# Facing a 73mm BB shell 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.

EDIT: After thinking about it, I've cross posted this in the frame building forum


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## Bikinfoolferlife (Feb 3, 2004)

To my way of thinking it'd be easier to get another frame...curious, though, do you own your own tool? can you sharpen your own? or you have extra cutting heads? or?


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## bad mechanic (Jun 21, 2006)

I do, yes. With a hardened steel cutting head going through aluminum using cutting oil I'm not too worried about dulling my tool. 

I don't want a different frame.


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## rockyuphill (Nov 28, 2004)

Are you trying to have the ability to use a e-type FD or a MRP chainguide with a Truvativ crank? You could just trim 2.5mm off of the drive side, you'd only need to trim the other side if you were going to sell it.


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## bad mechanic (Jun 21, 2006)

Nope, it's an eccentric bottom bracket for standard shells made by Forward Components.


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## PissedOffCil (Oct 18, 2007)

I'd be afraid of running out of threads for the BB. Apart from that I don't see any problem so I guess that if you have your own cutting tool you also have a threading tool if need arises...

Don't take my work for it though, frame builders would know better


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## bad mechanic (Jun 21, 2006)

There would be plenty of thread remaining. My main worry is it effecting the chainstay weld.

Unfortunately, no one in the frame building forum has answered.


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## PissedOffCil (Oct 18, 2007)

It looks there there is more than 2.5mm before the weld, no?


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## bad mechanic (Jun 21, 2006)

Nope. It'll get about 0.5mm into the weld on both sides.


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## mtnbiker72 (Jan 22, 2007)

I won't pretend to be an expert on frame building...BUT, I've looked at thousands of frames and seen many that the faced BB shell cut into the welds a little from the factory. My current frame has this as the downtube flares so wide that the BB facing cuts slightly into the welds on both sides (it's a 68mm wide BB).


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## LyNx (Oct 26, 2004)

Once the welds are good and you do a good job facing it, I don't see the issue. Do you really need to go to 68mmor could you not just stop before the weld? Is the frame expensive? If it's not a too expensive frame I wouldn't give it a 2nd thought. 

I'm thinking of doing this on my steel Karate Monkey frame, but on the non drive side only so I can have even spacing between the stays and both cranks running an external bearing without spacers.


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## PissedOffCil (Oct 18, 2007)

bad mechanic said:


> Nope. It'll get about 0.5mm into the weld on both sides.


Maybe marking where the 2.5mm mark lines up would help people estimate the risks...


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## alexkraemer (Jul 30, 2007)

I brought my Monocog into the bike shop yesterday to ask a very similar question - remove 2.5mm so I can space out my crankset and get better clearance (Monocog fixie conversion - chainline issue is a royal pain). The shop was really hesistant about taking that much material off, we figured out something else instead. This is on a steel frame where the welds weren't even close to the shell. Hope this helps!


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## Jim311 (Feb 7, 2006)

Isn't that going to totally screw up your chain line?


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## bad mechanic (Jun 21, 2006)

Jim311 said:


> Isn't that going to totally screw up your chain line?


Not for what I'm doing.

Also, if I ever wanted to go back to gears (HAH!), all Shimano external BB are made for a 73mm shell, and use spacers if it's a 68mm shell.


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## Jim311 (Feb 7, 2006)

Isn't the weld material actually much harder than the aluminum around it? It's going to be difficult to face once you get into the weld.


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## PissedOffCil (Oct 18, 2007)

Jim311 said:


> Isn't the weld material actually much harder than the aluminum around it? It's going to be difficult to face once you get into the weld.


I wouldn't be too concerned with this.


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## elake (Jun 24, 2006)

It's gonna take forever. Those facing tools aren't designed to take off a lot of material, i shudder to think how long it would take to remove 2.5 mm from each side, aside from tool wear. Probably better to get it milled off in a machine shop if you really must do it.


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## bad mechanic (Jun 21, 2006)

elake said:


> It's gonna take forever.


You'd be surprised. If I crank up the spring tension, it takes of quite a bit of material. The thing which slows the operation down is cleaning the chips off, and reapplying cutting paste.


