I've been searching for instructions on how to remove my old saddle from my seatpost. All the videos/guides I've seen have been about the newer kind of clamping mechanism where just one bolt needs to be loosened with an allen wrench. Mine is different- there are two bolts on either side of the saddle and none of my tools seem to fit. (It's an HL Aluminum Micro Adjust 27.2 X 300mm). How can I remove it??
Get 2 spanners that fit. Hold the bolts on both sides with the spanners and turn any one side to remove easily.
To put it back alternately rotate each side 1 or 2 turns.
You will get a spanner, just look for it in a hardware store or get adjustable ones.
I had a similar setup on my Bridgestone. Mine was a lot more antiquated as the seat post had a bolt mechanism similar to a handlebar stem :)
I found that the GMC Denali 21 speed uses an HL Aluminum Micro Adjust 27.2 X 300mm, so I assume your seat is very similar to those found on the Denali. In this case you will need a small crescent wrench that will fit between the guts and the side of the saddle. Some bolts are square and will stay put when loosing the nuts, but some are not and might require an opposing wrench on the other side. You only need to loosen each side a half turn and you can slide the seat off the post. The problem is that you will only be able to replace it with similar guts. You might be able to swap out the saddle itself if the rails match, but my recommendation would be to switch the whole saddle/post with a more modern set.