I was recently asked if I would be able to make a bike with an oversize bottom bracket and full internal cable routing, amongst other things. Although this project may come to nothing it has been on my mind for some time to use a T47 bottom bracket, my only personal reason for this would be that it gives the space to potentially run cables and cable guides right through. I have never been keen on partial internal gear cable routing which always looks untidy, and I have never been happy with the idea of drilling holes in chain stays.
I would choose T47 over press fit, do I need to say why, though I did make a press fit bottom bracket shell for a balance bike and it was OK as far as I am aware.
Ceeway do stock PF30 press fit bottom bracket shells, which appear to be the same dimensions as T47, so I presume it would be feasible to cut threads in a PF30 to make a T47. Paragon in the USA make both Stainless and Plain steel T47′s and Bear frame supplies do plain steel ones but both were out of stock at the time I looked. The upshot of this is I decided to make my own, how difficult could this be, well a lot more difficult than anticipated, but I got there.
T47 dimensions are a 46mm internal bore, a 47mm diameter metric thread with a 1.0mm pitch; devised, I believe, by Chris King.
I bought enough steel tubing for a couple of plain steel and stainless steel shells. Selection of raw materials is limited to availability from the suppliers who are prepared to sell small quantities. Paragon use 17/4 stainless steel, which is also used by manufacturers Reynolds and Columbus for their stainless bottom bracket shells and possibly head tubes. This isn’t readily available so I had to settle for 304 stainless steel welded tube 50.8mm o.d. with a 3mm wall. This costs less than £3 for material for one shell, plus postage. For the plain steel shells I chose CDS steel tube (stands for cold drawn seamless) 50.8 o.d. with a 6.4mm wall. It is a form of mild steel. Due to the weight of metal you are buying the stainless is half the price of the plain steel.
I commenced by turning down the outside of the plain steel tube just far enough to clean the surface, and left the stainless tube outer surface as it was. I cut and faced the tubes to slightly over 68mm and bored out the internal diameters on the lathe to 46mm then put a relief channel in at 15mm to allow to run in a thread I had purchased a surprisingly cheap T47 bottom bracket for test purposes (thank you Merlin Cycles), which has a thread length of 12mm, which I presume must be standard. I then used a single point threading tool to cut the threads, Right and Left, to the 47mm guided by test fitting the previously mentioned bottom bracket cups. I ended up scrapping one of the plain steel shells because the first threading wasn’t up to standard. Of course the threads are cut individually so it is necessary to remove and reverse the shell in the lathe leading to a potential for inaccuracy in alignment. I therefore turned a dummy axle to slide through the inserted cups to ensure the bottom bracket was properly aligned.
I also mirror polished the stainless shells. This process only reduces the outside diameter from 50.8 to 50.75mm.
The lathe is a relatively quick way to sand down the shells for mirror polishing with emery cloth strips but care needs to be taken.
Another aspect to using a different standard for a component is the extra tooling required, and this is what drives me to make as much of my own as is feasible. A future issue is going to be whether I will need to purchase, or get access to, T47 thread chasers and facers. These seem to be disproportionally more expensive than the usual British standard thread cutters. I needed to make some adapters so I can fit the larger bracket shell into my frame building jig. These simply take the form of larger washers to go around the existing bosses. I thought I would try a makeshift heatsink arrangement at the same time so have made the adapters to loosely screw into the shell, rather than just push fit, in the (perhaps vain) hope that they may preserve the threads so no further thread fettling will be required. I used the remaining bit of bronze to fashion a sprung heat sink to slip into the middle of the bottom bracket shell to complete the job. All the commercially available bottom bracket heat sinks I have seen completely occlude the bottom bracket so are not suitable if you wish to do any brazing in the jig as obviously it is held in position by a bolt going through.
All I need now is a project using a T47 bottom bracket.