Jing Gong Mp5k Gearbox Rebuild Help

[Jimbodini42]

Hi

my Jg MP5k was pretty pap but I liked the full metal body so I decided to upgrade the hop, re grease the gears and put a higher velocity spring in as my FIRST effort into tech AEG. 

Long story short I cocked it up and was a nightmare for over a week of trying to get trigger spring back in and re assemble the gearbox. 
 

However Now I’ve got it rebuilt it fires once on semi, then sticks and trigger won’t pull

back. Flick it onto full auto and it fires normally and then allows it to fire on semi, but just once again. Repeat. 
 

Two questions with this - 

1) How can I solve this and can I do so likely without having to take gearbox apart again? 

2) If need to sort gearbox, would buying a new gearbox likely insert back in be an easier method? 
 

Thanks in advance 

 

[Zarrin]

I believe I had a similar issue the first time I reassembled the Galaxy mp5k - for me it was the positioning of the trigger spring - there are two ways the spring could naturally sit, one was correct and ensured the trigger and switch block returned as it should, the other, possibly the way you currently have it installed caused issues. Unfortunately I've been in lock down away from home (and all my gear ?) so i don't have it to hand to take pics or experiment for you.

The problem is the mp5k v3 trigger and switch block assembly is kind of notorious for being crappy - i'm wondering if there is a problem with the switch block canal or even a weak trigger spring itself. Though I would start with reviewing the installation position of the trigger spring. I know you said it was a nightmare getting it back together though I would imagine you have learnt quite a bit and second time in shouldn't be as bad.

 

[Jimbodini42]

Thanks for input! 

Can I check I’m not being dumb - 

If I were to obtain a fully functional second hand gear box that is assembled and has had a mosfet fitted then it would be a simple task of putting in said new gearbox and it should work? 
 

 

[Samurai]

If I were to obtain a fully functional second hand gear box that is assembled and has had a mosfet fitted then it would be a simple task of putting in said new gearbox and it should work?

If it was made for the same brand and same model then you have 80-90% that it will fit.

If it wasn't then 10-20%.

 

[Jimbodini42]

Cheers. 
 

What makes you say 80% if it’s same model? More out of curiosity than anything 

 

[Samurai]

Airsoft parts are made to the same tolerances as airsoft meters to real meters.

But unfortunately a small size difference can make a great performance difference, and there are no standard specs for them. Everyone just makes a somewhat similar part that they make fit with the factory parts of the same gun, but even their own next iteration of the same part can be slightly different. They make it on a different machine, different supplier even, different material and that adds up to the 10-20% all parts combined.

 

[Rogerborg]

Throwing my 2c in here for posterity.  Yes, these V3 two-piece MP5K triggers are a mare.  In this case there are several things it could have been:

A mis-fitted trigger spring that isn't pulling the trigger forward properly.  The short prong goes into a hole in the gearbox shell, the longer prong lies along the trigger, between the two pieces on top.

View attachment 59698View attachment 59699

Fitting these things is a bit of struggle, and it should be a struggle. If it's not, then something is wrong with the spring fitment.

The spring may simply be inadequate - I carefully unwound mine a fraction to get a stronger pull, but that then makes it even harder to fit.

Or, it could be slop between the two trigger pieces preventing the main trigger from pulling the 2nd piece back and down enough in order to pull it behind the trigger trolley.  There's loads of slop in mine, it's hardly precision engineering.  You could, if it came down to it, try using JB weld to add material to the part that slides into the 2nd piece to make it a tighter fit, but you'd be talking about very small amounts.

Alternatively, it could be the trolley return spring pulling it off to one side and making it harder for the trigger to reset behind it - if you look at the back of the trolley, you'll see there's a section in the middle where the 2nd part of the trigger is supposed to engage.  If either of them are offset, it'll be problematic.

This is how the trolley return spring should (IMO) be, with the ends perpendicular to each other.

View attachment 59700

However, in my Galaxy MP5K, the ends are  not like this, they are parallel.  This makes no sense as the post and the trolley hook are perpendicular, but it's what I've got.  This means that the spring wants to pull the trolley to one side, and of course I didn't take a note of which way it was fitted before disassembling, so it took a few tries to guess which way worked best when re-assembling.  Also, one end is definitely smaller than the other, and won't fit on the post, which I assume is deliberate, but can't quite figure out why.

