Umarex/S and T Tavor 21 - Complete (For now!)

[Asomodai]

Hello all! 

 

I decided to tackle this difficult project by myself due to the enforced time we have. 

 

The Tavor is one of the more complicated projects you can do. I bought this for 80 euros brand new. I upgraded the plastic rails to metal ones, but that was about it. Performance out of the box was ok. The Hop wasnt great. But otherwise nothing to write home about. 

 

I bought the following parts. 

 

Rocket 16TPA High Torque Vented short motor. 

ZCI 407mm 6.02 barrel. 

SHS 16:1 gearset and Chip Delayer

Maple leaf Macaron and 0hm nub

SHS 14 Tooth Piston

SHS Silent Piston Head

SHS Silent Cylinder Head

SHS Shim Set

FPS Ergal Tavor Air Nozzle (Specifically for the Ares, but should be ok for the S&T/Umarex

Perun Mosfet

 

I just wanted a nice solid build that would run nicely on 7.4v, but could go to 9.9v life or a low C rated 11.1v if required. Nothing too crazy. I am not a professional, in fact I am pretty terrible and make horrendous amount of mistakes. 

 

 

Here she is in all of her plastic glory.  I followed the video here to disassemble down to the hop and gearbox. 

 

 

 

 

Pretty much an M4 unit with a long feed-tube. The Gearing is poor on it, but will do for now. Standard M4 take-down. Though a small retainer pin needs to be popped out to release the outer barrel. 

 

 

Really odd stock hop arm and rubber. 

 

 

Unfortunately a Maple Leaf Macaron and 0hm nub does not appear to work here. The ohm nub sits too far forward to get the desired shape. Will need to think about a Prommy purple at this rate. (Unless anyone has any suggestions)

 

 

Gearbox removed, Fake bolt plate on top. 

 

 

The other side. 

 

 

Sans Plate. Look at all the additional gubbinz required to make the gearbox fire and select! 

 

 

All those springs! Glad I took a photo for when I put everything back together. 

 

 

Set out neatly. 

 

 

Stock internals. Not much to write home about. Microswitch trigger as standard, odd way of wiring through the gearbox, but does the job. Plastic Cylinder head is awful and not up to the task. Motor is a weak ferrite balanced motor. Piston is quite heavy. Bushings are solid enough. Tappet nice and clean, usual yellow snot inside. Appears to be standard AR latch. 

 

 

Cleaned out the box, used my dremel to polish out the piston tracks and tappet area. Note the bolt return catch in between the two halves. 

 

 

New air seal parts and gears gone in. The new Cylinder/piston head and piston managed to get AOE pretty much bang on out of the box. So no need to sorbo. Unfortunately I had to compromise on the gear shimming, The Bevel had to be run quite high to clear the pinion, so the sector and spur is a little grindy. But will probably quieten down. 

 

 

The gearbox is well known to not take aftermarket pistons because the large tooth gets caught on the rear end. Most people grind either the big tooth down or the gearbox. I wasn't experiencing this problem, but I decided to polish out the area on the gearbox and grind down the large tooth by less then a mm. 

 

 

New Air nozzle compared to the old one. New one is slightly shorter, but has an O-Ring. 

 

 

The Completed box. I have since put the outside gubbinz back on successfully. I did have one big problem in that the tappet wouldn't cycle correctly when the gearbox was tightened up. I had to take the box apart, polish the guide rails and sand the side of the tappet plate by about 1mm. It is now cycling freely, though i wouldn't mind getting a spare tappet plate eventually. 

 

Here is a quick test on a half charged 9.6v NimH

 

 

The next step is to install a Mosfet and deans connector. I will use the existing trigger contact wires to hook up. The current wiring harness is about 60cm long to reach the front of the gearbox. I will be cutting it down to just 10CM as I am changing the Tavor to be rear-wired. 

[Adolf Hamster]

how tight is the region around the hop? might be possible to convert something a bit better to sit in there if all that's different is the feed tube length.

[Asomodai]

37 minutes ago, Adolf Hamster said:

how tight is the region around the hop? might be possible to convert something a bit better to sit in there if all that's different is the feed tube length.

 

Fairly tight I am afraid. The bolt cover surrounds it. I haven't heard of anyone managing to replace the hop unit. I agree the feedtube can be replicated. 

 

I have a prommy purple in my P90 I was going to switch out with the Maple Leaf system so will go down that route first. 

[Adolf Hamster]

8 hours ago, Asomodai said:

 

Fairly tight I am afraid. The bolt cover surrounds it. I haven't heard of anyone managing to replace the hop unit. I agree the feedtube can be replicated. 

