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Old 06-29-2007, 01:37 AM   #1
DeanD
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Default AK component sizing info

I have seen several threads lately where people had barrels that did not fit their trunnion. Usually the barrel is loose. Be aware of the following info:

AK parts are gauged at the factory with go/no-go gauges and coded for nominal size, under-size, or oversize. The components are then matched for assembly. Number is usually on the top of the trunnion where the rear sight base sets. If my memory is right a 2 is nominal, 1 is under-size, 3 is oversize, 4 is way oversize. I have only encountered a 4 one time.

If you have a "1" trunnion you can lap the hole and install a "2" barrel. If you have a "2" trunnion a "1" barrel is generally ok but only a moderate-to-light press fit. If you have a "3" trunnion and a "1" barrel you are screwed, it will be loose. If you have a "1" trunnion and you try to put a "2" or "3" barrel in it you may crack the trunnion or just gall things up or whatever. Most barrels and trunnions are 2 or 1 that I have seen but I have heard 3's are frequently encountered.

Barrel will generally have 4 numbers stamped in sequence. They are gauged for reference in the following order for sizing and mating: FSB, gas block, RSB, trunnion/receiver. Some of the older front sights have a "1" or "2" on the back side.

I just had the same problem with a new/surplus FSB for a Bulgarian barrel. It was a 3 and a loose slip fit on the 2 barrel. Called a "spinner". I also had a problem when I ordered a couple of new barrel pins from K-Var and they were sloppy loose in the trunnion I needed one for. Same with the small pins for the barrel components, I have a zip bag with a bunch of them and they mike out from .1165"-.1205". The nominal size is about .1175" but I don't recall exactly off the top of my head.

Offhand I don't think all parts are stamped with a number code but barrels generally are. They may have sorted the barrel components in boxes or somthing without stamping them. The rules still hold.


Last edited by DeanD; 06-29-2007 at 01:53 AM.
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Old 06-29-2007, 11:19 AM   #2
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great bit of info!

A while ago I had a batch of ten kits that I neglected to electro-pencil number the barrels to the trunions before demilling, and I had a heck of a time getting everything matched up again. I noticed the numbers on the barrels, but couldn't figure out what they meant.


Where did you find this out?
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Old 06-29-2007, 03:18 PM   #3
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I was told about it years ago from a gunsmith and I have only run into one other person who knew about the manufacturing process and size codes; Jimmy Streetman of AUSA/Global Trades.

It was developed by the Russians of course. They were not only able to effectively mate parts and minimize scrap but utilize the statistical range of variance in the tolerances and still maintain a mathematical model that equated to an AK-47 that would operate in the extreme environmental conditions it could be subject to in combat. Pencil, paper, and slide rule no doubt in those days but as we all know they came up with one hell of a rifle that always works. Not all by accident.

I don't know all the details of this coding and I can't always find a number. Undoubtly there were deviations in habits and processes in the many licensed countries and manufacturers. But it all helps and is fun to know. Some pics:

An old Bulgarian AK-47 barrel, 1-2-1. Only three codes because it was a screw-in barrel. There are some other marks and there may be a size code in one of them that equates to the chamber depth. There is a 9751 and some other little marks also...might be factory or inspector, date code, no telling.


This is the bottom of the gas block. Note the "2".


This is the front sight. Faint partial "1".


This is a 1981 Polish PMKMS barrel. Has all four numbers because it it a press-in. I didn't look hard and it is painted but I didn't see the numbers on all the components. I think I always see a trunnion number on top as I recall.

Last edited by DeanD; 08-03-2007 at 01:21 AM.
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Old 06-29-2007, 03:22 PM   #4
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If anyone knows any other info on this please post it. I may have something cross-wired. I made a mistake one time before so you never know.
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Old 06-29-2007, 03:22 PM   #5
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Thats a really nice piece of info to know!!! Thanks for sharing ur wisdom!!!!!!!!!!!!
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Old 06-29-2007, 03:33 PM   #6
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well, That is worth knowing
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Old 06-29-2007, 03:40 PM   #7
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With your permission, DeanD, I will STICKY this.

