I’ve been asked a couple of times how I enlarged the jump packs of my Artscale Assault Marines so I figured I’d throw up a quick explanation. As with the regular backpacks I enlarge (by cutting up two and gluing the bits back together) the size difference is subtle but I feel it is enough to make it worth doing. Start with a regular back pack and a jump pack (the jump pack should be assembled, it’s easier in the long run). Remove the exhausts and vents from the the regular back pack, and the middle section of the jump pack with cuts shown here.
I use a craft knife, but a saw is probably easier when hacking through the jump pack midsection. Glue the remaining bits together, trying to keep the engines on either side aligned to one another so the finished thing is symmetrical.
You’ll have gaps but don’t worry about it too much because you’ll need to finished them off with a bit of Greenstuffing anyway. Two blobs of equal sized GS pressed flat into each side and the edges cleaned up with a clay shaper and presto… you’re all done.