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Semi PKM conversion (no vendor kit)

8K views 31 replies 6 participants last post by  kingofbeer 
#1 ·
I figured I'd start a new thread to focus on design specifics and semi conversion challenges.

For now, I'm toying with a sheetmetal shell that will need considerable bending work to get right - needs the rails bent, the lower "dimple" for the feed pawl arm, and I don't have any internal rails or the "ratchet plate" for the charging handle. I may throw some money at coldsteelsolutions if I can't figure all these details and use one of his sheetmetal shells instead. Time vs. money.

I figure I may be able to fabricate my own "ratchet plate" out of some steel tubing heated up and bent outward to the shape I need. Otherwise I'm not sure how best to go about fabricating this part.



About the internals.

I found a drawing of that Sig rifle I was thinking of - the STGW.57 It has a "firing lever" that the hammer strikes against. *Exactly* what I was envisioning!

Note how the hammer - #14 - strikes against the firing lever - #11.



This was found at the swissrifles site:

The Stgw
 
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#2 · (Edited)
Pookie, I went ahead and started a BIY PKM thread as well, but it's over on TheAkforum.com. I have been redesigning the firing pin striker mechanism. It's been keeping me busy. I had to scrap the idea of the tension spring trolley. There was no way to attach the tension springs forward of the bolt because of interference with the bolt.
I looked the PK receiver and carrier and screwed around with my idea. It would work but it's not something I'm crazy about. Too much fabrication and lots of small precisely made parts. On the other hand, I thought about your idea and the information you posted and went over the receiver and carrier fit for about an hour. I think your idea is the better one. Provided hammer can hit the "L" shaped transfer strut hard enough to set off the primer. I think I can modify an AK hammer to work ( if it wasn't so damn hot and humid in the garage I would be out there right now). You could keep the spur on the bottom of a full-auto AK hammer and that could interface with a modified PKM trigger group. I can see the whole thing in my head, out of battery firing could easily be prevented.
 
#3 ·
This has some nice information on it too. You can clearly see the shape and size of the hammer. You can also see the hammer strut with the coil spring as well as the hammer-strut-tubular-guide all of which would be very useful and easy to make. The coil spring and hammer strut reminds me of a Colt Python hammer spring setup.
 

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#4 ·
I just got a Russian PKM technical information and repair manual. (A couple of them from different years) It doesn't have a lot of dimensions but it does have rivet sizes, counter sinks and which rivet heads are suppose to be flush. It has some angles but very few measurements for the full auto fire control parts. Not very useful for semi auto but might help some layout for semi auto parts.
 
#6 ·
Do you mean the pistol grip? If you have the wooden one the metal piece at the top is usually pressed on the wood. Sometimes not tightly but sometimes the wood swells. The service manual shows there should be a 0.5 mm gap between the bottom outside edge of the metal and the wooden grip.
 
#10 ·
Okay I wondered. I thought you might have meant the trigger housing.

The operator's manual shows that better than the armorer's manual but it's not a lot of help other letting you know you have all of the parts. If I could read Russian I might be able to get more from it than just the diagrams.
 
#8 ·
Not too much working getting done this weekend on the PKM project with Mother's Day and all that. I did get a chance to work on it last night at about 11pm. I sawed the "ears" off of an AK hammer and about 4/5ths of the axis cylinder on one side. The modified hammer fits with plenty of clearance between the left side of the carrier and the receiver wall. I left the disconnect tab on the top of the hammer so that a modified AK disconnect can be incorporated into the design later. Looks like there is enough room in there to get this done. Pictures to follow on the hammer modification.
 
#9 ·
I had the trigger out once to remove the sear. As I recall the detent just needs to be rotated so it will clear the housing. Mine was really REALLY gunky so you may need to clean it to see how it removes :)

I'm also going to toy with some scrap on the mill today to see about making a striker plate assembly (for the firing lever). Hope to have it done by Tuesday ;)
 
#15 ·
Looks like good progress. I haven't had time lately to mess with mine. I'm still debating if to use a hammer or an Uzi type striker. I hear the commercial semi auto's use an Uzi type striker but trigger pull is worse than the open bolt full auto. I would think a striker should be no worse than releasing the bolt. I'll see what sort of system I can design. A hammer looks good as long as it can clear everything and not get damaged in recoil.
 