(and delete this post requesting your blessing)
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Old 07-13-2007, 10:17 PM   #8
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Awesome guys! Probably more than I need to know, but one can never tell! Thanks for the excellent posts.
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Old 07-13-2007, 10:50 PM   #9
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There may have been changes in use over the years.

I have one Polish PKM that has "2 18" vertically under the upper HG, but no other numbers. Also, a Romy "G" with no numbers whatsoever. The Pole was built in 1978 and the Romanian in 1984.
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Old 07-17-2007, 12:14 AM   #10
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I had a new Romanian virgin barrel and it was blued with no numbers also. Guess they figured we should own a mike or we shouldn't be messing with one of their barrels. I think the barrels tend to be numbered more consistently than the components do or they may have identified the components some other way in some countries.

I just received a nice Russian milled Type 3 kit in the mail today and the barrel and components are numbered and so is the bolt (front of the right lug). My Polish milled PMK bolt is numbered also I noticed.

I need to get all my AK's and parts kits out and see what has what.
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Old 08-05-2007, 12:21 PM   #11
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dean, that is some great info, thanks for posting
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Old 08-18-2007, 11:34 PM   #12
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Ive got a Bulgarian ak74 kit with a 4 barrel and a 4 trunion.
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Old 10-01-2007, 02:40 PM   #13
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damn good info!
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Old 10-01-2007, 03:04 PM   #14
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I have been noting the numbers on the Romy's while I've been demilling the parts kits and found a barrel pin with a "2" on it.
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Old 01-13-2008, 07:06 PM   #15
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Just checked my barrel and trunnion for the hell of it. Trunnion is a 3, barrel has a partial two stamp on it, and on the RSB there is a 2 stamp. There are no numbers in sequence though.
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Old 10-16-2008, 07:20 AM   #16
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great info, thank you
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Old 11-12-2008, 03:30 AM   #17
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Guys I deleted the pictures and forgot about this thread. I will take some more and post them as soon as I get time.

The numbers are handy, I just bought a 1977 E. German trunnion and it has a "2" stamped on top. Kool, because most barrels are a 2, the "correct" size.
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Old 12-25-2008, 10:29 PM   #18
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Quote:
Originally Posted by DeanD
...

It was developed by the Russians of course. They were not only able to effectively mate parts and minimize scrap but utilize the statistical range of variance in the tolerances and still maintain a mathematical model that equated to an AK-47 that would operate in the extreme environmental conditions it could be subject to in combat. Pencil, paper, and slide rule no doubt in those days but as we all know they came up with one hell of a rifle that always works. Not all by accident....
Once a while I come trough thread like that leaves me shaking my head.

Info given here is a basic knowledge in machine design and technology, used since forever in mass production, like automotive field (for example engine piston will be group selected by three criteria, weight, outside diameter and wrist pin diameter).
Keywords for you guys to search and educate yourself are "dimensional tolerances", "selection assembly" and "chain of dimensions".
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Old 12-26-2008, 07:32 AM   #19
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good info, thanks Dean.





Quote:
Originally Posted by WJ-Polish Guy
Once a while I come trough thread like that leaves me shaking my head.

Keywords for you guys to search and educate yourself are "dimensional tolerances", "selection assembly" and "chain of dimensions".
my favorite is "stacked tolerance". i would explain the concept after hearing " everything was in spec but the motor blew up."
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Old 01-03-2009, 11:23 AM   #20
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DeanD, do you happen to know any numbers associated with these tolerances. I have been writing down any measurements I get with the corresponding markings. I only have about a dozen and it doesn't seem like enough to make any conclusions.

