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There was a really nice write up with great pics that was floating around years ago and it really conveyed the ideas of what you're trying to accomplish with shimming. Some guys will remember the SystemA instructions that said to place this thickness here and that thickness there...but those in the know know that that's not good enough.
With the variations that will/might exist with every build it's really more essential to understand the concepts and then to apply what's needed to sort it out.
And the objectives are deceptively easy:
1. Maximize tooth contact
2. Minimize latteral movement
3. Avoid excessive friction from parts contacting each other
The essence of the concepts to achieve those objectives are likewise deceptively simple:
1. Use shims to space the gears from the mechbox shell and each other
2. Use shims to align the gears so that the piston rack is as fully engaged as possible
That's pretty much it....until you get into all the fine points such as how bushings/bearings are aligned, what points of the shell are likely to be too close to the gears, how other components in the mechbox may interact with the setup, tuning out the noise, tuning in a bit of drag, etc...
Then there's all the other nice parts of a mechbox that actually sends the BB on its way.
IMO, the best way to learn how to work up a mechbox is to read up a bit, try your hand at it and watch someone who knows how to do it...or better yet have them coach you through it.
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