Bottom Bracket Shell Modification https://raleightwenty.webs.com/apps/photos/ Bottom Bracket Shell Modification The bottom bracket shell before being machined, faced, and re-tapped to ISO specifications. https://raleightwenty.webs.com/apps/photos/photo?photoID=137983363 137983363 A detail from the bottom. Note the lack of a kickstand boss (it was mangled beyond repair). You can see the slight offset of the shell to the left of the picture (the drive side). https://raleightwenty.webs.com/apps/photos/photo?photoID=137983364 137983364 The original width was about 77.4mm, far too wide for a 73mm BB not to mention a 68mm one. I need more options than just the stock BB (mine was pitted beyond use anyway). https://raleightwenty.webs.com/apps/photos/photo?photoID=137983365 137983365 A simple setup to measure the BB shell offset from the seat tube. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983358 137983358 Holding a frame securely without damaging the tubes is often a challenge in framebuilding. I make and sell these tubing blocks in a range of sizes (www.alexmeade.com/Tools.htm). The Raleigh Twenty has a seat tube diameter of 1.294", not a standard diameter. This block is made specially for holding the Raleigh Twenty frame. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983353 137983353 Once the frame is secured, metalworking can begin. Of course the BB shell could be shortened with a hacksaw, but a milling machine is a tool for accurate removal of metal. Here the frame is clamped in a tubing block and bolted down to the milling machine table. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983359 137983359 Checking to make sure the frame is square to the milling machine table. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983350 137983350 Removing extra length from the BB shell. The amount removed is intentionally less than the final amount to allow fine-tuning with the BB facing tool. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983362 137983362 Still clamped in the tubing block, the frame is now held in a bench vise for re-tapping. Here the taps are just started. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983360 137983360 Tapping requires cutting fluid, and lots of it! Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983356 137983356 Here the taps are almost all the way in. At this point, the taps are past the original thread depth and are cutting new threads into the BB shell. Lots of torque is needed to turn the taps, so a good vise and a solid tubing block are essential. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983354 137983354 Taps are to the final depth. They are left in place to guide the facing tool. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983361 137983361 The facing tool is used to remove small amounts of metal from both sided of the shell. At this point, careful measuring is required to get the BB shell centered on the frame while also achieving a 68mm width. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983355 137983355 Done with the major work. All that remains is deburring and final cleanup of the inside of the BB shell. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983351 137983351 Pretty much dead on 68mm. Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983357 137983357 A Shimano BB UN-52 threads right in by hand. Perfect ! Photo and caption by Alex Meade: http://www.alexmeade.com/ https://raleightwenty.webs.com/apps/photos/photo?photoID=137983352 137983352