Gen 4 – valve lash

Valve angle grinds and lapping isn’t a science. It’s a hands-on art form. The engine can be manufactured on the best machines, in a mechanically sterile environment, and something will always come out a little bit off. In the machinist world, it’s called “tolerance”. The machinist shoots for whatever the XXXX.XXXX in/mm spec, which is impossible to achieve (for say in the incidence of 16 individual valves). The engineering gives a spec for a (very) little bit of wiggle room.

In the case of a valve and lash adjustment, the valve and seat are made on a machine. They can be a mechanically perfect valve and perfect seat, yet they don’t seal. Valve gets lapped. The length of the valve stem changes in relation to seat depth. Some stems end up longer or shorter.

The lash gets adjusted by hand. Valve “x” may have .0023 too much lash. The follower cup comes off. Machinist adds a .0020 shim/ or two .0010 shims, and a three pack of .0001 shims (arbitrary numbers here). It gets put back together, and now it’s .0004 too tight. The oil on the shims took up too much space. It comes back apart again, smaller shims are added or omitted. It gets close, and within its engineering tolerance, and that’s it. Ship it.

Even something like an complete new engine manufactured on the best machines, in a mechanically sterile environment, getting a compression test. (Again, arbitrary numbers here), 1, 2, 3, 4 can test out to 165, 160, 150, 155 respectively. This doesn’t mean that 1 is “more gooder’er than the others, and 3 is bad”. They are within tolerances. Ship it.