#16 ·
hcpookie came to my house this week and we had a very enjoyable and productive afternoon and evening. We shot down some design ideas and came up with some new ones. The last one pookie threw out as he was walking to his Jeep was the one I think I will try first. Dual springs to the right and left on the bottom of the receiver to take place of the one large manispring with the hammer coming up through the middle of the two springs to hit the back of the bolt/firing pin. I have to find two 1/4" 10lb springs about 8 inches long.
 
#18 ·
I have made some progress, the PKM mainspring pull back tension is 25.1 lbs. and a single AK spring pull back is 15.1 lbs. So what I am thinking is 2/3 of of each AK spring which will be about 10 inches which should be about 20 lbs. of return force, should be plenty to run the PKM carrier with the added weight of the spring attachments and additional friction. I will install 2 spring "buckets" which will be about 4 inches long, through the rear trunnion into the rear stock, which you will not be able to see from the outside.
In the third picture you can see the dual AK recoil springs and the AK hammer in the middle with plenty of clearance. I think it will have to be mounted with an axis pin running from right to left through the receiver just like an AK hammer is mounted. There is room between the bottom of the receiver and the bottom of the carrier to fit a modified AK hammer. When it gets laid back the hammer recesses back into the trigger housing.
 

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#21 · (Edited)
This is how I am making the dual mainspring keepers, these will get welded to a thin plate of 4130 which will mount on the back of the carrier.The two cut AK spring retainers have gotten welded on to a plate that attaches to the back of the carrier. They are angled out to lay into the bottom corrners of the receiver and will recess into two holes in the stock. This provides more than enough room for the hammer to work just like an AK hammer but the hammer will need to be just a little taller.
 

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#26 ·
Thanks Jerry, I will need two spring "buckets" that AK recoil spring barely fit in. They need to be about 4" long and very thin walled. I paln on going to Lowes later to find some kind of tubing, Im thinking 1/2" conduit turned down to a thinner wall it will also need a small lip on one end.
 
#27 ·
PKM rivets

It looks like only the long rivets are swell head rivets.

The flat head rivets have a counter sink under the head similar to a swell head rivet. The counter sink is shown as "90 degrees" which is in American measures would be 45 degrees from center. The flat rivets are the ones that fit flush in their locations.

.............................

Long rivets (swell head)

shank diameter = 5.0 mm

swell head is 1.5 mm high / long at 45 degrees from center

round head of swell head long rivets is 2.0 mm high past swell angle, radius is unknown at this time, round head diameter is 8.0 mm (+0.5 / -0.2), length of rivet from swell head - round head parting line to other end is 53.0 mm , round head adds 2.0 mm, usually radius of round head is shown but I don't see it for the long rivets. Round head is 8.0 mm diameter and 2.0 mm high. The aprox length of rivet after fitting to PKM is 40.0 mm

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The short round head rivets have no swell necks.

Lengths do not include round head.

Rivet 1 is 10.0 mm long, 8.0 mm diameter head, 5.0 mm diameter shank, head radius is 4.2 mm, head height is 3.0 mm

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Rivet 2 is 12.0 mm long, head dia = 8.5 mm, shank dia = 5.5 mm, head radius = 4.2 mm, head height = 2.5 mm

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Rivet 3 is 12.0 mm long, head dia = 8.0 mm, shank dia = 5.0 mm, head radius = 4.2 mm , head height = 2.5 mm

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Rivet 4 is 8.5 mm long, head dia = 8.0 mm , shank dia = 5.0 mm , head radius = 4.2 mm , head height = 3.0 mm

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Rivet 5 is 10.5 mm long, head dia = 7.1 mm, shank dia = 4.0 mm, head radius = 3.8 mm , head height = 2.4 mm


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Rivet 6 is 5.5 mm long, head dia = 5.3 mm , shank dia = 3.3 mm , head radius = 2.9 mm , head height = 1.8 mm


That was the plain round head rivets.


