[Kahn2112]

Good Morning! I'd like to thank the Admins and the members of this forum, it's simply awesome. I've been lurking for a while and finally jumped in. Sorry to have my first post a "newbie" question, but I'm very confused I bought a couple flats with no weld kits. They are bent and now welded less the weldment kit. I spent hours filing my weld marks off and generally sharpening up some edges. I'm pretty happy with the bend and the shape from what I'm able to see of others available and I've just put a light coat of paint to keep the surface rust off.

Question 1-- I'm not sure who the maker of this flat is, I don't think it's an LSC flat because the holes for sling attachment are not predrilled in the mag well and the cocking tube notches are not present on the LSC. Can anyone ID this flat so I know in the future?

Question 2-- The flat has a cocking tube notch already. Am I supposed to cut that out flush and slide the new cocking tube in that area then weld? If so, what kind of measurements are required. I've searched and can't really find a definitive answer. I'm supposing I'd use the new cocking tube to measure and cut the notch out.

Thank you all so much for your help and answers. With these answers, I can start ordering the other parts and get this build a little further. Hoping to have done before Thanksgiving Day! edit---Christmas Day!

 

[Kahn2112]

Any help at all would be much appreciated

 

[21HK]

Question 1-- I'm not sure who the maker of this flat is, I don't think it's an LSC flat because the holes for sling attachment are not predrilled in the mag well and the cocking tube notches are not present on the LSC. Can anyone ID this flat so I know in the future?

Sorry, no help with this.

Question 2--With this flat, are my options an MP5, or MP5K, or MP5K reverse stretch or MP5SD? Can I make any one of these or am I just limited to MP5? I see LSC offering a flat specifically for the MP5K and now I'm confused. I thought the receiver was the same for all with differences being in the cocking tube, trunion, barrel, bolt group, etc. and not the receiver itself.

I really think you need to take a step back and examine the details on the MP5 variants, especially the the receiver and cocking tube, so you'll have a better understanding of what your getting into. I think you got ahead of yourself....how is your receiver set up, one or two holes?

View attachment 68849

MP5 variants

http://www.hkpro.com/forum/hk-nfa-talk/232666-how-differentiate-mp5k-mp5.html

The Submachine Guns

Question 3-- The flat has a cocking tube notch already. Am I supposed to cut that out flush and slide the new cocking tube in that area then weld? If so, what kind of measurements are required. I've searched and can't really find a definitive answer. I'm supposing I'd use the new cocking tube to measure and cut the notch out.

There is a bit more to it than that.
http://www.hkpro.com/forum/hk-clone-talk/177514-hk-cocking-tube-gap.html

 

[Kahn2112]

Sorry, no help with this.


I really think you need to take a step back and examine the details on the MP5 variants, especially the the receiver and cocking tube, so you'll have a better understanding of what your getting into. I think you got ahead of yourself....how is your receiver set up, one or two holes?

View attachment 68849

MP5 variants

http://www.hkpro.com/forum/hk-nfa-talk/232666-how-differentiate-mp5k-mp5.html

The Submachine Guns


There is a bit more to it than that.
http://www.hkpro.com/forum/hk-clone-talk/177514-hk-cocking-tube-gap.html

Thanks for the help! Appreciate you very much!!! Sometimes a guy just needs the obvious pointed out - not BS replies (although very amusing) and mis-information about being a "premium" member.

21HK - You are a saint!! I have two holes and I see them clearly on the MP5K now. After all I've read, it never occurred to me that the MP5 and the MP5K receiver flats were different. Also understand this flat is an MP5K flat which is what I wanted for sure. I can order some parts. As far as the cocking tube - I appreciate that thread very much. Helped out a lot.

 

[Socalmp5]

It's a special weapons k flat. A lil bit of work needed on these compared to lsc, pm me, I'll give you some advice, my first build was a k with that same flat.

 

[Discogodfather]

The MP5k uses a different bolt carrier (which is shorter) and the cocking handle is also about .75" back from where it would be on a regular MP5. SW made these flats so you didn't have to buy the very expensive factory style cocking tube (which is built like any HK cocking tube with the detent integrated into the tube not the receiver). Todd's idea was that you could then find a G3 tube and cut it down and then butt weld it to his flat. He also made this awful plastic cocking rod that came with the kit.

One option is to just cut out the part of the receiver (literally cut away the cocking handle recessed area about .75" from the edge) and then just weld in an actual MP5k tube. This is what I would recommend, the tube is expensive but it will work. The other route is to get a G3 stub tube (which is cheap) and cut and grind that up into something that might work.

I used the LSC Mp5k flat which required the cutout and then welded the factory tube in. It's difficult and you can see by my welds that it challenged me, lol. Great rifle though and it has not failed on me in thousands of rounds.

 

[Kahn2112]

Thanks so much for your answers!! That helped a TON!!

Special thank you to SOCALMP5 and DISCOGODFATHER for passing on some very useful tips!!

