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?

 

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.

 

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.

 

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?

 

even the lapping the valves in can lie to you even with a experienced steady hand. You would even need to take into consideration what your valve guide clearance is, a looser guide will allow the valve to move around some to find a seating surface and if had nice tight guide, the valve job may not even lap. So whether or not it is accurate enough would be just the opinion of the person doing it. I am only saying my own experience I have seen alot of valve Jobs that would not even lap in on brand new heads. The difference in pressure loss will depend on how much cylinder pressure your engine builds the more pressure you have the more loss you will have. So the more power you have the more you will lose. Not to mention if the valve and seats are not as concentric as you can get them you are losing alot of cooling, that is the number one place a valve dissipates heat from. The sealing of the valve is even more critical on the intake side, that can cause huge power loss. If you have just one slightly bent intake valve, from whatever reason you will know it from the power loss, a slightly bent exhaust can be much harder to see. Now just think if all valves are not bent and have .001" runnout both on the seat and valve for a total of .002" included runout which is acceptable tolerance for most and is better even than most factory Valve jobs times that by the number of valves. .002" included runout is not going to be good enough for a good cylinder head guy trying to make power they will want included runnout of around .001" even with carbide pilots bolting fixture down and so on it can be very hard to get .0005" runout on a machined seat with the valve seats as hard as they are these days there is still cutter deflection. A valve seat with .002" included runout will lap in fine with a looser guide like most factory guides are but if you have a nice tight guide it may not lap in so easy. Compared to the one with .001" included runout you will make more power and the more power you make the less you will lose. I'm not saying that just because you have 002" total runout you are going to have to make it better but it does help alot and will last longer. I'm only saying just because a seat lapped in doesn't mean it has the best concentricity.