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Posted: 1/17/2008 8:35:03 PM EST
[Last Edit: 4/3/2008 12:47:02 PM EST by myfakename]
Whether you are putting a new one on to replace an out of spec part or to add a compliance part, the process is pretty easy for a homebuilder. You need a drill, 3/16" and 1/8" drill bits, hammer, and anvil or bench vise (although a chunk of steel would work fine), center punch, file, and of course a new piston and gas piston rivet.

Here is the carrier and old piston. The hardest part of this project, for me, was locating the piston rivet. I took some emory cloth to clean it up and eventually found the old pin.

Here is the rivet marked. It was oriented horizontally with the carrier.


Next step is to center punch it and drill the concave rivet portion out. I used a 3/16" bit to take the top off.


Once it is drilled, punch it out and unscrew the gas piston.




Clean the threads in the carrier and screw the new one on. It may take some effort. Mine did so you may want to chase the threads first. I clamped it in some padded vise jaws and turned the carrier onto it. Note: the new one did not wiggle like the old one which may or may not have an effect on operation. I personally like the wiggle. Added: after firing several rounds the piston regained its' wiggle. Joy!

I left a small gap (like the original) between the piston and carrier. Now you have to drill a new hole in the gas piston. Clamp it in your vise (use soft jaws or a rag) and make sure you drill it straight so you hit the hole on the other side (that's what she said)

Once it is drilled, put the new rivet in, flip it over (new rivet head down on the flat of the vise) and start hammering. You want the new rivet to peen over into the hole. Flip it over and check the other side.



Once it is flattened enough, filling the concave holes, grab your file and blend it into the carrier. It should almost disappear.


When it is smooth, use some sandpaper or emory cloth and clean it up a bit. As a final touch, I hit it with some cold blue.


Now if only my barrel pin was as easy

mfn


Link Posted: 1/17/2008 9:48:49 PM EST
Wow. You've just made my life easier. Thanks.
Link Posted: 4/2/2008 11:46:36 PM EST
great write up! i know some people dont mess with them because they are not sure about it.....think this will help with that
Link Posted: 4/4/2008 9:32:15 AM EST
thanks mods for making this a sticky.....i am sure it will help some other people out there
Link Posted: 4/5/2008 12:09:04 PM EST
Great thread, thanks a lot. The only part of this that would worry me is drilling the hole through the piston nice and straight.
Link Posted: 4/6/2008 8:47:36 PM EST
Thanks for the help! Only addition I could add is to drill half of the retainer hole in the piston, flip the assembly over and drill the balance. I did this because my ability to drill straight can be called into question and this gave me a minor margin of error!
Link Posted: 6/27/2008 9:23:34 AM EST
I was just wondering about this.

Link Posted: 9/3/2008 6:17:43 PM EST
[Last Edit: 9/3/2008 6:21:56 PM EST by AC12]
So, where do you get the rivet? I have done everything else. USA grip. USA trigger parts. Thanks, AC

Pic
Link Posted: 9/3/2008 6:22:08 PM EST
I wondered how the hell to get that apart.



Thank you
Link Posted: 9/20/2008 2:48:46 PM EST
What is the thread size in the carrier where the piston screws in ? I need to get a tap to chase my carrier.

Thanks
Link Posted: 9/25/2008 12:10:39 PM EST

Originally Posted By AC12:
So, where do you get the rivet? I have done everything else. USA grip. USA trigger parts. Thanks, AC

Pic


Also wondering where to get the rivet.

Thanks.
Link Posted: 9/26/2008 10:18:02 AM EST
Link Posted: 10/12/2008 9:26:37 AM EST
This tutorial is pretty decent, but it's lacking a few subtle but (in my mind) important details. I would like to amend it with the following information:

First, to answer a previous question - the gas piston / carrier are threaded for 12mm x 1.25. You might want to run a tap into the carrier to ease the installation of the new piston (most likely, the carrier is slightly out-of-round from the original riveting process).

Now on to the missing details.. it's worth noting that while the holes in the bolt carrier are ~0.120" (3mm) the hole through the gas piston is actually ~0.140" (3.5mm). This difference in size exists to assure that the piston is able to float (wiggle) when installed. And yes, the float is important - it helps to compensate for misalignments resulting from manufacturing tolerances. DO leave a gap between the piston and carrier as in the original installation - 1/2 turn back out from flush is fine.

So - make your first drilling (through carrier & piston) with a 3mm (#31) drill. Then, remove the piston and enlarge the hole to 3.5mm (#27 or 9/64" drill). Pass a 1.25mm 'thread restorer' file (or a die) over the threads to remove any flash from the drilling. Now reinstall your piston and rivet / finish as above.

Tip: Before enlarging the hole, use a #2 center drill (aka combined drill and countersink) to put a slight chamfer on both ends of the hole in the piston. This will reduce or eliminate the tendency of the #27 / 9/64" drill to catch and bind up as it enters and exits the hole. Enlarging a hole puts a lot of stress on the edges of the drill flutes; we want to keep the abuse to a minimum.

Hope this is helpful.

-L
Link Posted: 1/15/2009 9:06:20 PM EST
[Last Edit: 1/15/2009 9:08:11 PM EST by Gunplumber]
good points made in the last post - I'd like to add too that the size of the gap is important. The overall length of the bolt carrier and piston measured from the L side impact surface is important. the gap is 0.4mm max, with OAL 294mm min and 295 max. Max deflection is 5mm from ceneterline, but I've never seen one nearly that loose.

Maybe someone can check my math here

1.25 mm per revolution = .8mm in 360 = .4 mm in 180 degrees = .2mm in 90 degees.


