve will seat better. I have used the blue dye on the surface on a set of ported heads and they seemed fine, but had a wise old man tell me that I should do a hand lap to ensure they were right. It's probably 6 of 1 half dozen another, but I did it anyway. Never had an issue. I have however wondered which surface would be more desireable to have, rough or smooth. There is always the people that say the turbulent air that a rough surface gives you is better, then you have the smooth people.
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The only real way to know if the valve seat has good runout is to use a runout gauge.
Now a good head guy can tell from experience from sound and feel. But you can have a valve lap In that does not have perfect runout you also have to consider the included runout of both the seat and the valve. But from my experience the best way to ensure a concentric seat is to cut the valve job then stone in the seat angle with a valve seat grinder which then seat angle may be a little wider than you want it so you go back in with a stone for the top cut and grind the top cut a little to narrow the seat angle back to the width it was when you machined it. Then you blend in the valve job I blend my seats in where they are eccentric (not the seat angle but in the venturi) then after regrinding all the valves I lap in all valve seats not only to check concentricity of the valve and seat but to see where it laps in on the valve. That is how I do a valve job the same way most real proffesional cylinder head porters will. It takes time but it is worth it.
Now a good head guy can tell from experience from sound and feel. But you can have a valve lap In that does not have perfect runout you also have to consider the included runout of both the seat and the valve. But from my experience the best way to ensure a concentric seat is to cut the valve job then stone in the seat angle with a valve seat grinder which then seat angle may be a little wider than you want it so you go back in with a stone for the top cut and grind the top cut a little to narrow the seat angle back to the width it was when you machined it. Then you blend in the valve job I blend my seats in where they are eccentric (not the seat angle but in the venturi) then after regrinding all the valves I lap in all valve seats not only to check concentricity of the valve and seat but to see where it laps in on the valve. That is how I do a valve job the same way most real proffesional cylinder head porters will. It takes time but it is worth it.
Last edited by matt d.; 06-14-2010 at 01:07 AM.
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And are we talking about a big difference in pressure leakage with a valve job that looks good with a blue-check but does not have perfect runout, and one that does have perfect runout? Would we be able to see a hp/efficiency difference between a good/okay seal and an absolute perfect seal?
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Is it not accurate enough to use the prussian blue dye to check if the seal is good?
And are we talking about a big difference in pressure leakage with a valve job that looks good with a blue-check but does not have perfect runout, and one that does have perfect runout? Would we be able to see a hp/efficiency difference between a good/okay seal and an absolute perfect seal?
And are we talking about a big difference in pressure leakage with a valve job that looks good with a blue-check but does not have perfect runout, and one that does have perfect runout? Would we be able to see a hp/efficiency difference between a good/okay seal and an absolute perfect seal?
Last edited by matt d.; 06-14-2010 at 11:31 AM.