On Thu, 01 Jul 2004 08:52:20 GMT, Jeremy Collins
<
[email protected]> wrote:
>OK, so I've got 28/38/48t chainrings, and 170mm cranks. I've
>seen a reasonably cheap (< 30 GBP) Shimano chainset with these
>specs, and as you say they recommend a particular BB.
>
>Now this is where I'm in the dark. How do I know whether a
>given BB will actually fit the bike? The BB goes inside that
>little lateral tube at the bottom of the frame - mine currently
>has a little bearing assembly threaded into each end with a
>spindle in between, and all the BBs I've seen for sale don't
>seem to look like this.
Measure the length of the tube that the BB goes into, and measure the
end-to-nd length of the BB shaft. The specs for a BB are in two bits;
the first is typically either 68 or 73 (the mm length of the tube),
and the second is the length of the shaft (also in mm). The 68 or 73
needs to be a match for the tube on the bike, and the other number
(which is somewhere over 100, often around 113) needs to match the
requirement of the cranks. A number of different BB designs are
supplied; the majority of local bike shops will probably carry
cartridge-style units, which most likely look very little like what
you have now. The appearance is not the important thing. The chances
are near to certainty that your bike will accept the ommon types of
cartridge BB without a problem. However, if the length of the
existing shaft is within 2mm of what's required for the replacement
cranks, and if it's not in bad shape, there's no reason not to
continue to use it.
One more thing; when measuring the shaft length, if the shaft has
threaded studs on each end, ignore the studs; measure just the
distance between the outboard ends of the square tapers.
You will probably need either one or two more tools to install the
replacement BB if you're going to swap it to a cartridge unit. The
tools required will be determined by the type of bearing you buy; ask
the lbs about it.
>The spindle and one of the end bearings are in this photo:
>http://homepage.ntlworld.com/jd.collins/images/chainset.jpg
>
>> The Park Tools site already mentioned covers it reasonably well; if
>> the current cranks aren't loose, and if you don't want to risk
>> damaging them in removal, you'll need a crank extractor.
>
>I got a Park crank extractor yesterday and it worked perfectly.
>
>> Since you're
>> new at this, a torque wrench might be a good idea for tightening the
>> bolts when reinstalling. Most cranks go on to a tapered shaft, and
>> tightening the bolts correctly requires stopping at a point that's not
>> apparent to the untrained hand.
>
>Noted, thanks.
--
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