-myers
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Old 04-23-2009, 09:09 AM   #21
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Default component sizing info per Ding

This clarification was provided by Ding:
The numbers read from top to bottom(muzzle to breech).1-2-1...1 being FSB,2 being the gas block,1 being the RSB. Some have 4 numbers, the fourth being the barrel number. If you look at your parts, the FSB, RSB, gas block they will have numbers stamped on them.. The FSB has it on the back area you see looking down the barrel. The gas block will have the number on bottom lug area and the rsb will have it stamped on the left side or on the journal ring.
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Old 04-23-2009, 09:43 PM   #22
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Durock, I think you mis-understood the question.

The question was not, what are the numbers, rather what are the tolerances associated with the numbers.

Ie, I have barrels marked 2-2-2-2, and some 1-3-2-1...

What is the difference in the barrel journal for a barrel marked 1 vs 2?

-myers
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Old 05-19-2009, 10:59 PM   #23
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I was always going to mike some parts and see what the differences are but just never have. I have built about all the AK kits I have had so I don't have much around.

Group sizing parts is nothing new and no revelation nor was it intended to be. The numbers on the parts signifying the sizing is good information to know though. I once had a trunnion that had the number signifying a larger diameter hole and I went to put a barrel in it and it just fell in. Could have saved money and a lot of headaches if I had looked at the trunnion before I bought the barrel, the number was right there on top.
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Old 11-30-2010, 10:01 AM   #24
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Quote:
Originally Posted by DeanD
I was told about it years ago from a gunsmith and I have only run into one other person who knew about the manufacturing process and size codes; Jimmy Streetman of AUSA/Global Trades.

It was developed by the Russians of course. They were not only able to effectively mate parts and minimize scrap but utilize the statistical range of variance in the tolerances and still maintain a mathematical model that equated to an AK-47 that would operate in the extreme environmental conditions it could be subject to in combat. Pencil, paper, and slide rule no doubt in those days but as we all know they came up with one hell of a rifle that always works. Not all by accident.

I don't know all the details of this coding and I can't always find a number. Undoubtly there were deviations in habits and processes in the many licensed countries and manufacturers. But it all helps and is fun to know. Some pics:

An old Bulgarian AK-47 barrel, 1-2-1. Only three codes because it was a screw-in barrel. There are some other marks and there may be a size code in one of them that equates to the chamber depth. There is a 9751 and some other little marks also...might be factory or inspector, date code, no telling.


This is the bottom of the gas block. Note the "2".


This is the front sight. Faint partial "1".


This is a 1981 Polish PMKMS barrel. Has all four numbers because it it a press-in. I didn't look hard and it is painted but I didn't see the numbers on all the components. I think I always see a trunnion number on top as I recall.

Could you "please" repost your pictures? They have disappeared from Photobucket and are not viewable anymore. Thanks for sharing.
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Old 12-22-2010, 11:37 PM   #25
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Hey guys, just took a look at the kit I got in the mail today and I have a "4" marked trunnion.

Barrel I have is a new US made barrel. It just slips right in

I am just SOL? I'd really like to build this 74 but don't relish the idea of buying another parts kit. I'm trying to stay on a budget.

The kit is a Bulgarian 5.45x39mm from Apex.
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Old 12-23-2010, 04:17 PM   #26
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You could have a barrel turned for you, because your trunnion does not match the nominal size you are limited in your options.

There are other "fixes" that I do not believe to be proper. I am sure someone will chime in.

-myers
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Old 01-27-2011, 12:54 AM   #27
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was looking @ some old pieces i had bought some years ago for a build, then noticed the FSB is wider (24.44mm)than the top of the trunnion (22.73mm) bottom of the bbl reads 1,2,1,SN sequence?, 1. trunnion reads 2...really new to this, have done a few ARs, and 1 FN Fal but 1st AK..any ideas?
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Old 08-05-2011, 12:13 AM   #28
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The "undersized" barrel journal can be carefully knurled only in the area where it truely makes contact with the bore of the trunnion. But know this - knurling alone is not the answer. The only correct way to increase a diameter slightly is to first knurl the part to increase its diameter (a medium diamond knurl should suffice) and then machine the higher points of the knurled surface true, round, and to the correct dimension. This can only be done with a lathe in the hands of a skilled machinist or toolmaker.

