My ZZ4 350 had 4,500 miles on it when I did the change. I had several good mechanics tell me that I could use my existing springs with my Thumpr in the ZZ4, particularly since I don't race it or get into the real high RPMs. I bought the COMP beehive springs anyway. You should replace the pushrods, and if you do the springs, will also need the spring retainers (#787) and the spring seats/locators (#4693). If you buy the COMP Thumpr, you can get it from it from Summit for about $40 less than from COMP. One more thing, if you change the springs you will need new valve stem seals.