Shimming Eccentric Bottom Brackets

Recently, I put together a 17" 2006 Zion 660EBB Singlespeed frame.

It has an eccentric bottom bracket with dual locking set screws.

The Problem

One huge problem with this system is that the eccentric doesn't even come close to fitting the shell snug. It has room to rock around like they were two unsuited parts. This will lead to problems in the future if not soon.


The shell measures 2.141" to 2.150" (slightly distorted due to welding)
The eccentric measures 2.126" (54mm)


By having a well greased snug fit, the bottom bracket will have much less room to rock around and creek. Also, it will require much less clamping force with the screws to keep it in place. This is nice for long term service, as well as keeping the eccentric from spinning when hitting rocks.

The Standard

Bushnell Eccentrics (one of the finest made) is and expanding eccentric that is very light. It is also very expensive. It's initial diameter is 1.118" and is designed to expand from 1.120" to 1.125". A very close margin.

The proper ream size of a tandem bottom bracket shell is 1.125"

Reaming the Shell Round

For the best possible fit, I will be reaming the bottom bracket shell round.

I will use an adjustable hand reamer (MSC # 02239135) Industrial size M. This will eliminate the distorted shape and make the shell easier to shim.

A 12 point socket helps drive the reamer. It may take about 50 rotations, so don't use a rickety setup.

The reamer requires the use of a cutting oil as well. Motor oil is fine in a pinch.

Shiming the Eccentric

I will fill the gap using some stainess steel shim stock.

A 0.010" shim and a 0.0015" shim. Together they will take up 0.023" of diameter, bringing the eccentric to 2.149".

Greasing and Installing

Marine Grease. Use marine grease.