KillerCrate buys Genuine GM Springs in Bulk and carefully rate / match them in-house on their high-end spring machine. Each box is coded, and you get what you pay for.
KillerCrate recommends changing 602 Crate Valve springs every 300 laps; however, this depends on the length of track and rpm you are running. Springs last longer on short tracks with a rev chip and last a little less when you are running a big track at 6400 RPM!
KillerCrate is, the most trusted name in Crate Matched Springs.