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Flat head with counter sink / swell rivets

Flathead rivet 1 is 11.0 mm long including the head, 45 degree counter sink swell is 2.5 mm long, hed dia = 10.0 mm , shank dia = 5.0 mm

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Flathead rivet 2 is 9.0 mm long including head, 45 degree swell is 2.5 mm long, head dia = 10.0 mm , shank dia = 5.0 mm

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Flathead rivet 3 is 6.0 mm (+1.0 mm) long including head, 45 swell is 1.5 mm long, head dia = 6.0 mm , shank dia = 3.0 mm

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Flathead rivet 4 is 9.0 mm long including head, 45 degree swell is 1.8 mm long, head dia = 7.6 mm , shank dia = 4.0 mm

That's the flat head rivets with counter sink.

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#28 ·
The next step is to mock up a new stock so that I can build a jig to drill the spring holes in the stock. The mock-up will be just a block of wood about 5 inches long and the same inner and outer dimensions as the stock. I will only get one chance to drill my real Russian PKM stock.
 
#29 ·
hey look at me 5000 posts! :)


Alright. Thinking more about the dual-spring setup, I got my receiver shell out and toyed around some.


I think the dual-spring setup will work BETTER than my Swiss-style striker-plate.

If dual-springs are used, they will have some MAJOR problems to overcome. Namely, there is not enough space to compress an AK-style recoil spring before it bunches up and prevents the backward motion of the carrier.

I think I figured a way around that.

Looking closely at some of my pistols, there is a spring oprod design that uses two tubes one nested in the other, and two springs - a smaller spring nested on the smaller tube, and a larger spring around both the larger tube AND the small spring. Both springs work together and move about halfway each - the oprod (recoil rod?) moves completely back. A very clever design!

Based on that dual-spring setup, I mocked up this conceptual in Visio:






This design would let both pieces of spring be shorter than the overall travel, and yet should provide enough travel to let the recoil cycle completely.

I discovered that a VZ-58 striker spring and an AK recoil spring should work together in this design. I can lathe up some tubing and a section of steel rod the guide rod pieces. The smaller spring will require an 11/64" size guide rod. 3/16" won't do it. I may get some drill rod as I've had problems finding this size of rod before. I don't know if the VZ guide rod is available, but if so that would work as-is.

There is perhaps 1/2" or so that can be cut from the bottom corners of the buttstock so that the springs have a bit more room to compress. Any further and you'd expose the spring.



SO.

This should give us some room to use an AK-style action, although the hammer will need to be longer. Perhaps a FAL-style action could work? US-built FAL parts are available and that would be three US parts right there.


Build update -

I received my plate steel in the mail so I can make a bending jig of sorts for the upper rails! Hopefully this weekend I can get some rails bent!

I still need to figure the best way to bend that "spine" on the rails. Seems like I'll need to bend the rails OUT to about 90 degrees, then place a wedge of some sort inside so that I can bend the rails all the way in. I am concerned that my receiver shell doesn't have enough metal to make the "spine", so I may abandon it.

After looking at some more close-up pics of the receiver, the "spine" on the outside of the rails is not always present - seems some countries or perhaps newer models don't put that extra work into the receiver. Maybe I'll just bend them like AK rails and be done with it.
 
#30 ·
If dual-springs are used, they will have some MAJOR problems to overcome. Namely, there is not enough space to compress an AK-style recoil spring before it bunches up and prevents the backward motion of the carrier.


This should give us some room to use an AK-style action, although the hammer will need to be longer. Perhaps a FAL-style action could work? US-built FAL parts are available and that would be three US parts right there.
I have the same problem with a .45acp carbine, at first I cut the double hook trigger down to a single hook, and cut the unused sear surface off of the hammer, and most of the other one, mostly to reduce locktime... Then I started thinking, and then thought that the dual recoil springs would be a good idea, so then I came up with using unmodified AR15 HTS set, the FAL parts are more expensive and use a plunger type hammer spring, they would be harder to use, so I'm gonna use the AR HTS...
 
#31 ·
Pookie there is vert little room between the rear of the carrier and the front of the rear trunnion when the carrier is in the full backstroke position, maybe an inch and a half. However, you have about 4 inches that can be bored out of the rear stock before you get into the void. I plan on making use of that.
One other thing I was looking at is that maybe you can use the PKM spring but just relocate it off to one side of the carrier and drill a hole through the rear trunnion and into the stock about 4 inches deep. The total compression length of the spring would only be reduced by 2 inches.
 
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