Not sure what series were talking about. Their off road series of shocks can be completely rebuilt. The 6100 series was a little tougher and needed special tools until I worked with Bilstein to add a shrader to the top of the shock body. This made them easy to rebuild and re-valve and allowed for a reservoir option. That is whats cool about the 8125's, you can pull them apart and re-valve them with a 3mm allen wrench and a 19mm socket. Recharge them and your done. The word by-pass gets thrown around a lot, lets just say that the Bilstein "Digressive" piston controls fluid exchange like the buzz word, but doesn't need to over complicate their shock to do so. The piston design make for a very smooth transition through the entire stroke of travel and bottom out is controlled by the components in their shim stacks that again are patented and no one else has.