Left crankarm pinch bolts don't fail often, but when one strips or walks loose on a Hollowtech II crank, the whole non-drive arm becomes a problem — and finding the correct M6 x 15mm replacement for a Dura-Ace crank isn't something you want to guess at. This is Shimano's genuine service part for the FC-7800 left crank clamp, and it happens to cover a much wider swath of the Shimano catalog than the FC-7800 name suggests. Same bolt is listed as an "A" match — meaning identical materials, finish, and dimensions — for FC-7900, FC-7950, FC-7803, FC-9000, and the entire XTR M9000/M980/M985/M9020 series.

The FC-7800 was the first-generation Hollowtech II Dura-Ace crank, and the two pinch bolts on the non-drive arm are what hold the whole assembly together laterally. The center fixing cap preloads the bearings, but it's these two clamp bolts that keep the crankarm from creeping off the splined axle under pedaling load. If you've ever pulled a crank and found the bolts backed off a quarter turn, that's the failure mode — and it's why the correct torque and a properly seated hex key matter here more than on most fasteners.

Install torque is 12-14 Nm per Shimano's service instructions, and both bolts should be alternated up to final torque so the clamp closes evenly around the axle. Recheck after your first long ride. Use a 5mm hex and seat it fully before applying torque — these bolt heads are shallow and easy to round if the key isn't bottomed out. Sold individually; most installations need two per crank, so plan accordingly.