One problem is if the face of the bolt carrier isn't perfectly perpendicular than you can get different readings at different spots. I just use the OAL measurement and 294.5 usually equates to 1/8 turn. I did 1/4 turn before I got the gauge, which mathematically seems to put me in the middle of the .4mm max, but is longer than the 294.5 gauge midrange. Maybe its a function of the TAPCO piston length.






Link Posted: 1/15/2009 10:42:32 PM EST
Originally Posted By AC12:
So, where do you get the rivet?

Pic


Ive used a regular nail...Find one that fits the hole, cut it shorter...then peen over one side...then the other side. No need to buy a rivet when you probably have a box of nails just sitting there in your home..
Link Posted: 1/15/2009 10:54:05 PM EST
Originally Posted By Fishpaw:
Originally Posted By AC12:
So, where do you get the rivet?

Pic


Ive used a regular nail...Find one that fits the hole, cut it shorter...then peen over one side...then the other side. No need to buy a rivet when you probably have a box of nails just sitting there in your home..


I just don't see how this works.. and I've tested it out at one point.

Nails are made of hot-worked tempered wire, this is why they don't easily collapse (or mushroom all that much) when struck with a hammer. I tried to peen over several different nails (not in bolt carriers) and they easily resisted the levels of force that I'd be willing to apply.. especially when valuable parts are close by.

On the other hand, the available rivets are easily beaten into shape with a series of light, well-placed blows. But the ones supplied by AKbuilder do need to be shortened quite a bit before setting. I do this in a minute on the bench grinder.

-L
Link Posted: 1/16/2009 8:57:49 AM EST
Originally Posted By Gunplumber:

Maybe someone can check my math here

1.25 mm per revolution = .8mm in 360 = .4 mm in 180 degrees = .2mm in 90 degees.


Maybe I'm misunderstanding, but a 1.25mm pitch advances by 1.25mm in one revolution.

So, 360' = 1.25mm / 180' = 0.625mm / 90' = 0.3125mm

Also, I'm unclear on what you meant by "max gap is 0.4mm". Is this the gap between the front of the carrier and the flange on the piston - i.e. the gap that is opened when the piston is backed out from flush?

Interesting points on the OAL of the carrier / piston. Are the 294/295 figures from a blueprint, or did you derive them in the course of your own work? I've always done the 1/2 turn out and pin, and haven't ever detected any functional issues. But I'm going to begin checking this dimension, now that you've mentioned it. Then of course, the position of the gas block also comes into play..

-L

Link Posted: 1/16/2009 9:35:09 AM EST
[Last Edit: 5/29/2009 9:45:06 AM EST by Gunplumber]
Maybe someone can check my math here
1.25 mm per revolution = .8mm in 360 = .4 mm in 180 degrees = .2mm in 90 degrees.

Maybe I'm misunderstanding, but a 1.25mm pitch advances by 1.25mm in one revolution So, 360' = 1.25mm / 180' = 0.625mm / 90' = 0.3125mm.

oops - I plugged the math in as if it was an english thread - revolutions per inch = revolutions per milimeter,

Also, I'm unclear on what you meant by "max gap is 0.4mm". Is this the gap between the front of the carrier and the flange on the piston - i.e. the gap that is opened when the piston is backed out from flush?.

yes


Interesting points on the OAL of the carrier / piston. Are the 294/295 figures from a blueprint, or did you derive them in the course of your own work?.

Various manuals, John Baum is still translating this one for me, as my german is poor, but I can pick out key words.

Anwendung und Bedingungen zum Prufen der Lange und des Spiels am Gaskolben in der SchloBfurug MaB: 295+1 mm

Vorrichtungs, - Werkzeug - und LehrensatB (VWL) Nr. 1 fur SchutBenwaffen


Link Posted: 5/28/2009 10:16:56 PM EST
Awesome thread. This should be tacked.
Link Posted: 5/29/2009 5:50:14 PM EST
AC12, et al,

I have about 60 rivets left if anyone needs them. I bought a box of 100 1/8" x 3/4" stainless steel countersunk solid rivets from McMaster-Carr, because I was having problem searching the AK sites for them. (Why they don't have optional "mechanically inclined buyer" piston kits that include the rivet, I'll never know?!?!) I did my piston and had 99 left. I gave a bunch to my local FFL, who helps me out quite a bit. The rest are just sitting here awaiting use. If you need one e-mail or IM me. Price: Free to a good home!!! Just remember me if you have any good parts kit deals, and maybe give me first right of refusal.

Bigun
Link Posted: 6/2/2009 6:27:51 PM EST
Originally Posted By 2ndAmendican:
(...) I bought a box of 100 1/8" x 3/4" stainless steel countersunk solid rivets from McMaster-Carr, because I was having problem searching the AK sites for them. (Why they don't have optional "mechanically inclined buyer" piston kits that include the rivet, I'll never know?!?!) (...)


Curtis at AK-builder.com includes a rivet with the purchase of a gas piston. Loose rivets were $1.25/ea last time I checked, but he tends to give price breaks on rivets to owners of an AK-builder rivet press.

-L

Link Posted: 6/9/2009 6:03:33 PM EST
Just put a U.S. piston on my carrier and it was easier than I thought. I tried to get the gap to look the same as the original, but I'm not sure how accurate it was. I need to take it to the range to ensure smooth operation. Thanks for the write up!
Link Posted: 9/18/2009 5:40:00 PM EST
I just used this to change the piston in my MAK90. I just measured the OAL of the entire carrier and threaded the new piston in to match the old length. The TAPCO piston was pretty hard to drill through, but the results were great! Took the MAK out and gave it a torture test today and it worked flawlessly.

Thanks!
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