HW77 / 97 rebuild
Re-assembly:
This is basically a reversal of the above.
Begin by assembling the piston in the compression tube (remember suitable lubrication), making sure that the underlever slot in the piston lines up with the square hole in the compression tube. Fit this assembly into the action with the piston slot uppermost, in line with the underlever slot in the main action.

Refit the spring and spring guide, correctly lubricated, into the piston.

Now you need to refit the end block. With the block in your gloved hand, push against the end of the spring guide to bring the block up to the main cylinder. Turn the block to engage the threads. This is easiest done by pushing with the gloved hand and turning the block with the other. Screw the block into the action, taking care to look at the square hole in the cylinder and the slot in the piston to make sure they are not going out of alignment with the slot in the action. They can easily do so, but to stop or correct any movement I find it easy to hold a screwdriver in the slots to prevent the piston cylinder and piston tube from rotating. When you have screwed the end block in as far as you can by hand you may well need to apply some leverage to get it up tight. You can check this position by noting that the sides of the trigger housing slot line up with the action and the threaded bolt can be screwed into the block.

I now prefer to refit the underlever. Place the safety slide / antibear trap slide mechanism on top of the action and refit the underlever cocking arm end into the square hole and piston slot. Refit the underlever into the pivot at the end of the action and tap the pin home.

You can now secure the safety slide to the end of the action with the threaded bolt. Ensure the spacing washer is still in place. You need to push the safety slide a little against its spring pressure but it is easily done; refit the threaded bolt. Once fitted, just release the underlever and make sure the safety slide moves freely.

Now to refit the trigger. Refit the safety catch, remembering to put the spring on the catch before you push it through its locating hole in the end of the action. Hold the trigger unit with the left side slightly up and feed the trigger unit into the trigger housing, all the time keeping the safety catch pushed in against its spring. As you feed the trigger unit home, bottom right corner first, rotate the left edge down a little and it will fall easily into place. When the unit is seated, take two punches and place them through the holes in the action from the front to back, all the time holding the safety catch in. Then take the shorter pin and replace it through the action and trigger unit from the back towards the front. This will drive the punch out; the pin should go in quite easily. If it jams or refuses to go, do not force it; you will have the holes slightly out of alignment. Jiggle the punch to bring the holes into alignment.
Fit the long pin.

Make sure the captive nut at the top right of the trigger unit is in place ... a trigger guard screw fastens into this.

Refit action into the stock. Refit trigger guard. Job done. Refit sights and test rifle.
Cheers, Phil