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## grivooga (Mar 14, 2009)

elake said:


> It's gonna take forever. Those facing tools aren't designed to take off a lot of material, i shudder to think how long it would take to remove 2.5 mm from each side, aside from tool wear. Probably better to get it milled off in a machine shop if you really must do it.


elake is on the right path. The proper tool for this is an end mill. Most time consuming part would be figuring out how to properly fixture the frame to the table.

Facing tool would probably work though. If it was my tool and my frame I might try it but I wouldn't do it with someone elses tool or frame. Just not what it was designed for.

I wouldn't be concerned about nicking the weld beads. So long as you aren't cutting a lot out. Just file smooth any sharp edges left when you're done to help cut down on stress risers.


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## elake (Jun 24, 2006)

bad mechanic said:


> You'd be surprised. If I crank up the spring tension, it takes of quite a bit of material. The thing which slows the operation down is cleaning the chips off, and reapplying cutting paste.


Then by all means have at it and let us know how it goes.


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## alexkraemer (Jul 30, 2007)

bad mechanic said:


> Not for what I'm doing.
> 
> Also, if I ever wanted to go back to gears (HAH!), all Shimano external BB are made for a 73mm shell, and use spacers if it's a 68mm shell.


Incorrect. Shimano road components are designed for 68/70mm shells, not for 73mm. There was an article about this on Velonews where Leonard Zinn stated that road and mtn bb's were the same, and Shimano sent him an e-mail indicating that the road BBs are wider than the mtn BBs, and therefore not interchangeable. I also just installed some shimano road cranks on a 68mm shell, and there is no room for spacers.


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## bad mechanic (Jun 21, 2006)

alexkraemer said:


> Incorrect. Shimano road components are designed for 68/70mm shells, not for 73mm.


I'm aware road BB are narrower, but we're on a site called _Mountain_ Bike Review, talking about the BB shell of a _mountain_ bike frame. I didn't see a reason to qualify my statement.


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## rideonjon (May 27, 2009)

elake said:


> It's gonna take forever. Those facing tools aren't designed to take off a lot of material, i shudder to think how long it would take to remove 2.5 mm from each side, aside from tool wear. Probably better to get it milled off in a machine shop if you really must do it.


i've done this to a frame and it didn't take very long at all just use a generous amount of A-9 and clear the chips with an air gun as you go.and measure often you might be surprised how fast it removes material.
cheers,
jon


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## jtmartino (Jul 31, 2008)

Somewhat off topic, but is that a 1999 Schwinn Homegrown Pro frame?


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## bad mechanic (Jun 21, 2006)

jtmartino said:


> Somewhat off topic, but is that a 1999 Schwinn Homegrown Pro frame?


It is. Good eye; I'm impressed.


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## elake (Jun 24, 2006)

rideonjon said:


> i've done this to a frame and it didn't take very long at all just use a generous amount of A-9 and clear the chips with an air gun as you go.and measure often you might be surprised how fast it removes material.
> cheers,
> jon


I'm curious, can you give on estimate on how long "it didn't take very long at all" is?

Are we talking 20 minutes per side? 40 minutes? an hour? two hours?


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## PissedOffCil (Oct 18, 2007)

bad mechanic said:


> It is. Good eye; I'm impressed.


Impressive indeed!


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## rideonjon (May 27, 2009)

elake said:


> I'm curious, can you give on estimate on how long "it didn't take very long at all" is?
> 
> Are we talking 20 minutes per side? 40 minutes? an hour? two hours?


well i'll put it this way,if it takes you 2 hours to do one side ,you shouldn't be doing it.leave it to someone who know's what they are doing.


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## bad mechanic (Jun 21, 2006)

elake said:


> Then by all means have at it and let us know how it goes.


Got it done today. If you don't take into account all the measuring and setup, I was able to do both sides in about 20-25 minutes, using a trick or two to make it cut faster.


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## alexkraemer (Jul 30, 2007)

bad mechanic said:


> I'm aware road BB are narrower, but we're on a site called _Mountain_ Bike Review, talking about the BB shell of a _mountain_ bike frame. I didn't see a reason to qualify my statement.


I wasn't trying to nitpick - I had just read the article that day so I wanted to give you a friendly heads-up.

Glad to hear the machining went well - let us know how things turn out!


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