 

[Kahn2112]

I got around to working on this yesterday as I received the German cocking tube, trunion and barrel. I began by grinding/ shaving the SW cocking tube recess down to the bolt carrier tube and test fitted the cocking tube as I went along. I went back just enough to match where the SW cocking tube was on the flat originally. After a while, it slid right in snugly!


Fitment looks good to me - what do you think?



No binding inside on the bolt drop. Carrier slides smooth.


I know the Trunion needs to go in a hair more - I'm just dry fitting at this point. One thing I learned for sure is the trunion has to go in before the cocking tube can. I was thinking of tack welding cocking tube in place, but after dry fitting - that's a definite no-no. The ledge on the back side of the trunion really caused me trouble inserting the cocking tube. That's when I took a harder look at the "receiver side" of the cocking tube where it's inserted and found that the upper left was mis-shaped causing the tube to cant down and to the right just enough (when fully inserted) to cause it to bind on the ledge. Once I got that round again - the tube slid right over the ledge very snug. I used a 20mm piece round stock I had to reshape the cocking tube and that worked out very well.


And here's where I'm at right now. Yes I know that's the wrong trigger housing for the MP5K


Here's where I'd like to be:


Here's some misc parts I need / looking for:

Rear Stock....researching how to attach that rear stock (A3 I believe). I'm looking but cannot find a definite answer how to fit to MP5K
Paddle Mag - the whole kit. I've bought the jig from HKParts
Mag release
magazines
Triple tree
cocking tube cap
correct lower housing - would love a pictogram

 

[Discogodfather]

Nice progress, but the image links are not coming up for me (seems like a photobucket maintenance thing, I will check back). Sounds like the exact same issues I had, the cocking tube is a tight fit against the trunnion and has a tendency to want to tilt upward. I can't say I solved that problem because my cocking tube is tilted upward a bit, maybe a degree. Maybe sanding down some of the material and more tack welding would have made it more straight. I also had a tough time fitting the cocking rod, this has been the case with many other HK builds I have. I ended up making an aluminum rod that goes into the cocking tube and presses against the inside of the receiver to act as a guide and a heat sink for running the weld bead. That has helped alot.

A trick I also use for better fitment is to get a very long drill bit (maybe a .375" 10" bit) and wrap some sandpaper on the end (I use the adhesive backed sandpaper) and then wrap it until you get a nominal diameter, then put it on a power drill and use it like a lapping device to help smooth the transition from receiver to cocking tube and eliminate and burs, weld slag, etc. on the inside.

 

[Discogodfather]

This is a great tool to have in the box- a simple aluminum rod that is tuned to the cocking piece diameter and the receiver diameter. It acts like a heat sink during welding and also aligns the tube perfectly the the inside diameter of the receiver. I lived without this one on the few builds and can't remember why I didn't male one sooner, it makes life much easier. I made this one for a tight fit, I have to hammer it a bit to get it in and out.

 

[eodinert]

The rear stock you show won't work on a K...that's a reverse stretch. A reverse stretch uses a standard MP5A3 stock.

 

[Socalmp5]

You're on your way!

 

[Kult]

What diameters did you machine the rod to?

This is a great tool to have in the box- a simple aluminum rod that is tuned to the cocking piece diameter and the receiver diameter. It acts like a heat sink during welding and also aligns the tube perfectly the the inside diameter of the receiver. I lived without this one on the few builds and can't remember why I didn't male one sooner, it makes life much easier. I made this one for a tight fit, I have to hammer it a bit to get it in and out.

 

[Discogodfather]

Diameters are .7300" and .7840". I think it varies between tubs and receivers, so it helps to be able to get a custom fit.

 

[Kahn2112]

Well I've made a tool to press my barrel in....I think I've gotten pretty close, just looking at the roller positions. I'm waiting on my feeler gauges to come in. What do you think? Am I a little too high just based off the rollers not seating in the "valley"?

The picture is taken thru the mag well I marked the center of the rollers on the BCG to get as close as possible.

Also looking for the two rear plates at the rear of the receiver. If I have to cut them out of 16ga sheet metal and bend them that would be ok too.

Last question (today!) - what about the insert that holds the pin at the top rear of the receiver? I'm thinking of using the insert that would normally go on the bottom, welding it in at the top and cutting away the rest from the inside. Or, would a better plan be to cut off a little and weld these as "ears" and file off the inside as required? I worry about alignment for the pin that way..... What's normally done here?

 

[Kahn2112]

Well I got my feeler gauges in and I was off! Way off. .030 or better. So back to the press I went. I got down to .020 pretty easily and was really hoping not to overshoot my .018 mark but the dreaded POP came in just when I didn't want it. I overshot it down to .014. I started over and I remembered reading somewhere on here that if you tap on the trunion while under pressure from the press that it might move just a hair. That's what I did. After I got close I put under pressure and tapped for 3-4 minutes. Worked beautifully. Got me right into .018 after just three checks. I fitted back into the receiver and that tiny bit of difference actually made the holes line up better for the hanger off the cocking tube. I can actually see now that the triple frame will have clearance for the cap on the cocking tube. I was getting worried that tolerance would be way too tight to work without shaving off somewhere. Now I don't think I need to! Also, the BCG has closed the gap on the visible side. Now I can just barely see the bolt head and the carrier fills up the ejection side nicely.




