[blykins]

On a lot of the Comp Cams stuff that I use, you'll see flat tappet cams with lash specs between .020-.026". On the roller stuff, you'll see .016-.018" on a lot of lobes. Then of course you have a little creative license either way on that.

Lash grows when hot, so if you open valve lash way beyond the lash ramp design, you take a chance of bypassing that ramp, which would allow a lot of "hammering" to go on with your valvetrain, instead of having a "gentle" open and close.

Conversely, setting it too tight can cause issues with hanging the valves open when cold.