The interference fit between the journal and the trunnion bore should be between 0.0005" (medium fit) and 0.0010" (maximum fit). Since the knurled surface offers less metal-to-metal contact I would increase both limits by 0.0002" or so.

Done carefully and precisely - this will work and cannot be detected from outside the rifle. Gee - I've even seen knurled dowel pins installed in the front sight block or gas block.

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Old 08-12-2011, 11:04 PM   #29
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I just measured a batch of ten (10) virgin barrels of Romanian origin and compiled the results in an MSExcel spreadsheet. All of these barrels are marked 2-2-2-2 and were hand-selected as this is the target tolerance group. If anyone wants a copy of the spreadsheet - just send me a PM.

At room temperature with a 0.001mm division calibrated micrometer I determined that the average diameters of ten (10) samples are:

Trunnion Journal: 23,056mm
Rear Sight Block Journal: 17,567mm
Handguard Retainer Journal: 16,978mm
Gas Block Journal: 15,055mm
Front Sight Journal: 14,550mm

The journals of the barrel are ground on these barrels, and what surprised me are the errors of roundness as it was occasionally as much as 0,003mm. So, I went for the maximum material condition present.

It would be neat to get my hands on some "1's" and "3's" to see the level of variation, but I certainly don't want to purchase any of these.

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Old 02-10-2012, 01:07 AM   #30
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DeanD,
On original drawings АК, on some details there are tables. In these tables values of the sizes and number of groups are written down. If the worker makes the size of 1st group brand "1", and if from the second that "2".

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Old 02-27-2012, 02:44 PM   #31
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Wow, I'm getting ready to build a few kits and didn't know all this. I had thought that the trunnions and barrels where all the same dims.

What about these new US made virgin barrels that one can get? I just recently bought four of them for my builds that I'll be needing for. I'm guessing that I'd have to mic everything before assembly?

I know a guy that bought a few Polish kits with US made virgin barrels from Apex. Now, is he going to experience the same problem? He checked and found no markings on his barrels.

If can one can supply some answers, it would be much appreciated.
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Old 03-04-2012, 11:57 PM   #32
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To MyMalteseFalcon, yes, you should either measure all of your parts or test-fit them to some degree. I have purchased a few USA-made barrels over the years and they are machined to the class "2" tolerance group for the most part.

Many AK, AKM, and AK-74 parts are stamped with the tolerance grade: "1" undersize, "2" correct size, "3" oversize, and "4" seriously oversize. There is perhaps about 0.001" (0.025mm) or so difference between each size class. That does not sound like much; but it means a too-tight fit, a normal fit, or a "spinner" fit if the parts are not matched to the barrel journals.

I've never seen a USA-manufactured barrel marked with the tolerance class. An example of eastern European marking is successive numerals stamped above each other. The barrel I normally seek is marked with numerals "2". The numeral closest to the breech describes the breech and the numeral closest to the front sight describes the front sight. The numbers between these describe the gas block and the handguard retainer. One may see a "3" in the stack, or perhaps a "1" or a "4". That tells one the location and tolerance class of the particular journal.

I have seen barrel blocks that were not marked, and they measured and fit as "2" tolerance class.

Me? I precisely measure, record, and fit every barrel component I touch.
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Old 04-04-2013, 11:43 PM   #33
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This is a great thread and I'm glad I read through it. My SAR-1 has "2 2 2" stamped under the barrel, and the fsb and rsb both have "2" stamped on them. I can't find a stamp on the gb.
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Old 04-05-2013, 12:49 PM   #34
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If it is any help, my gb was stamped(2), very faintly, on the rear-underside portion on the rectangular flat area where the rear pin is inserted. Not sure the country of origin, since I built this last one from many parts I have purchased from multiple sources over the last year.
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Old 07-05-2013, 11:40 PM   #35
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I just got a bulgy 74 front trunnion that is a #4..... Fuck Me!!

They are out there, and this one did bite me in the ass

I'll pin it out at work to find the diameter and let you guys know just for the hell of it
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