See how much more into the "valley" the rollers are on the right picture? That's where I needed to be.....



Few More Questions...........

Can anyone to chime in on the two rear plates at the rear of the receiver? If I have to cut them out of 16ga sheet metal and bend them that would be ok too, what do the experts do?

What about the insert that holds the pin at the top rear of the receiver? I'm thinking of using the insert that would normally go on the bottom, welding it in at the top and cutting away the rest from the inside. Or, would a better plan be to cut off a little and weld these as "ears" and file off the inside as required? I worry about alignment for the pin that way..... What's normally done here?



Last question (today!) - Has anyone engraved the bolt carrier on the visible ejection side? Just a thought....

 

[Chopstix Kid]

I got around to working on this yesterday as I received the German cocking tube, trunion and barrel. I began by grinding/ shaving the SW cocking tube recess down to the bolt carrier tube and test fitted the cocking tube as I went along. I went back just enough to match where the SW cocking tube was on the flat originally. After a while, it slid right in snugly!


Fitment looks good to me - what do you think?



No binding inside on the bolt drop. Carrier slides smooth.


I know the Trunion needs to go in a hair more - I'm just dry fitting at this point. One thing I learned for sure is the trunion has to go in before the cocking tube can. I was thinking of tack welding cocking tube in place, but after dry fitting - that's a definite no-no. The ledge on the back side of the trunion really caused me trouble inserting the cocking tube. That's when I took a harder look at the "receiver side" of the cocking tube where it's inserted and found that the upper left was mis-shaped causing the tube to cant down and to the right just enough (when fully inserted) to cause it to bind on the ledge. Once I got that round again - the tube slid right over the ledge very snug. I used a 20mm piece round stock I had to reshape the cocking tube and that worked out very well.


And here's where I'm at right now. Yes I know that's the wrong trigger housing for the MP5K


Here's where I'd like to be:


Here's some misc parts I need / looking for:

Rear Stock....researching how to attach that rear stock (A3 I believe). I'm looking but cannot find a definite answer how to fit to MP5K
Paddle Mag - the whole kit. I've bought the jig from HKParts
Mag release
magazines
Triple tree
cocking tube cap
correct lower housing - would love a pictogram

Your cocking tube is not far enough back. The proper inset is approx 0.875" from the leading front edge of the trunion.

In case you did not know, the "where you'd like to be" gun was built by us It is a reverse-stretch. The gun you are building is a K-gun. You will not be able to attach an A3 retractable stock (unless you Frankenstein your gun).

 

[Discogodfather]

The tabs and insert at the top are really nothing special but I was able to find them in a kit, so no fabrication. Very easy to make the top insert to factory spec on a lathe, it's basically just a stepped insert that protrudes about 1/16
" outward on each side. The tabs for the rear of the receiver are fine in 16 or even 18 gauge. They are structurally important because any stock that fits inside the receiver is going to put a lot of outward pressure and deform it without the tabs (with a ton of force). If you need exact dimensions let me know.

 

[Kahn2112]

My cocking tube is a little over 3/4. Looks like I'll be filing a little more this afternoon. How critical is the cocking tubes position or length from the leading edge of the trunion? Being off 1/8" effect the cycle? I'd like make it as close as possible of course . I need to get this straightened out before I weld it.

I love your reverse stretch--looks great and I really like the A3 retractable stock. I read a lot and it seems it's nearly impossible to attach to a K. So guess what me next build is?

I figured I'd have to make those rear support plates. Kind of looking forward to it actually !! Might do something a little more than just the plain plate. Might do some engraving there.

As for the rear pins- I have a lathe, but I think I'll go with my original idea with the tube I already have. Just need to work it a little.

 

[Discogodfather]

On an MP5 cocking tube length is not particularly critical as there is no lockout claw in the mechanism. If you compare it to a G3 or a 33, there is no "lever" just a stub with a little ball detent to keep it from flapping about too much. The problem is not going too big of a gap but too small. If the carrier is at all impeaded and there is no clearance to the coking tube the action will not fully go into battery, and you will also get false bolt gap readings. Most people don't want a big 1/8" gap either because it feels too loose. I try to go for a minimal gap of around .020-.030" but some say .050-.060". Again, like most rollerlock problems, this one will not effect the gun cycling and seeming to work.

If you get stuck in a build where there is not enough of a gap then you can turn the coking tube down a bit to get the right gap. If you went too loose and you need to make up some space then you can always weld in a section, but bear in mind that this is a hardened part and you are going to anneal it. This could lead to wear problems down the road.