O-ring,​​44.​​7mmx2.​​4mm 180459

Removed the screws holding the top cap on
Replaced O-ring and piston ring and worked great!

If you've already diagnosed the issue, mine being that the nail gun wouldn't fire a nail, then this repair is very simple. I have the Bostitch BT-1855 18 gauge nailer. First you will need to remove the cap and (4) Allen head screws at the rear of the gun (part #181917, or item 2 if you are looking at your factory schematic which is the exploded diagram. Once you have removed the cap be sure to set aside the frame cap gasket (item 3, part #175682). Concealed inside the cap are items 4-9 on your schematic. You should see a clear bumper, spring and O-rings. You shouldn't need to replace these. You will need to remove and replace items 10-16 on your schematic. They are as follows: 10-piston driver (#180451) 11-piston ring (#180457), 12-cylinder sleeve (#180458), 13-O-ring (#180459), 14-check seal (#180460), 15-bumper (#180461), 16-driver guide (#180462). I pulled everything out and lined it up just as it went into the nailer, so as not to forget the original placement and mechanics. Remove your replacement parts from their packaging. Assemble items 10-16 together, then place the completed assembly into the gun; rather than putting each individual item back in place. Reattach frame cap seal and frame cap. Test fire gun. All told it should take about 15-30 minutes to complete this repair. You should only need the appropriate size Allen wrench and maybe a small flat head screw driver for prying some of the O-rings out.

1. Took nailgun apart.
2. Replaced piston.
3. Reassembled.