I have indeed heard of under shiming your springs- it's a great way of avoid recutting all your valve seats whan someone has gotten a bit carried away lapping in some of the valves, but you want to match spring rates.
.. But lets be honest- if you're deepening your valve seat to avoid buying a bucket shim, and have to buy a spring seat shim instead what exactly is the point? Surely it's easier, and more sensible to simply buy a bucket shim in the first place
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