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Old 12-23-06, 01:06 PM   #1
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Bottom bracket conversion 68mm to 73mm

Right so I've got my hands on a trek 7000 mtb frame and I'm in the process of building it up. Originally I had planed on using a bottom bracket I had lying around (for 68mm shell)but the trek frame is for a bottom bracket with 73mm shell.

The axle itself is more then long enough to accomodate the cranks so as I'm broke after buying all the other bits I was wondering if it was possible to get spacers of some sort to fit in both sides of the shell between the bottom bracket itself and the cups to make this set up work at least until I have the funds for a new one.

I can't see any major problems as the XTR/XT hollowtech chainset uses a simular system for 73mm bb shells, but I was wondering has anyone done something simular or would I be signing my/my bike's own death warrent if I did this?

I hope someone can help!
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Old 12-23-06, 02:52 PM   #2
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Actually, it is a bit of backyard fix, but some washers/spacers of suitable size inside the left-hand cup should do the trick. There is only 5mm difference total (or 2.5mm each side) in the widths. The interference fit of the left cup over the housing of the cartridge should keep the bearing unit from flopping around inside the shell. If you have a left cup without a flange, you could keep try it without the washers by turning it until things snug up, but I think you probably will run out of thread on the inside of the BB first.

Last edited by Rowan; 12-23-06 at 05:50 PM.
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Old 12-23-06, 06:04 PM   #3
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Wait a minute. The bb shell is already too wide and you want to add spacers? What am I missing here?
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Old 12-23-06, 07:16 PM   #4
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Quote:
Originally Posted by HillRider
Wait a minute. The bb shell is already too wide and you want to add spacers? What am I missing here?
Heh +1
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Old 12-23-06, 07:18 PM   #5
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Isn't the trek 7000 a hybrid series and not a mtb series?
meaning 700 wheels and not 26
you said you just got the frame, so keep this in mind if you haven't already considered it
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Old 12-24-06, 02:26 AM   #6
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Quote:
Originally Posted by HillRider
Wait a minute. The bb shell is already too wide and you want to add spacers? What am I missing here?
The spacers go inside the left cup is what you missed.

Frame = 73mm shell
BB cartridge = 68mm
Difference in width = 5mm (frame shell is wider).

Ergo, there would be a need for 5mm of spacers to fill the gap on the *inside* of the lefthand cup where its snugs up against the cartridge.

You are assuming that the spacers go on the outside. I did make it quite clear in my post that it was *inside* the cup or ring.

This is assuming Hollowtech I and not Hollowtech II.
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Old 12-24-06, 07:16 AM   #7
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Quote:
Originally Posted by Rowan
The spacers go inside the left cup is what you missed.

Frame = 73mm shell
BB cartridge = 68mm
Difference in width = 5mm (frame shell is wider).

Ergo, there would be a need for 5mm of spacers to fill the gap on the *inside* of the lefthand cup where its snugs up against the cartridge.

You are assuming that the spacers go on the outside. I did make it quite clear in my post that it was *inside* the cup or ring.
Assuming you do all that, how's the clearance between the left crankarm and the bottom bracket and the left crankarm and the chain stay going to be?
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Old 12-24-06, 08:51 AM   #8
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OK thanks for the help so far and as for the responses:
It's an old trek 7000 mtb frame (1999?)not the hybrid 700 series
the drive side cup does have a flange
and the chainline/crankarm clearance is fine I got truvativ isoflow cranks which have a curved crankarm so no problems there

Ok so I was messing around with it and I tightened it as much as I felt possible without stripping threads and it seems fine, I almost thought it fit but I think there is still 1or2mm gap difference but the cups sorta compress in the shell and hold it tight,no rattling around or movement at all.(Even though it looks like the drive side has an extra 1mm of the spindle )
Would this be safe? I can't see anything going majorly wrong and I don't care about the bb itself but I really don't wanna ruin the frame

I measured the shell(assuming 73mm) and my other bike's shells(all 68mm) and there doesnt seem to be a 5mm difference,it seems like the trek has a 70mm shell,I dunno should I just grease it all up and pray for the best?
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Old 12-24-06, 09:15 AM   #9
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Quote:
Originally Posted by Rowan
Ergo, there would be a need for 5mm of spacers to fill the gap on the *inside* of the lefthand cup where its snugs up against the cartridge.

You are assuming that the spacers go on the outside. I did make it quite clear in my post that it was *inside* the cup or ring.

This is assuming Hollowtech I and not Hollowtech II.
I certainly was assuming the spacers go outside of the bb shell as that's the only type I've ever heard about. Those are the type Shimano provides with their 73 mm MTB-type bottom brackets to allow them to be used on 68 mm bb shells.

Where do you get spacers that go inside the nds cup to make up the width difference the other way? And, yes, it has to be for either a Hollowtrch I or a square taper spindle.
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Old 12-24-06, 09:26 AM   #10
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Yes thats what I ment sorry for not explaining it better
I was planning puttin spacers inside the shell to get the opposite effect of the hollowtechII spacers,ie making a 68mm b work with a 73mm shell.
I was talking to they guy who owned the frame before me and he had the XT hollowtech II cranks but he said he only needed 1spacer.......that doesn't make sense to me its like the frame has a 70mm shell??Smaller then 73 but bigger then 68...
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Old 12-24-06, 12:05 PM   #11
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Quote:
Originally Posted by HillRider
I certainly was assuming the spacers go outside of the bb shell as that's the only type I've ever heard about. Those are the type Shimano provides with their 73 mm MTB-type bottom brackets to allow them to be used on 68 mm bb shells.

Where do you get spacers that go inside the nds cup to make up the width difference the other way? And, yes, it has to be for either a Hollowtrch I or a square taper spindle.
As I said, a backyard fix. Reread the OP's original post, although it is short on the details as far as BB and crank spec. It's a bicycle... you can manufacture your own spacers if need be, or go to the hardware shop and find something that will work... I've used things like spring washers for stuff like this in the past (bent them with a benchvice and mutligrips to eliminate the sprung part). A round file is a handy device to have in a workshop, too, to increase the internal diameter of ordinary washers. The internal diameter only needs to be just greater than the spindle. Broaden your horizons -- anyone would think bicycle mechanics was rocket science!

As it is, to the OP, I reckon that in your circumstances, your fix with just screwing in the left cup is OK. The idea of the cup is to support the left end of the cartridge from moving around. If the friction fit from the left cup is reasonably tight, it will perform that function. It probably would be better if the left cup or ring were made of metal rather than plastic, but I think it should be OK. Your Q on the pedals might be a bit off.

Now ask me if ES-71, 73mm BB cartridges will fit in a 68mm BB shell...
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Old 12-24-06, 03:08 PM   #12
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Well, of course you can cobble up nearly anything at home with ingenuity and some simple materials but the original posting seemed to make it sound like there were commercial spacers to do what was being discussed.
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Old 12-25-06, 04:00 PM   #13
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Yea sorry should have been more clear I was sorta asking if there were commercial spacers out there for this type of thing or what others would do in this situation.
Anyway I've just tightend the cups as much as I could,seems fine, thanks for the help I'll let you know if(or when) it fails catastrophically!!
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