I will assume that you already cut the the track for the shaft and the trigger pocket, if not do that first. Once that is done, insert the shaft into the trigger mech. and insert the mech. with the shaft installed into the trigger pocket, this will assure that the trigger is installed at the correct depth so the shaft can be loaded properly.
Make a mark across the mech. as it sits in the pocket, using the top of the gun as your guide, this will be you reference to cut the line release slot and the holes for the screws of the mech. Measure the distance from the top of the mech. to the reference line you drew and make the cut for the line release at the proper distance accordingly, at this time drill the holes for the screws, this is crucial because if you do this wrong things wont line up, I clamp the trigger onto the wood exactly where it should be and use it as a guide to drill the holes, once you drill the first hole insert a screw into it to prevent the mech. from moving as you drill.
Insert the trigger mechanism back into the pocket and install the screws, make sure your line release slot lines up exactly as the one in the trigger, install the shaft and make sure it locks in position, fire the trigger and assure the shaft fires. If you have done it correctly the shaft should go in and out of the trigger freely, if not, it has to be corrected before you go further.
To pin the line release is what I think the most challenging part, first load the shaft so the sear drops in the line release slot, insert the line release all the way into the slot until it contacts the sear and at the proper angle which is the line release pointing towards the back of the gun as it would be when the shaft is loaded, mark the angle of the line release on the top of the gun and remove the line release from the slot and lay it on the mark you just made, mark the pivot hole of the line release on the top of the gun and drill the hole for the pivot screw. I would recheck this last step several times to make sure you got it right. If done properly the shaft should fire with the line release in place and loaded with line, if the gun does not fire when line is loaded on the line release plug the hole and do it again.
Good luck, if you need more help send me a PM.
Steve
http://www.poseidonsub.com/
