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I have a bike with worned thread on the BB bracket shell. I talked to a mechanic, and he said that is difficult to repair the shell. The main problem is that he and most bike mechanic here has no tap to do the job. Actually, i tried to google m35 tap but found little result; and actually I have no idea what kind of material type to do the job. And the drive side and the other side use differwnt orientaion tap; right handed on one side and left handed on the other. It seems there are some welding shop that may add some material to the shell for retapping but little to no workshop that have the right tap.

I am wodering, what if I add material to the shell face and run a drill with the correct size diameter to drill a press fit size shell. That way I wouldn't have re tap the shell. Simple change from threaded to press fit BB bracket.

Any thought?

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  • Find a proper bikeshop, really. Around here one can even buy a semi-pro tap for less than 100 euros. The profi one costs less than 2x as much. That is peanuts for bike shops even if they need to be resharpened very often. kovys.cz/k/rucni-naradi/zavitniky-zavitova-ocka/… (and it is built locally, not imported from China) Actually, I would not want any frame from a bikeshop that does not have a tap and a facing tool. Nov 26, 2021 at 15:39

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It is impossible to convert a threaded BB shell to one of the common press fit standards, e.g. PF30, BB86. I assume you have an English (aka BSA) threaded BB with a 33.6-33.9mm diameter. All the press fit standards have larger diameters for the bearing seats. Many also require wider BB shells than the BSA (which is 68mm for road bikes). Even if you could machine the BB correctly, you would be affecting its structural integrity.

I would keep searching for a bike store that has the correct taps for your shell. I don't know where you are. A shop with the taps should be possible to find. If you come across a metal framebuilder, they are also likely to have the correct taps.

There is one edge case where someone might be able to convert between press-fit and threaded. If you have a metal frame with a PF30 shell and you have the correct taps, you should be able to cut threads for a T47 threaded BB. Few metal frames have PF30, however. Additionally, my understanding is that cutting threads in titanium frames may be challenging, and not every shop might have the correct tool for that. Also, Mavic appears to have made BBs designed for damaged BSA shells. You would ream the threads and then press in the BB. However, those are vintage items, and if you found one, you’d be stuck with whatever the square taper spindle length was. Mavic may not have been the only manufacturer of such items. Also note Nathan’s comment that some non-standard (or maybe single-manufacturer standard) press fit systems may have existed.

Additionally, if the threads in an English BB are truly damaged beyond repair, I believe it is possible to re-thread the BB to Italian. You would then use an Italian BB and add 1mm spacers on each side. Italian is a rare standard, but surprisingly many high-performance BBs are offered in Italian thread due to Italy’s long ties to the sport. However, this operation would require the right cutting tools.

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There have been various pressfit shell standards (Singer, Klein, Fisher, others) that are dimensioned similiar to an ISO shell and where convesion might be possible in some sense, but they all involve special shouldered BB spindles and the whole idea takes far more expensive and elaborate tooling than the BB taps you're in need of to begin with.

There are threadless square taper wedge-fit cartridge bottom bracket units that exist to be a cheap and simple solution in situations like this. They have their own caveats, mostly that they sometimes have problems with creaking. Shops can get them.

What could in theory be done with a torch is take some old cups, file or machine tap-like grooves in them for chips, and then heat and quench them to harden. Depending on the severity of damage, that could give you a tap that does its thing well enough.

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  • Your comment to me jogged my memory of a type of BB I heard of that’s designed for damaged BSA shells where you ream the threads and press the BB in. Mavic seems to have made this, although there may be others. That said, the OP would be limited to square taper cranks, and I don’t know what taper lengths these were made in. bikerecyclery.com/…
    – Weiwen Ng
    Nov 25, 2021 at 18:12
  • The Mavic one had you cut a 45 degree countersink into the shell, and then the cartridge tightened up against that. Not a bad concept but they're vintage parts now. Nov 25, 2021 at 20:11
  • Found this recently: Box says "M-Wave Repair Bottom Bracket," here's an example link that I hope works in the future as well: messingschlager.com/en/products/bottom-brackets_t55/… Different axle lengths are available, it seems new enough that it might even be in stock. I don't know how it wedges itself against the shell threads, but you can see in the pictures that the cups are not threaded into the shell anyway, but into each other. This might be workable, though I personally wouldn't try getting any KOMs with it.
    – jayded-bee
    Aug 18, 2022 at 14:18

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