 

I have a prommy purple in my P90 I was going to switch out with the Maple Leaf system so will go down that route first. 

 

maybe even just a better made regular style then if you can't fit a rotary.

[Asomodai]

55 minutes ago, Adolf Hamster said:

 

maybe even just a better made regular style then if you can't fit a rotary.

It would also need that pin at the front to hold the Outer barrel in place. 

 

I do have some spare G&G gearing somewhere actually which I could use. Though I would prefer a more standard arm. 

[Adolf Hamster]

6 minutes ago, Asomodai said:

It would also need that pin at the front to hold the Outer barrel in place. 

 

I do have some spare G&G gearing somewhere actually which I could use. Though I would prefer a more standard arm. 

 

ahh, i misread that, thought the pin was holding the inner barrel in lol.

 

would a standard hop arm fit? kinda hard to tell the scale.

[Asomodai]

12 hours ago, Adolf Hamster said:

 

ahh, i misread that, thought the pin was holding the inner barrel in lol.

 

would a standard hop arm fit? kinda hard to tell the scale.

 

Quite possibly! I have used the stock hop nub and a Prommy Purple for now!

 

 

 

I spent the morning wiring up the gearbox. Used the stock 18AWG wiring for the trigger contacts and 16AWG for everything else. The basic Perun Mosfet is excellent as the two current wires are on the front and single trigger contact wire is on the back.

 

 

This was wired to the front as stock through a 60cm wiring loom. Decided to change to a rear mount as there is plenty of room for a battery above the fake bolt. Fits a 1450mah 7.4 stick with room to spare. It is a little difficult to close the buttpad due to the thick wiring, but placed in the correct way it will sit flush without pushing the motor towards the bevel. 

 

 

Forgot to post a photo of the gearbox post upgrade yesterday. 

 

I had some motor height issues and I forgot to put the Motor shim back in after I had reassembled it. So I had to disassemble again and put in a narrower shim then stock (The Motor height was perfect without the shim, it's slightly grindy now but its reasonable enough!). 

 

I also changed the Maple Leaf out for the standard odd hop nub and an old Prometheus Purple that I grabbed out of my Cyma P90 (Which got the Maple Leaf and 0hm nub which thankfully fits perfectly).

 

 

Here it is in its glory. A fully charged 30C 7.4V Lipo results in a snappy trigger response which rarely locks up on semi when trigger spamming (Kind of par for the course with bullpups). It has a nice 23 RPS. It's a bit loud as the gearbox is right next to my recording phone. 

 

FPS is coming out at a steady 277FPS on .3's which equates to roughly 339 FPS on .2's taking into account Joule Creep. With actual .2's it's more like 328.  

 

Hopefully I'll get to test the range and accuracy soon. But (hopefully) The gearbox is complete and pretty much the project! I may add some foam into the body cavities in the future. The Silent piston head and cylinder combo didn't exactly do much! 

 

----------------------------------

 

Working on the Tavor was not the nightmare I thought it would be. The initial take down is quite difficult as there are lots of bits to take off in a somewhat specific order. But the gearbox itself is pretty easy to work on due to the quick change spring system, a AR latch that stays put and relatively simple wiring. The AOE was correct as soon as I put in the new Piston/Cylinder/Head combo needing no changes. 

 

I can take down the Tavor right down to an open gearbox in about 10 minutes, reassembly is about the same sort of time. 

 

 

 

[Adolf Hamster]

Glad it all seems to have worked out nicely.

 

I must admit im not a fan of the silent piston head thing, tried one a while back (think it was an shs) and couldnt tell the difference, except in fps due to the shocking compression, maybe just got a bad one.

[Asomodai]

5 minutes ago, Adolf Hamster said:

Glad it all seems to have worked out nicely.

 

I must admit im not a fan of the silent piston head thing, tried one a while back (think it was an shs) and couldnt tell the difference, except in fps due to the shocking compression, maybe just got a bad one.

 

I am more relieved that I managed to do it in the first place. For the first time ever in a project I have not had Nozzle/Feeding issues.  

 

Yeah. It is not really any quieter. Maybe more of a thud then a thwack sound when the piston head hits the cylinder. 

 

I am happy to say though that compression is 100% excellent. 

[matas17]

@AsomodaiDid you somehow manage to get trigger pull shorter?

[Asomodai]

36 minutes ago, matas17 said:

@AsomodaiDid you somehow manage to get trigger pull shorter?

 

The trigger pull doesn't bother me at all so I didn't bother. With the motor and mosfet upgrades its very snappy as it is.