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All DMTs have a break-in period, where the diamonds sticking out are knocked off. Even a DMT 325 will feel pretty smooth when broken in.

Practising on a GD is a good idea, though they are not...

Welcome to SRP! I wonder what kind of car you drive?

As for your question: you can even lap the Norton 4K/8K with a DMT 325, the latter being one of the most used lapping stones on this forum....