The hinge holds the upper cowling to the firewall. To remove the cowling, you pull the hinge pin out and the cowling can then be removed.
I have struggled with this seemingly easy step for a couple weeks. The instructions appear to have an error in them. The index holes that they call for do not line up with the holes in the firewall flanges and the top skin.
I also was concerned that the holes in the hinge that they call for in the instructions will be too close to the edge of the hinge. I have seen two different tolerances for edge distance: 2 diameters and 1.5 diameters (from the center of the hole to the edge). In this case, that would mean an edge distance of 3/8″ or 9/64″. My first attempt was a little shy of that smaller distance. Perhaps a more experienced builder would have ignored this and continued on but it bugged me.
While doing research on this topic I found Stu’s Aircraft Factory on Youtube. He built an RV-8. He has lots of interesting building tips. He mentioned in one video that the piano hinge supplied by Van’s is MS20257-3 and is susceptible to having broken knuckles (eyelets) over time. He recommended substituting MS20001P-3 extruded aluminum hinge. About 4x more expensive, but I decided to go with it since I had several broken knuckles on the RV-8 and RV-10 that I had owned.
I am confident in saying he was right and the added expense was worth it. Look at the difference between the two parts in the following pictures.
I spent about an hour putting the hinge on and match drilling it. I am happy with the result so far.