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 Standard

Generic sizes most comonly available measure generic 54mm (2.126").

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 2.118" and is designed to expand from 1.120" to 1.125". A very close margin to generic 54mm (2.126").

Phil Wood NFES814 Eccentric and NFBSE8 Shell are designed around a 2.142" size, althought custom versions of this are available.

Easton Sports has another standard used in their shells. Their eccentric measures 2.155"

So the proper ream size of a tandem bottom bracket shell is in question. This would help explain the array of problems that people are having with these.

Reaming the Shell Round

For the best possible fit, I will be reaming the bottom bracket shell round. I have reamed many bottoms, and I suggest grease for most.

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.

The reamer is huge. OAL 410mm.

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.

After a light cutting pass, the distortion in the shell is obvious.

Shiming the Eccentric

I will fill the gap using some stainess steel shim stock. It is best to use a single shim so that it is as thick as it can be for durability sake. I prefer stainless shims over brass for the same reason.

A 0.010" shim and a 0.0015" shim. In the end, just the 0.010" shim was used and it was a very snug fit.

The greased shim with a bend to help installation

Hardware Replacement

The two round head cap screws were replaced with Stainless Steel Cap screws for use with a larger wrench and matirial to grab in a worst-case situation. Now a 5mm allen wrench can be used.

Greasing and Installing

Marine Grease. Use marine grease. Its what I use when reaming bottoms.

Test fitting.

I also need to find a way to rust proof the edges.

Park SPA-2 tool is used to roll the cam.

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