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I'm putting a gotoh tune o matic bridge on my mustang and was wondering what the best degree of shim would work best.

I found it was always a trial and error thing but you'll be surprised how much a thin piece of card for a shim changes the angle. Start with a piece of card from something like a cigarette packet or cereal box.
You only need a 10mm strip the width of the neck pocket, at the body end of the pocket.

I try to do this with the old strings still attached. Just loosen them, then the neck plate, then just gently lift out the neck and put the shim in place. Whilst the strings and neck is loose put the TOM into place and make sure it is at its lowest point using the bridge post screws.
Put it back together and tune the strings.
If the action is still high double the shim thickness, if the action is too low just raise the TOM until you are happy with it.
It may take a few attempts to get it right.

You need a big shim. I tried myself and it was lots of trial and error. 4 bits of business card cut down worked ok.

You

I tried it on a Mustang and it was a ridiculous shim that didn't feel safe, but yes, trial and error, and certain TOMs have different heights themselves.

I have a Japanese made Kurt Cobain Mustang that came stock with the Fender TOM clone. Would they have cut the neck pocket at the proper angle at the factory or would this guitar also benefit from a shim?

Ricey wrote:I have a Japanese made Kurt Cobain Mustang that came stock with the Fender TOM clone. Would they have cut the neck pocket at the proper angle at the factory or would this guitar also benefit from a shim?

Without ever examining one you would have thought so in this day and age of CNC machines. But, ya know...

Yeah they’re peng. They’re clearly built with that bridge in mind.

Will work on a Squier but there will be a big gap.

OP, What Mustang you got?