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Bilstein has a new ID system. The previous "Sport" shocks are now "B6". The previous "HD" are now "B5". For a Z51 you probably want the B6 but if you want to loosen up the shock tightness a little, quite a few here have been happy using the B5 on their Z51 cars.
Save yourself some money and buy them at RockAuto.
And the part numbers are actually B8 and B6, for Sport and Heavy Duty, respectively.
I'm thinking of going this route once my current shocks wear out.
I have a Z51 and chenged my OEM's to Bilstein B8, and like the ride. I was thinking about going B6 to soften the ride a bit, but I'm glad I chose B8. Much nicer ride than the OEM's for sure.
I bought mine from my son who sells parts throught his business and he could just barely beat Rock Auto prices.