Funny you posted that Rad, the Mk2 uses that exact layout and 25mm fan!
But here's how the mk1 went together.
Started by removing the whole unit from a HL tank and undoing the 2 screws that hold the smoke box to the pump tray. I've also cut the red and black wires that run into the pump motor. The whole pump, tray and piston can be put in your bits box as they aren't needed (I rarely throw anything away)!
Next I cut off the piston tube at apx 45 degrees making sure the screw mounts are still intact.
Get your 30mm square 12v fan (.99p on eBay) and test it to ensure you know which way it's blowing. Sounds daft but I mostly superglue these together so it's a major PITA if you get it wrong. All mine have have blown towards the label so place it label side down.
Using a couple of 3mm bolts (screws etc whatever you have to hand) bolt the fan to the 2 screw mounts. These were a little too long so I dremelled off the excess.
I then cut a strip of .5mm plasticard as wide as the fan (30mm) and superglue it carefully to one edge. Make sure no glue gets onto the fan!
Bend it around the bottom of the piston tube (I put a dab of s.glue here too) and up to the other side of the fan. Cut off the excess. Notice the gaps? We'll sort those later.
Now glue another bit of the plasticard strip over the end making sure all the edges line up with no gaps. A bit of extra glue should fill any you find. You could cut the end to follow the curve around the piston tube but I find its best to leave it square so it'll stand up on its own.
Now for the gaps. Cut a final piece of plasticard with a section removed that will fit over the piston tube. It'll look like this.
All glued up.
Last thing to do is solder the black & red fan wires to the corresponding wires that ran to the pump. With that done I screwed the PCB to the underside so it was out of the way, this also screws the bottom section of plasticard to the piston tube for added strength.
All done and ready to test as seen in the YouTube clip above
Once you're happy it works you can refit into your tank using the single original screw mount on the bottom of the smoke box.
Regards
Nic
http://www.crispyandtherocketman.com