Press-fit bottom brackets behave the way they're supposed to only when they're installed straight. A bearing pressed in cocked, even by a degree or two, runs with side-load on the bearing race; the bearing rolls rough out of the box, develops a flat spot on the load side within a few hundred miles, and starts the creak-knock-replace cycle that gives press-fit BBs their reputation. The 1721BB is the kit that takes the geometry guess out of the install. The press-rod centres the drift to the BB shell; the drifts are sized to the bearing's outer race so the press load lands on the race surface, not on the seal or the cage. The result is a square install on the first try, every time, without the part-of-bearing damage that makes press-fit BBs unforgiving. How it's used Identify the BB standard in the frame (BB30, PF30, BB86, BB92, etc.) and select the matching drift from the kit. The kit ships with drifts sized to the most common press-fit standards, packaged in a tray that shows immediately whether anything is missing. Thread the press rod through the BB shell from the non-drive side. Place the appropriate drift over the bearing on each side. Tighten the press rod to draw the drifts toward each other; the bearings press into the BB shell at matched depth, both sides simultaneously. The simultaneous double-side install is what keeps the bearings square to each other; pressing one side at a time and then the other lets the second bearing pull misaligned to the first. What stops the press is mechanical, not a published depth figure. On direct-seat BB30-family shells the bearing lands against a machined step or a retaining clip; fit the clips before the bearings, because there is no fitting them afterwards. On PF30 and BB86 / BB92 the cup flange lands against the shell face. Press until you meet resistance and the part is fully seated against its stop. Compatibility BB30 (30 mm spindle, 42 mm shell) and PF30 / PF30A (30 mm spindle, 46 mm shell) BB86 / BB92 (press-fit 24 mm Shimano-pattern spindle) Most other current press-fit BB standards using common drift sizes Not for threaded BBs (BSA, BSA30 threaded, T47, Italian); for those use the appropriate notched-cup wrench like the 16 Notch External BB Wrench (1609/2BI-US) Specs Press rod and drifts: premium flex plus carbon steel Surface finish: chrome-plated Drift sizing: common press-fit BB standards (BB30, PF30, BB86, BB92) Includes: press rod, drift set sized to common standards, packaging case for organised storage Built in Zreče Unior has been forging hand tools in Zreče since 1919, and sponsors professional road and mountain-bike race teams. The 1721BB is the kit that takes a press-fit BB install from a guess to a procedure; the simultaneous double-side press is what separates a bearing that lasts the design life from one that develops a flat spot in the first month. Pro tip from our mechanics A creak from a press-fit BB isn't always the bearing. Before pressing in a fresh BB, clean the BB shell threads (or shell bore, on press-fit) thoroughly and inspect the bore for ovality with calipers. A worn or oval shell will reproduce the creak on the new BB regardless of how square the press is. If the bore is worn past spec, the next step is a frame-warranty conversation with the manufacturer rather than another BB install. For the diagnostic order on BB-area noise (what to rule out before assuming the BB is the source), see Creaky bottom bracket? Check these first →.
