Hairline is a superb crack on the right side of Elephant Rock, one of the very best in the Valley. The name Hairline is an odd choice of a name for a crack that is never smaller than .75", and most of the pitch is glorious 5.8 hand jams. The approach is somewhat involved but not really that difficult if you're familiar with the layout of Elephant Rock. This just serves to keep the crowds away and judging from the lichen on this thing, it gets climbed on a yearly basis at most?!
This climb can be approached from above or below. From below requires an arduous slog up a hillside to the base and then a death-defying step across (or maybe a leap) from a tree branch to gain a small ledge at the base of the crack.
The more sane approach is made from above and i think is best done with one 80m rope for fixing and a 60m rope for leading back up. First you rappel down Killer Pillar and pull your ropes. Then navigate down a steep brush filled gully. There is one last big tree near the bottom of the gully before a fork. The left fork (towards the main wall) leads to the top of Crack of Deliverance, an ugly 5.9 gully climb. The right fork (on the other side of the intruding ridge) leads down to Moongerms and Hairline. I think if you fix a 80m rope at the last big tree here you will just barely make the foot ledge at the bottom of the climb. I'm not positive though. If it's short I guess you can make a 2 piece anchor in the middle of the climb and then rap once more with the lead line. Or you could just bring two shorter ropes to fix, if you want to be on the safe side.
The climb starts with some 10c thin hands or maybe liebacking. I did this on a self-belayed TR and my 70m rope was a little short, so I didn't get to climb the first 20ft. But the crack quickly widens to hands and the going gets easy. This leads up to a big arching overhang above a slab. Lieback and jam up the crack, stopping to brush the lichen off of footholds as you go. Also extend your gear so you don't get too much rope drag. The crack splits the very overhanging headwall and this is the crux. With small hands and good chimney technique it's not too hard. Larger people will struggle with bad jams in the back of a shallow flared chimney. Once you're over the top you can cruise up a dirty slab to a tree on a ledge.
If you came in from below you will probably want to rappel back down from this tree (2 ropes plus webbing to leave on the tree). If you came in from above you should still have your fixed line leading up through some dirty class 4/5 stuff. Ascend the rope to get back to the bottom of the class 3 gully. Bushwhack back up to the base of the Killar Pillar and lead out on one of the routes. The easiest is The Hundredth Monkey, an 11b sport bolted route that's comparable in difficulty to Hairline, so that shouldn't be an issue.
Pro to 3", with lots of cams from .75" to 2".