here's a list of the items you'd need to get :
1) P/N 73512 (hpi pro 4 stick pack holder), qty : 1 set
(this is a set of various parts, each marked with numbers. you'll only need the parts marked 4 (2pcs), 6(1pc), 7(2pcs) and 8(1pc) from this set. i'll refer to these as 73512-4, 73512-6, 73512-7 and 73512-8 respectively)
2) P/N Z082 (flat head screw m3 x 8mm), qty : 2
3) P/N Z352 (button head screw m3 x 8mm), qty : 4
4) P/N Z224 (washer m3 x 6mm), qty : 4
5) P/N Z305 (screw post m3 x 8mm), qty : 2
top with battery clip view
1) cut out P/N 73512-7 (2pcs) from the set. dremel/file away the little "tooth" at the bottom of this part, so that it can sit flat.
2) attach the 2 pcs of P/N 73512-7 to the chassis, using P/N Z082 screws and using the existing unused holes along the centre line of the chassis, as in positions as shown in the "top view" pic.
: note the direction of the P/N 73512-7. this is the opposite way of how it's mounted on pro 4. this is to prevent the stick pack of getting in the way of the pinion and spur gear on the cyclone.
3) cut out P/N 73512-8 from the set.
4) put your stick pack on the chassis, and push the left side of the stick pack flush against the 2 pcs of P/N 73512-7 installed in step 2.
now place P/N 73512-8 flush against the right side of the stick pack, in the position as shown in the "top view" pic. mark out on the chassis the location of the 2 holes for mounting P/N 73512-8. the 2 holes must be located at a safe distance away from the side-by-side battery slots of the chassis.
5) remove the stick pack, remove P/N 73512-8, and drill the 2 holes marked out in step 4, using a 3mm diameter drill bit. make sure the bit is the type for drilling metal and is sharp (use a new drill bit if you can).
you'll need to clamp down/secure the chassis properly when drilling, to prevent any warpage of the chassis. i also use a vacuum and point it towards the drilling to suck away the cf dust.
along each hole (which will be very close to the side edge of the chassis), parallel to the battery slots, i use a needle file to file a slot from the side edge of the chassis, inwards towards the hole. this creates a "u-shaped" notch along the side edge of the chassis. this is to ease the positioning and mounting of P/N 73512-8 later on.
if you use CA glue to seal the edges of the cf chassis you'll probably want to seal these 2 notches too.
i'm not sure if the description of the notches make it hard to visualize, but i hope you can make out the 2 notches in the "underside view" pic above.
6) place the left side of the stick pack flush against the 2 pcs of P/N 73512-7, and then adjust and mount P/N 73512-8 using the P/N Z352 screws and P/N Z224 washers, as in the "underside view".
7) Cut out P/N 73512-6 and the 2 pcs of P/N 73512-4 from the set. dremel/file away the little "tooth" at the bottom of P/N 73512-4. screw in P/N Z305 screw posts into the 2pcs of P/N 73512-4, and then mount the 2 pcs of P/N 73512-4 onto the chassis using the existing slots in the chassis, and using P/N Z352 screws and P/N Z224 washers. with a stick pack on the chassis, adjust the positions of P/N 73512-4 until they hold P/N 73512-6 well, and then tighten the 2 pcs of P/N 73512-4 into location.
at the end of this you should see what you see in the "top view with battery clip" pic.
that's it! the above instructions may seem like quite a lot of work but it's actually a pretty simple straight-forward change. main thing is to be careful with the drilling. the disadvantage is that there will be 4 button head screws on the bottom of the chassis. but i have my ride height set pretty high and run on pretty smooth concrete ground so this isn't an issue for me. if you have access to machining equipment that can make accurate countersunk holes into a cf chassis that would be a great way to solve that.
anyway hth and good luck
! if you have any questions just post back.