At the head of the valley the road gives up on the open air and goes into the mountain. The Cristo Redentor tunnel runs under the pass, about three thousand two hundred metres up, and marks the boundary between Argentina and Chile. Just beside it, the old road goes over the top, a narrow, steep gravel route that was the original crossing, famous for the statue of Christ the Redeemer placed on the frontier in 1904 as a sign of peace between the two countries after a border dispute.
The border crossing is a serious business. Both customs and immigration are handled in a large complex on the Argentine side, and the queues can be long, especially on weekends and holidays. Bring passport, vehicle papers and any insurance documents, and allow a couple of hours at the worst times. Fuel is not sold here, so make sure your tank is adequate, and do not expect much more than a snack bar.
The weather at this height is harsh. Even in midsummer, temperatures can drop and a storm can blow in without warning, and in winter the pass is often closed by snow and avalanche risk. The authorities close it frequently, so check status before setting off; the mountain does not negotiate. If it is open, the ride up is a great experience, with the air thin, the light brilliant and the peaks all around. The surface is good, with a few rough patches near the tunnel.
Think about altitude as well. A fast climb from Mendoza can make you tired and give you a headache, and a bike may run richer and weaker in thin air. Take it slowly, drink water, and rest if you need to. If you go over the old road when it is open, you will find a rewarding detour for photographs, with the statue standing alone against the sky. On the Chilean side, the road drops through a dramatic series of hairpins, and the reward is a warm, green valley far below.