HK Ump45 Teardown

Casing:
We’ll start by removing the barrel stabiliser so it isn’t flexing when we pull the box out. Rotate anti-clockwise until it slides out, you may need to give it a bit of a wiggle.

Next, push the two main pins out of the case - Take a size 4 Allen key and gently but firmly push until it slides out. You can also remove the pin for the stock but it doesn’t really matter.


With the pins out we can remove separate the two halves of the gun. Gently pull the lower half of the gun from the top. Be mindful of the barrel and wires, make sure they don’t catch on anything.

Gearbox: Use a size 2 Allen Key to remove the 6 long screws from the casing - but be careful. If they’re tight, push down firmly and slowly begin to turn. For the sake of your mental health, don’t strip the heads.


The only things left connecting the gearbox are the mag connectors and trigger switch. Both magazine terminals pull upwards and out of their housing - make sure to mark which is which, one of my wires had heat-shrink on it. I’d suggest a piece of tape if nothing else.

Now the trigger switch. It’s held in place by a plastic cover and will unclip by pushing the latch inwards. Make sure to calculate it’s launch trajectory, there’s a good chance you’ll never see it again. The switch will then lift upwards and out of the way.

Gearbox:
The first half of the of the teardown is done. Now onto the box - We’ll start by removing the motor with two black Phillip Heads - then it’ll slide straight out. Unlike the rest of the screws - these two are held in by two nuts on the other side - try not to lose them.

If you’re looking to replace the motor (optional), remove the other two Philips head screws to separate the motor from the casing and re-solder the wires to the new motor.

Now for the gearbox - before you do anything, remove the spring. Take a massive flat head to the back of the casing, push down a couple mm and turn 90 degrees. Gently begin to withdraw the screwdriver.

That only leaves the 8 Philips Head screws keeping the box together. I believe they’re a size two head, use one of the flatter heads though. They’re cheap screws and will strip if you’re not careful. Remember some of the gears have little plastic washers on them - but they should stick to the plastic with the grease.


You can remove the barrel and t-piece at this point.

It doesn’t really matter what order you do things in now so in no particular order, we’ll take the trigger out - it is sprung loaded so remove it slowly.

Next I’d take the cylinder, plunger and tappet plate out - be mindful of the return spring.
It’s a good idea to grease the inside of the cylinder every few months.

The only thing left is the gears, it’s also a good idea to grease these every few months.
Remember, when you’re pulling the lowest gear out, there is a little spring loaded niblet underneath to stop the gears back spinning. If you forget about this you’ll either hear a loud whine from the motor when you let off the trigger.

Congratulations
You’ve completely disassembled your UMP45 and JinMing Gen 8 Gearbox.

Now just do everything in reverse to put it back together. But first here’s a few tips:

To reseat the gears, you’ll need to start with middle one, then the lower one and the little niblet, then the upper one.

[b]For the niblet[b/]: Ensure the position of the spring is the same as pictured below, place the niblet into its respectful housing and push the end of it so it’s facing as far downwards as possible, hold it there with a fingernail or screwdriver, then put the gear in. You’ll know you’ve gotten it right when you can turn it one way but not the other - when you’re testing it, push down on the metal pieces to keep them in place.

When putting your cylinder and plunger back:
Line up the two holes in the cylinder head with the two standoffs on either side of the case and make sure the plunger ligned up with the rails on either side of the case


Trigger (Be patient, it may take you a while):
Below is a photo of how it should look when you put the spring back in. You’ll know you’ve got it right when it doesn’t fall off gravity.



To put it back in, align the right angle of the spring with the opposing right angle of the case, keeping it there: wiggle the metal piece into its correct hole - avoid playing with it until you have the switch back in and the plastic cover back on.

That’s all, feel free to ask questions and leave feedback. I’ll try and reply as fast as possible.

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Having just finished building my UMP, this post was a brilliant guide, thanks for putting this up :smiley:

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So it’s your standard gen 8 box by the looks of it. What FPS did you manage to get out of it?

Got the silver wire kit in, turnigy battery, alloy barrel, green o-ring, and as far as FPS no bloody idea as I havent bought a chrono yet

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At the time I was getting just under 200fps. Although I’ve thrown a ChiHai 37,000rpm motor and 1.4 Spring, Green o-ring. Full metal all round with bigger battery and beefed up wiring.
I haven’t had a chance to chrono it because I’m having troubles with my spring guide moving in the gearbox housing and bending my springs…

Is the spring guide metal with bearing or the original nylon/plastic with no bearing?

Heavier springs need the bearing at both ends (in the guide and in the piston) so the spring can rotate freely as it cycles… if one end doesnt have a bearing, the spring will wind up until it bends.

Have you tried a 1.3 spring? Is your 1.4 spring constant or uneven? noidea

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