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TM MP5K

#1 User is offline   aotsukisho 

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Posted 18 May 2020 - 01:57 AM

Got this older Marui MP5K for a low bid cause why not. Would be interesting to have a full auto capable AEG SMG sidearm.

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Surprisingly it came in the original box with most if not all of the original paperwork and such. NiCd battery, old school speedloader, Tamiya to mini-Tamiya adapter for chargers, and two MP5 midcaps. The MP5K lowcap mag was not included, but I don't mind.

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Chrono shows that this MP5K has seen some better days. Not sure if it was the compression or whether this is standard for Marui way back when, but I'm getting 200fps with 0.20g.

14.9rps is actually very good for a stock AEG on 7.4v lipo.

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I thought it was failing compression due to age, but look how consistent those readings are! 2% variance on FPS. I believe this chrono session was two bursts of full auto, hence the kind of high variance on RPS.

This will be a fun AEG to work on. I don't have much experience on v3 gearbox yet, but it would be cool to see how far I can take this with modern parts.
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#2 User is offline   aotsukisho 

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Posted 18 May 2020 - 05:10 PM

Cracked it open last night.

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I'm surprised how much detail that went into molding the plastic body of this replica.

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View from the other side.

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Popping off the front pin to remove the handguard/grip

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I believe this is an M4 set screw threaded through the front assembly, which is metal

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Exposing the inner barrel, and the spring holder for the charging handle. The bar going across is only held in place by the front assembly, so I will remove it for now to not lose it.

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Motor plate coming off, this one is v2 style since the MP5K does not use a motor cage like the G36 or AK series.

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Tokyo Marui EG700 motor, still going strong after all these years

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Two more push pins and the rear plate comes off

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The lower push pin is actually a screw assembly for reasons that we'll see in a bit

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I removed these two screws but they are unnecessary for takedown. Fuse holder also pops off

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Don't forget this screw for the fire selector, needs to come out in order to open up the gun

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Fire selector removed

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Lower receiver can be wiggled off

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View from the other side. MP5K unique selector plate, as full size MP5 replicas use V2 gearboxes.

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Yanking out the gearbox shows a huge buildup of grease from the cylinder port, this will be cleaned off before continuing

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Heavily adjusted photo to show the second prong, but these are the contacts for the battery connector up front. I do not have a screwdriver that fits under there, so I will use my soldering iron as they are threaded into plastic.

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All wires removed for a freshly empty shell

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Mag release lever hinges on a brass tube that surrounds that false takedown pin mentioned eariler.

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Hop unit out, and the hilariously short inner barrel is seen.

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Removing the base of the gearbox, it is a replacement for the motor cage normally seen.

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Holding down the cylinder with my screwdriver so it doesn't blow up. Looks fairly standard for a Tokyo Marui gearbox

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I noticed that the pickup tooth was noticeably thicker than the other teeth, and had a strange wear pattern on it. The top was extremely shiny, I am guessing from being polished from the second tooth on the piston coming into contact with it.

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These gears will need to be replaced, they feel like aluminum and have these crimped on bushing spacers rather than the shafts being machined separately.

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Complete parts teardown. I will probably have to replace the inner barrel with a tighter bore to increase FPS as much as possible, as I'm not sure a stock decade-old Marui gearbox will handle the required M160 or so to get this thing up to 400fps

Parts are on the way!
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#3 User is offline   aotsukisho 

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Posted 19 May 2020 - 04:48 PM

Small updates from last night

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Comparing MP5K tappet plate to standard SHS v3

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Replacement spring guide, Marui stock one is all plastic

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Learning from past experiences - loctite thet tip of the spring guide on if you don't want to have to tear down the gearbox to do it later

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Replacement, more durable cylinder head

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Cleaned out the gearbox shell for bushing gluing

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The front assembly has a strange inner barrel stop, which looks like it will block any use of inner barrels longer than the stock 110mm. I will see if I can drill this area out or something to gain a slight length gain. If I can have an inner barrel extend all the way to the tip, that will gain me 8mm...

I still think it's hilarious this thing only has 110mm of inner barrel to work with. This is shorter than the G&G ARP9 and even the standard 5.1" 1911 or Hicapa
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Posted 23 May 2020 - 12:53 PM

Been working consistently on this project, I am not sure which direction I wanted to take it but the reality is any MP5K build will be limited by the batteries it can use. I am having trouble finding high-current capable li-ion or lipo cells that can fit in the 17.3mm diameter battery compartment. Perhaps I will find a good battery down the road but for now I will have to consider alternatives.

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This is a comparison photo I've taken to show just how short the MP5K inner barrel is. From top to bottom:

Tokyo Marui MP5K - 110mm brass
G&G ARP9 - 125mm aluminum
Krytac PDW - 155mm brass

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SHS bushings for this, I didn't need FLT as this won't be a super serious build

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Glued in, using the stock Marui gears to align them in the hole.

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Grinding off tooth 2 and part of tooth 3 for AOE correction

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CA glued the rack back in the piston. I'm not sure what's up with this batch of SHS/GMT pistons but the rack doesn't sit fully flush with the piston head mating surface.

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Radiusing done on the front of the cylinder window. I've heard V3 gearboxes don't need this but can never be too safe. I will have to run a heavy spring in this gun to try and pop out bbs with that tiny barrel.

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Polished the brass cylinder, will be reusing this one in the final build especially since I don't have another 50% ported cylinder.

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GMT 13:1 gearset going in. I'm pretty sure these are just the next batch of SHS gears, they look and feel the same just they arrived coated with less oil than before.

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Testing function of the piston with the shell closed, it slides back and forth effortlessly on the gearbox tracks. Very nice

I don't think I will be using a Gate Titan in this gearbox, so I can start shimming the gears and getting it sealed back up. I think I will have to use an external battery on this so I am able to use the space underneath the front handguard to hold an external MOSFET unit. I have not decided if I want to go with a small simple one like the Gate NanoAAB or just use one of the multiple Warfets I have laying around. Either way, wiring it up will be the same with the trigger bypass.
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