Lapping in valves

[snowman]

Originally posted by Mark McClure
I have been under the impression that lapping in valves is a way of reseating them in the seat. The more you lap the lower the valve is seated and the more the face is matched.

......

The valve seat, in the case of a 3 angle grind is say 0.050" wide, the valve face is approx 2 to 3 times as wide, eg 0.150" wide. Consequently there will be a ring of contact on the valve face that is 0.050" wide, usually near the outter third of the valve face. If the valves are lapped properly they will define a ring or groove that does not have any ridges inside of the contact area, but there will be a sharp step both on the top and bottom of the contact area. They will seal perfactly untill the temperature changes and like I stated before the vavle gets longer, the seat larger in diameter and you now have the contact surfaces sitting on top of one of these ridges. Valve manufacturers send out info on this to machine shops that include a good picture of what is happening

[snowman]

The followin has been lifted from a repair manuel for a very late model engine. The info is typical of the entire industry for late model engines. Black, blue, green, even pink felt tip markers can be used instead of blue dye.

[CharlesJones]

Thanks for posting this Jack - it all sounds like useful stuff, but why does it say that you can lap refaced valves but should never lap new valves "under any condition"? Is there something about the surface of new valves which would damage the seat if you tried to lap them?

[snowman]

What I have read is that there is some kind of special coating on the face of new valves. Other thing I have heard is that new valves are so perfact and concentric that the only thing you can do to them is to make them worse.

[Mark McClure]

Jack,

That all make sence now ........ I was missing the bit about the ridge above and below the contact area after AGRESSIVE lapping.

I am just about to tear into my engine and in a cost saving effort my first though was to lap the valves to make sure I had a good seal. I now think that I should have them cut. OK the price of the rebuild has just gone up.

Thanks for the good information!...........Cheers

[snowman]

An additonal note for those taking notes. THe specs for the angles, margins, and seat widths given are for valves that are approx 1" dia to 1.3" dia and for a particular engine. Do not use these specs for your engine, use your manufacturers specs for your year and model.