MP5 LDT FPS upgrades

Hi, Just recently I was asking about return springs and if hardened return springs will help the FPS. I discovered that the Hardened return spring from tac-edge does preform better than a regular return spring by about 20-30 FPS.

The link to that topic is here: Hardened v Tightened v Normal Return springs, what's the difference and what's better

I am using LDT MP5 (Metal gearbox) and with the replacement hardened return spring it’s doing anywhere from 170 - 190 FPS compared to a regular which is 140 - 150. Before my return spring broke its was 220 - 250. I would like to see if can get it similar or better.

I have a bit of a Theory that I don’t know is true and its kind of hard to explain with just words but I will try my best. So… If you are using a longer return spring that isn’t using the entirety of the coils to do its job could you increase the air seal (pressure) in the cylinder to then make the return spring use more of its coils making it work more efficiently. Idk if that makes sense so I can explain it again if you would like me to. I am not really great with this kind of stuff.

I’m planning to increase the air pressure with one of these, does anyone have any doubts about me using this:

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If it is Ldt and suits yes it will increase air seal, make sure to lube the nozzle stem and orings .
The brass as you can imagine are heavier than alloy and may require a heavier return spring to get it closed fast enough.
I make my own so can’t recommend a supplier.
Normally I would cut off the existing loop coil and re-bend the loop to add tension if I was to use a standard return spring.


ok cool, thanks for the info! I have been to afraid to cut off and re-bend the return spring as I don’t know if I could bend it too hard and it break after being used in the gearbox. Should I not worry?

On another note will higher air pressure then affect the ROF or just the overall strength of the motor. The motor looks like it is pretty fancy but it has no label, although I think its a high torque. Should I worry about changing the motor/ROF if the air pressure increases.

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The rof is just how fast the system cycles and it has little to do with the air pressure

The motor you have with it is the black can with red cap? I find them pretty good but they are no neo motor

The harder springs are fine like I said before but tend to break, depends though on your setup harder spring helps overcome sealing issues. Softer ones last longer and are softer on everything making all components last longer. That goes for your main spring too.

The Tac edge ones you mention I have not used so cannot vouch nor deny so take what I say as I dunno. :rofl:


Yeah that’s the one. No clue about its specs or anything though. I guess it will do if it’s just air pressure then.

Yeah I found that the coils were larger in the hardened but the overall tightness between them was actually worse than the og return spring. Wish I could find another one of those but I wouldn’t really know who would be selling specific return springs. I don’t know if it was you or someone else who said to also try “Tightened” return springs from GBU.

M4A1 has a good cylinder head nozzle kit aswell. The brass one is a very tight fit in the ldt cylinder but the below works quite well.

After you get back to 220-250fps change out the spring for a ausgel 1.18 should get you around 300-310. That’s all I’ve done to done and added a hop up. Probably the most accurate blaster I have.

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Ah ok so its pretty much the same thing but a better fit. I will give it a look, thanks for pointing it out, I didn’t even know it was a thing. Do you think that this will get the FPS up to around 220 or do you think I will have to do some extra.

Also can the Motor keep a decent ROF with that spring. I’m curious as I will maybe end up changing out the spring for something a little weaker than that because my field has a 300fps limit. I’m trying to reach for around 280-290. I just don’t know what the motor in it is capable of but essentially I’m going for somewhat high FPS and ROF.

Do you have the full metal version or nylon with metal gears version?

Getting either the brass or the one I recommend cylinder head will be a tighter fit then the nylon one that’s comes with and while it’s apart you should look at replacing the the o ring on the piston head. From memory I used the SHS black o ring from M4A1 but maybe get a green aswell and see which one gives you a better seal. I think the standard spring is equivalent to a M80 which should keep you under 300.

Agreed…… seems strange to only be getting high 100’s to low 200’s in a metal LDT MP5 that were originally punching out 280+ :thinking:

Certainly sounds like a simple air seal check of all components should have it sitting at the desired FPS without having to change any major parts other than a couple of o-rings :+1:

Do you still have the blowback feature connected? If so, try taking a FPS reading with the cocking handle locked back. Apparently it shoots harder that way (haven’t tested though). I have a metal LTD MP5SD and with an o-ring change and minor spring upgrade, it shoots an easy 320.


@arc and @DocBob Yes I do have the metal gear box version. I only ever got 250 with it though. When I was replacing the return spring I checked the seal and even added some silicone to the O-ring. It’s doing 170fps at the moment so I would say that the return spring is definitely the main cause of the problem.

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I guess that could make sense since the gearbox is what makes it work. I will have to try that with reading next time I can.

I would say that something hasn’t been assembled properly if you had 250 before you opened it up, and now have 170? :thinking:

I have the all metal LDT MP5 from WAT and put in a 7.3mm stainless barrel, stainless steel Tee piece, one piece stainless cylinder/head, green piston O ring, alloy nozzle with twin O rings, sweetheart fixed hop up, 1.18 spring and upgraded Mosfet (for the mag prime function but found after soldering it in I was sent the wrong one with no mag prime).
It runs 300 to 310 FPS consistently and is my straightest shooter at the moment but the standard motor I am running gets very hot.
It went into safety mode and shut down on me today at Donnybrook towards the end of a game after several long bursts of sustained fire. It is my back up blaster anyway and I was only using it as my primary HPA M4 was out of air.
It was running about 270 FPS out of the box so I just looked where I could improve the air seal to squeeze more out of it, however tightening the tolerances I found has created more friction and heat.
I recently had a faulty set of SHS gears put in that shredded during the first magazine after installation and it is also very fussy on the gels it likes to feed, it has spent more time on my workbench or with a tech than on a field so am seriously considering converting it to HPA.
It is a great CQB gun when running though.
Hope this helps you look at what might work with your build.


Awesome build specs and you are only a few little steps away from the last couple of upgrades to make this beauty a good reliable AEG without having to spend a motsa on HPA.

Sounds like a motor modification along with the correct wiring and battery supply should have your heat issues sorted pretty easily :ok_hand:

Hi DocBob,
Thanks for the feedback.
I’m running a quality 11.1v battery and the wiring was upgraded with the mosfet install.
What motor would you recommend?

Hey @Girds ……
I always ran Chi Hai motors.
Good performance at a great price.
The Chi Hai Red or Purple motors run rare earth magnets and are a great balance between speed and torque.

Torque is important for bigger springs and less stress on your motor/wiring/batteries.

High speed motors are for speedy boi builds with lighter springs/short stroke gearboxes depending upon how they are built for the desired output.

With your 1.18 spring, you can afford to step up to a purely higher speed motor, if RPS is your only desire, which I think is not really required in any long barrel type blasters, as this can create a whole new world of issues in something that you are already happy with the performance.