# Facing a 73mm BB shell down to 68mm?



## bad mechanic (Jun 21, 2006)

Due to a compatibility issue with a component I want to run, I'm considering the possibility of using a bottom bracket facing tool to cut my current 73mm shell down to 68mm. Of course the material would be taken evenly off both sides.

In measuring it out, it would just start to shave the edge of the chainstay weld's bead on each side. Now granted, the weld bead itself it pretty wide, but still.

Has anyone done this, or give me some good information regarding this?

I'll see if I can throw up a picture of it this evening.


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## bad mechanic (Jun 21, 2006)

Bump.


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## themanmonkey (Nov 1, 2005)

If we knew what the compatibility issue was there might be a work around that is a better option. Can you trim your BB down to 68? Sure. You might have issues with the chainline or ring and chainstay interface problems to deal with too.

If I was to do it, I'd trim off as much as possible from the non-drive side and as little from the drive side as possible. Don't fret about alignment as that had nothing really to do with the spacing of the BB shell. As long as your wheels, HT, and ST are perpendicular to the axis of the BB you're good.


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## bad mechanic (Jun 21, 2006)

It's for a EBB for a conventional BB shell. It needs a 68mm shell as to accommodate it's extra width. Since I plan on using an Hollowtech II crankset, which are all 73mm wide anyway and use spacers for a 68mm shell, I should be ok so far as chain line and spacing. Since the EBB takes up the extra space, I shouldn't have any issues with it interfering with the chainstays either.My main concern is effecting the integrity of the welds.


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## Linnaeus (May 17, 2009)

Cut away, you're not going to do any harm by trimming the edge of those welds.


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## NorseRider (Feb 9, 2004)

I've trimmed down a few 73mm shells (on some very high quality frames) for use with the super-duper-light Extralite cranks (that require a 68mm shell) without any issues.

Truls


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## Brujo (Jan 21, 2010)

i did this on my trek 6500, the bb shell stripped and i couldnt find a threadless bb in 73 mm.

i file mine down with a set of hand files, just running a sharpy over thhe area to file and filling evenly taking teh ink off once all was filled away i checked the surface with a level make sure sure my mesurement was correct and ink it up again an file. i also had to file a 45° chanfer into the bb shell to get the bb to grip properly.

my Recomendation is to take of 2.5 mm from each side to properly center the bb, getting the correct spindle length is the key to having the proper alingment of the crank and cassette, so if the bb is 73x113, then the new bb should be 68x113, if you removed even amounts from both sides opf the bb shell the new bb will center itself and the spindle will be in the right place for the crank to align.

i also ended up shaving some of the weld away, but the frame held up on a few rough trails and a a couple 2 foot drops, and im a big guy at 230# and 6 feet


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## scooter2468 (Aug 5, 2008)

themanmonkey said:


> If we knew what the compatibility issue was there might be a work around that is a better option. Can you trim your BB down to 68? Sure. You might have issues with the chainline or ring and chainstay interface problems to deal with too.
> 
> If I was to do it, I'd trim off as much as possible from the non-drive side and as little from the drive side as possible. Don't fret about alignment as that had nothing really to do with the spacing of the BB shell. As long as your wheels, HT, and ST are perpendicular to the axis of the BB you're good.


I'm not sure what your reasoning for not cutting the drive side down might be, but I can tell you from personal experience I suffered hip and knee problems by having a bb 2mm out of alignment trying to correct a chainring clearance issue. I would absolutely, positively never suggest to someone that it's okay to have the cranks, and therefore the pedals, an unequal distance from the center of the bb shell. It may work mechanically, but bio-mechanically it's not a good idea.


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## themanmonkey (Nov 1, 2005)

scooter2468 said:


> I'm not sure what your reasoning for not cutting the drive side down might be, but I can tell you from personal experience I suffered hip and knee problems by having a bb 2mm out of alignment trying to correct a chainring clearance issue. I would absolutely, positively never suggest to someone that it's okay to have the cranks, and therefore the pedals, an unequal distance from the center of the bb shell. It may work mechanically, but bio-mechanically it's not a good idea.


You stated the reason to do it in your own post, clearance issues. Doesn't do the OP any good if he trims the BB shell and then the arms or rings dig into the chainstay.


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## scooter2468 (Aug 5, 2008)

themanmonkey said:


> You stated the reason to do it in your own post, clearance issues. Doesn't do the OP any good if he trims the BB shell and then the arms or rings dig into the chainstay.


