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I'm very new to mountain biking, and even newer to bike maintenance. Appreciate your time on a question that I'm sure is simple; I just can't find the right words to Google it.

I have a fairly nice Trek bike. The dropper post on it was losing pressure; I could add more and it'd lose it over a few hours or days. No one local wanted to service it, so I bought this for now:

Race Face Aeffect R Dropper Post (31.6/150mm)

I could probably have REI install it for a few bucks, but I'd like to take the opportunity to learn. Since there was a prior dropper post installed, it already has a cable & remote on the handlebar. I should just need to swap out the connection.

It was easy enough to disconnect, but now I'm looking at these two obviously incompatible interfaces and am totally lost (click for full size):

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In order: (1) New dropper post, (2) connection that used to go into old dropper post, (3) Included bead that looks like the cable goes through it, and fits into the hole on the new post.

Additionally, the new post came with an instruction sheet (available here). Steps 15 and 17 look relevant, I just don't have enough experience to understand what they actually mean.

Obviously, I need to remove the current assembly on the end of the cable, and put a new (simpler) assembly on it. Doesn't look too difficult. I just can't find any name of that connection to find the 10,000 YouTube how-tos I'm sure exist on it.

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The aspect of the new post where the cable is secured is called the bottom cap assembly. The little black piece that holds the inner cable is called the cable bushing or, simply, cable holder.

You're going to have to remove the inner cable from both the old post's mechanism as well as the outer cable housing. It's important that the end of the inner cable is in near pristine condition in order for it to be fed through the new post's cable holder (picture 3) and then the outer housing starting at the seatpost end and up to the lever.

Generally I would use a new inner shift cable and shift housing in a situation like this. I would store the old cabling with the old seatpost if it appeared to be in good shape. However, I have bulk packages of inner shift cables and housing to work from. If you choose to use the old cabling, make sure there will be adequate length of both to connect the lever and new seatpost mechanism while having enough slack that the cable run doesn't interfere with turning the handle bars.

Note that the new dropper's inner cable is run starting at the seat post where it first must be threaded thru the cable bushing, then the outer housing with ferrule. Continue threading the inner cable so the free end exits the outer housing at the lever side. When the inner cable is routed thru the cable holder/bushing (as shown in step 15 of your linked instructions) and outer housing (represented by step 16), you'll place cable holder/bushing in its spot on the lower seatpost mechanism (as shown in step 17). It generally can only come in from one side which has a space for the inner cable to slide thru unobstructed. As emphasized in step 15, it's important that the barrel attached to one end of the inner cable is fully seated in the cable bushing/holder.

Next secure the ends of the outer housing with their ferrules in their respective places. First, at the lower seatpost mechanism, then, at the handlebar side, you'll pull inner and outer cable slack generated by inserting the seatpost to desired height (the instructions refer to this as "climb height"). I will route the inner cable thru the lever body prior to setting the outer cable ferrule in place in the lever. When that ferrule is set, simply tug the end of the inner cable to remove any remaining slack while you secure it in the pinch bolt of the lever.

All this gets a bit fiddly keeping everything in place, pulling the extra cable toward the lever side as you install the seat post down the seat tube. Keeping the inner cable taut will assist in keeping the barrel end of the inner cable seated in the bushing. The seatpost ferrule will stay in place as long as you don't pull the outer housing by itself and make it slide on the inner cable.

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