Tip for finding correct thickness of shims needed (with less trial and error):

Take the existing shim(s) out of the injector, re assemble, and then pop test the injector. Record the reading ( it will vary by injector). This allows you to know the baseline pop pressure created by the spring itself. Then, you can simply do the math to determine the correct thickness to get you desired pop pressure. My 2 cents.

I do some what of the same but I keep all the original parts together. This usually gets you in the Ball Park. Unfortunately each injector has separate base line from the others. A size X shim that will raise one injector 100psi will raise another only 50psi.

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