Yes, but he may not have the same problems. When I did it, I was trying to get away w/ running a 46t inner ring on a road bike with a bb spindle that was already 5mm too short. The OP probably has way more than enough clearance for mtb crank/inner ring unless his drive-side chainstay is massively curved for tire clearance.


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## themanmonkey (Nov 1, 2005)

scooter2468 said:


> Yes, but he may not have the same problems. When I did it, I was trying to get away w/ running a 46t inner ring on a road bike with a bb spindle that was already 5mm too short. The OP probably has way more than enough clearance for mtb crank/inner ring unless his drive-side chainstay is massively curved for tire clearance.


*scooter2468* I've fit a hole bunch of folks over the years and have never seen the problem you experienced caused my a drive-side being off my a couple mm and are using as justification. I'm not saying that wasn't your problem, but 2 mm difference in "centering" should have been helped with a cleat movement or BB spindle change. I've seen similar problems to yours with really wide "q-factors." In general those problems can be alleviated with stretching of the ITB. Just because you had a problem doesn't mean that the OP will have the same problem.

Bikes have less and less clearance with wider BB shells because the stays are moved out from what they would be with a 68 shell. Add this to the need for lower "q-factors" and you have lots of interface issues. Add to that is that you will also encounter chain-line issues, though 2-3 mm really shouldn't make much of a difference in reality.


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## scooter2468 (Aug 5, 2008)

themanmonkey said:


> *scooter2468* I've fit a hole bunch of folks over the years and have never seen the problem you experienced caused my a drive-side being off my a couple mm and are using as justification. I'm not saying that wasn't your problem, but 2 mm difference in "centering" should have been helped with a cleat movement or BB spindle change. I've seen similar problems to yours with really wide "q-factors." In general those problems can be alleviated with stretching of the ITB. Just because you had a problem doesn't mean that the OP will have the same problem.


You're right, the OP may not have the same problem. But, he might. In my experience I definitely had the problem. It started shortly after adding a spacer to the drive side bb cup and went away shortly after taking the spacer back out. Draw your own conclusions. Keep in mind, we're not talking about just moving the drive side out by 2.5mm, we're also talking about moving the non-drive side in by 2.5mm, and while all of this could perhaps been accounted for by moving my cleats around, you run the risk of foot related issues by having the pedal in a less than optimal spot under the foot. My bottom line, I want everything where it's supposed to be, not a few millimeters away.


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## bad mechanic (Jun 21, 2006)

Since I will only be using Hollowtech II cranks, I can face both sides down by 2.5mm and not have to worry about interference, as all Hollowtech II mountain cranks run at 73mm width.

Thank you everyone for the replies thus far, it looks like the unanimous consensus is to cut away without worry. The facing tool is already on the way!


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## djr8505 (Apr 23, 2008)

I hope it goes well. On a side note, do you think it would be possible to cut a head tube slightly shorter?

Don


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## Smokebikes (Feb 2, 2008)

From the pic it looks like you will not affect the weld on the CS.......the only real concern I'd have is for the tool that I was using.........your cutting quite a bit of meat off the BB and so you will certainly have an affect on the tool. If your borrowing it you may have to send it in to be resharpened before giving it back.


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## bad mechanic (Jun 21, 2006)

Smokebikes said:


> From the pic it looks like you will not affect the weld on the CS.......the only real concern I'd have is for the tool that I was using.........your cutting quite a bit of meat off the BB and so you will certainly have an affect on the tool. If your borrowing it you may have to send it in to be resharpened before giving it back.


It's my own tool. Any reason I couldn't use a dremel to get it close, and then mill it flat using the facing tool?

EDIT: I just asked Park Tool about it, and they suggested using a belt sander to get the shell close, and then finish it with the facing tool. They also suggested a guy who could resharpen the facing tool for $20.


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## smudge (Jan 12, 2004)

If you were to ask the builder about facing into the weld, my bet is that the official answer would be "don't do it"

All I can do is provide anecdotal experience. I did it on an Ellsworth Specialist and it was fine for the nine years I owned it.


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## bad mechanic (Jun 21, 2006)

It's a '99 Schwinn Homegrown, so no worries about the builder. I just want to make sure the shell won't drop off the frame, the chainstays won't fall off, and the shell won't ovalize.


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## Linnaeus (May 17, 2009)

It's an Al frame. Any good tool steel will cut that for a _long_ time before becoming dull. I wouldn't be afraid to just use a cutter to take all the material off. Or if you have a mill just bolt it down and face it off to start.


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