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'18 Transition Sentinel
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Discussion Starter · #1 · (Edited)
Sorry, this is a dummy question, but I see these terminology used here & there.

What is exactly to "face" & "ream" the BB shell ?
(Also, what is exactly to "face" & "ream" & "press" the headtube, for that matter?)
I tried to look it up in a bike-maintenance book, but I didn't see it (or missed it).

Is this something that a person must do when getting a brandnew frame, or only if/when needed? Does it need to be done differently for different component standards?

I've never bought a frame-only before, and I guess it was all done with my complete's(?) so that I haven't (had to) "face" or "ream" whenever re-installing BB myself, etc.

Thanks in advance,
- PiroChu
 

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It's not a dumb question.

"Facing", which is done to bottom bracket shells, head tubes and, now, disc brake posts, is to shave/cut the mounting surfaces so they're perfectly flat and aligned to one another as parallel surfaces. So, the bottom bracket shell and headtube get shaved on both sides, just on the edge of the tube, to make a great fit for the BB or HS that goes in there. The disc brake tabs are shaved/cut to be parallel and on the same level as one another, and also to be dead parallel to the axis of the brake rotor.

It's not done to all bikes or frames, due to cost and some headset/BB/brake designs that are pretty tolerant of minor imperfections. It never hurts, though, when done right.
 

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as for "reaming", or more appropriately, "chasing", that's where you run a small cutting tool along the threads of a bottom bracket shell to clean out any paint or other impurities that may have clogged up the threads during the building process. you can also use a wire brush-like thing to remove such impurities from the seat tube, and i suppose, head tube.

just helps all the parts, like bottom bracket and seat tube, to sit better in the frame.
 

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Some clarifications

PiroChu said:
What is exactly to "face" & "ream" the BB shell ?
(Also, what is exactly to "face" & "ream" & "press" the headtube, for that matter?)
- PiroChu
Some definitions -

Facing - a material removing operation that creates a flat surface which is perpendicular to a reference surface.

Reaming - a material removing operation that creates a highly accurate hole diameter.

Chasing (as applied to chasing a finished bottom bracket) - an operation performed on accurately sized threads to remove foreign materials (paint, metal chips, burrs) from an accurately threaded hole using a tap (taps are thread cutting tools).

Pressing - a fastening technique used to join two parts together. Typically, one part contains an accurately sized hole and another part contains an accurately sized outside diameter which is slightly larger than the hole. Because the two parts cannot be put together by hand due to the interference (slight mismatch in size), a mechanical press is used to force the diameter into the hole. The interference is what holds the two parts together.

Putting the terms together -
A bottom bracket should be faced on both sides to ensure that the shoulders (where the cups of the BB hit the frame) are both flat and perpendicular to the bore (threaded hole). This is important because you want the BB cups to thread straight down (perpendicular to the bore) and you want them to seat 360 degrees around the shoulder (the flat part). Also, you want clean threads so you can screw your BB cups into the bore without getting hung up on chips, dirt, and anything else. Chasing the BB ensures the cups will turn in smothly with no "hang ups".

As far as a head tube/headset goes, to ensure you have the right size hole to give a proper interference fit for you headset, the head tube should be reamed. The reaming is done for both the top and bottom cup areas. As with the BB, the shoulders of the head tube (where the headset cups rest against the head tube) need to be flat and perpendicular to the bore (the head tube). This is accomplished by facing the head tube. Once the headtube is reamed and faced, it is ready to have the headset cups pressed in. A headset press is used to force the headset cups into the head tube.

On most frames bought seperately (like the kinds bought by MTBR folk), the manufacturer will prep the frame by reaming and facing the headtube and facing and chasing the bottom bracket. It's in their best interest to do this so that the consumer can just put everything together and get good performance out of the attached components. The tools required to prep the frame yourself are extremely expensive (were talking hundreds of dollars) and require some skill and knowledge. You can generally tell if this has been done simply by looking at the frame. Reaming and facing are finishing operations which typically produce very smooth, if not shiny (depending on material) surface finishes. You'll know if the threads require chasing if the BB cups constantly get hung up and are difficult to turn in by hand (most times you can fix this simply by turning a brush through the threads). If in doubt about your frames prepping status, always ask the manufacturer or bike shop from where you got the frame.

I hope that was helpful and not confusing.
 

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'18 Transition Sentinel
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1,015 Posts
Discussion Starter · #5 ·
Wow, that was absolutely the best reply post! (So much so that I can't possibly have any more further questions that I'd usually do.) Thank you very much.
 
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