Finally, I know it's considered anathema to get grease anywhere near the contacts, but I felt compelled to lube the trolley in mine as it's so problematic.  Indeed, out of the box the trigger was sticky and didn't want to reset on semi, so I sprayed silicone lube in without disassembling, which sorted it.

If all this was done to fit a mosfet, I'd suggest doing it from outside the gearbox, which is just about possible. One contact is fully exposed, the other can be got at, or just attach to the existing wire.  Unless you really need to go into these gearboxes, I'd recommend avoiding it.

 

[Zarrin]

Throwing my 2c in here for posterity.  Yes, these V3 two-piece MP5K triggers are a mare.  In this case there are several things it could have been:

A mis-fitted trigger spring that isn't pulling the trigger forward properly.  The short prong goes into a hole in the gearbox shell, the longer prong lies along the trigger, between the two pieces on top.

View attachment 59698View attachment 59699

Fitting these things is a bit of struggle, and it should be a struggle. If it's not, then something is wrong with the spring fitment.

The spring may simply be inadequate - I carefully unwound mine a fraction to get a stronger pull, but that then makes it even harder to fit.

Or, it could be slop between the two trigger pieces preventing the main trigger from pulling the 2nd piece back and down enough in order to pull it behind the trigger trolley.  There's loads of slop in mine, it's hardly precision engineering.  You could, if it came down to it, try using JB weld to add material to the part that slides into the 2nd piece to make it a tighter fit, but you'd be talking about very small amounts.

Alternatively, it could be the trolley return spring pulling it off to one side and making it harder for the trigger to reset behind it - if you look at the back of the trolley, you'll see there's a section in the middle where the 2nd part of the trigger is supposed to engage.  If either of them are offset, it'll be problematic.

This is how the trolley return spring should (IMO) be, with the ends perpendicular to each other.

View attachment 59700

However, in my Galaxy MP5K, the ends are  not like this, they are parallel.  This makes no sense as the post and the trolley hook are perpendicular, but it's what I've got.  This means that the spring wants to pull the trolley to one side, and of course I didn't take a note of which way it was fitted before disassembling, so it took a few tries to guess which way worked best when re-assembling.  Also, one end is definitely smaller than the other, and won't fit on the post, which I assume is deliberate, but can't quite figure out why.

Finally, I know it's considered anathema to get grease anywhere near the contacts, but I felt compelled to lube the trolley in mine as it's so problematic.  Indeed, out of the box the trigger was sticky and didn't want to reset on semi, so I sprayed silicone lube in without disassembling, which sorted it.

If all this was done to fit a mosfet, I'd suggest doing it from outside the gearbox, which is just about possible. One contact is fully exposed, the other can be got at, or just attach to the existing wire.  Unless you really need to go into these gearboxes, I'd recommend avoiding it.

Good diagram up top explaining the trigger spring dude -  when I first reassembled mine and encountered issues, I think i had the bottom prong sitting in front of the trigger as opposed to down the middle where @Rogerborg has the arrow pointed. I took it apart again, reassembled as above and it solved the stuck trigger issue. As indicated above, this bit is by default really fiddly and it's actually far easier to position it incorrectly than it is to get it right.

 

[Jimbodini42]

Thank you both! It’s in pieces in the garage but last day of work today before the summer so I’ll be taking this advice when I get it out next week! ??‍

 

[Rogerborg]

I think i had the bottom prong sitting in front of the trigger as opposed to down the middle where @Rogerborg has the arrow pointed.

It's not obvious where it's supposed to go.  In fact, it feels like it doesn't want to go in the right place, and you have to apply a fair bit of pressure and twisting to get it in.

Just to be clear, the short prong goes into this small hole, above and to the front of the hole for the main trigger piece.

View attachment 59723

Thank you both! It’s in pieces in the garage but last day of work today before the summer so I’ll be taking this advice when I get it out next week! ??‍

No worries, I just want to you know that it's not you, it's the design.  Plenty of us have struggled with these.  This guy goes to the bother of fitting an optical mosfet to deal with the sloppy MP5K trigger pull, only to wreck it when re-fitting the trigger.