The Kalashnikov rifle is assembled with tight dimensional control of many items. The barrel and its associated parts are assembled with interference fits and exacting alignment. The headspacing of this rifle is a very exacting operation.
“When a man’s only tool is a hammer – everything looks like a nail!”
Well, ain’t that the unfortunate truth? It breaks my heart to see costly rifle parts abused due to poor fits and poor tools and techniques for disassembly and assembly.
There is much speculation and blabber from internet know-it-all’s regarding the proper interference fits for these parts. What I will show below is information lifted from original Izhevsk blueprints, so, let’s look at the block and journal tolerance classes of a Soviet era AKM rifle:
Front Sight Block Bore Front Sight Block Journal
0. 14,465mm-14,480mm 0. 14,485mm-14,505mm
I. 14,500mm-14,515mm I. 14,520mm-14,540mm
II. 14,515mm-14,525mm II. 14,540mm-14,555mm
III. 14,580mm-14,600mm III. 14,605mm-14,620mm
Gas Block Bore Gas Block Journal
0. 14,965mm-14,980mm 0. 14,985mm-15,005mm
I. 15,000mm-15,015mm I. 15,020mm-15,040mm
II. 15,015mm-15,025mm II. 15,040mm-15,055mm
III. 15,080mm-15,100mm III. 15,105mm-15,120mm
Rear Sight Block Bore Rear Sight Journal
I. 17,500mm-17,525mm I. 17,530mm-17,555mm
II. 17,525mm-17,545mm II. 17,555mm-17,575mm
III. 17,585mm-17,605mm III. 17,615mm-17,635mm
Breech Block Bore Breech Block Journal
I. 22,980mm-23,000mm I. 23,025mm-23,045mm
II. 23,000mm-23,020mm II. 23,045mm-23,070mm
III. 23,065mm-23,085mm III. 23,110mm-23,130mm
Note! Some Countries have as many as four (04) tolerance classes and not all agree on the values of each class. Suffice it to say that Tolerance Group II is the most prevalent class.
So; from the data presented above we can deduce that the proper values for the interference fits are:
Front Sight Block: 0,005mm (0.0002”) – 0,040mm (0.0016”) interference.
Gas Block: 0,005mm (0.0002”) – 0,040mm (0.0016”) interference.
Rear Sight Block: 0.030mm (0.0012”) – 0.055mm (0.0022”) interference.
Breech Block: 0.025mm (0.0010”) – 0.065mm (0.0026”) interference.
You cannot make measurements at this level of precision with a cheap Chinese digital caliper or even an expensive Swiss digital caliper. This level of precision requires the careful use of a micrometer and at the very least a telescoping gauge or a dial bore gauge. Blocks are often stressed from demil operations and can be a bit out-of-round so many internal readings at different points must be taken sometimes.
When fitting these parts my suggestion is to aim for the somewhere between the low limit and the middle limit of the interference fit value if possible.
Working on a RPK, AK74 or Krinkov? Just apply the same fits to the particular areas of those rifle assemblies.