how do you measure for a replacement bottom bracket

ohiobiker said:

I want to install sealed bearing bottom brackets in my tandem, chopper and replace them in my road bikes.

What is the correct way to measure the bottom bracket in the frame for the replacement part. I know they can be different lengths but are there different diameters. I checked a couple of catalogues and I only see 70mm in the square taper style. If there are different sizes do I measure the ID or the OD as well as the length. Thanks.

Your bottom bracket has to match your frame and your crankset.

Measure the width of your frame's bottom bracket shell. If it's 68mm or 73mm wide, you will need an English thread bottom bracket. If it's 70mm, you will need an Italian thread bottom brcket to fit.

The spindle length is determined mostly by your crankset. If you are keeping your old crankset, measure to the ends of the square tapers and match the replacement as closely as you can.

The square tapers can vary too. Generally there's Campy and otherwise so, if you have a Campy crank you need to get a Campy bottom bracket. Otherwise a Shimnao bottom bracket will work for other square taper cranks.