Drop-In Rotary Hop-Up unit for the S&T Tavor TAR-21 AEG.

Been working on this one for a while! The stock Tavor hop unit, while good, is a monster to adjust on-field, especially with gloves on.

You will be re-using your hop-up barrel clip, barrel centering ring, barrel O-ring, and the cross-pin for the hop arm off the original hop unit.

The additional hardware you'll need is 1x JIS 1.8x12 (or US #013) O-Ring, and a 1mm ID 1mm CS O-Ring (green gas fill o-ring).

You'll probably want to install an aftermarket nozzle as well, as the stock nozzle in these guns suck. 21.1mm is the nozzle to use.

The body of the hop-up unit is not printable via FDM processes. You will need either SLA, SLS, or other similar processes (MJF, DMLS, etc) to create this part successfully. I also recommend printing the knob in FDM or SLS, as grinding SLA on SLA will rapidly wear the parts.

- Printing -
SLA prints should be in an ABS-Like, tough resin.

The hop arm should be printed in whatever format you use for the body, but the knob can be FDM, SLS, SLA, anything. The only thing that should only be FDM or SLS is the assembly clip, as it needs to have flexibility. The assembly clip should be printed in 0.1mm layer height, as it should be 1.7mm thick.

- Assembly Instructions -

1. Install a JIS 1.8x12 or US #13 O-Ring on the body grooves closest to the feed tower.

2. Install your barrel with bucking, then the barrel clip. Finish with the brass centering ring, and the big hop-up tension spring (slips over the barrel).

3. Place your nub of choice into the nub pocket, then place the hop arm on top. Install by pressing the crosspin salvaged from the original hop unit through the holes in both parts. Secure with a dry 1x1mm o-ring to keep it from slipping out. If the cross-hole has shrunk in printing, use a 2mm (or Wire Gauge 46) drill bit to open the hole back up to size.

4. Install the hopup knob, pressing it over the o-ring. Make sure to line up the flat tail of the hop arm with the cam groove in the knob. Secure in place with the assembly clip in the exposed grooves behind it.

5. Reinstall the hop-up into your AEG and test!

- Assembly Note -
You will not be able to install the hop unit unless the nozzle is retracted. Feathering or drag flicking the trigger should let you get the nozzle into a workable position. The rotary arm interferes with bosses in the shell, so don't try and force it or jam tools against the nozzle to try and push it back.

This design does not use a clip to hold the hop unit to the barrel, it's totally unnecessary. Just line up the top rib with the slot in the barrel while you install everything.

- Parts Notes -
The crosspin can be replaced by McMaster 90270A222 if you grind the threads off.

- Amazon Parts Links -
JIS 1.8x12 or US #013 O-Ring: https://a.co/d/ig9jQFA
1x1 O-Ring: https://a.co/d/1s8gCJ0

- Troubleshooting -

1. If you are using a tapered-lip bucking (ex. Maple Leaf Macaron) and not a straight-lip (ex. Prometheus Purple) you may find that the BB's won't pass the bucking lips without a bunch of force. This may require you to pass a 7.3-7.4mm or Letter L drill bit through the bore of the hop unit, as the diameter shrunk during printing. The model is 7.4mm and will still work down to about 7.3mm.