06-16-2009, 8:56 PM
Join Date: Mar 2008
Location: Contra Costa County
Now we move right along and take barrel - trunion measurements.
NOTE 01 - factory kit Yugo barrel and trunions are just right and do not need to be fitted. Fact is the M70AB2 virgin kits have all parts - RSB, Lower Hand Guard retainer, Gas block and FSB matched up and they dont need to be trimmed / turned down. Using another trunion (milled receiver in this case) may need fitting.
NOTE 02 - You will often see numbers stamped on your parts (FSB, BG, RSB, Trunion) ranging from 1 to 4. These are production limit control numbers and have nothing to do with quality or accuracy as some believe. This method of grading production is very common in any mechanical production shops. It tells the operators when to change out old tooling so the product remain within tolerance.
1 is for under-size, 2 for standard, 3 for over size and 4 for way over size. Parts were grouped accordingly and assembled.
The factory/Arsenal also mixes and matches parts so you may end up with a 1 and 2, its no big deal. This does not take away from your value or originality.
Here my measurements from the trunion come in at .8985"
And the Barrel at .9075. The difference is just too great.
Ideally the barrel should be turned in a lathe between the centers (using live center) so you may also check that the outside diameter is concentric. BUT we have to do this without a lathe.. so here goes..
We need to bring the difference between the barrel and the trunion/receiver down to .003 from .009. (The barrel stays slightly over sized). We do this by enlarging the trunion hole! Here are our $6 fixings that will replace a lathe.
Double sided tape
Drill bit anywhere between 1/4" - 1/2"
Using the material above, fashion yourself a drum sanding device that will barely fit into the trunnion. And sand away for 10-15 seconds at low speeds periodically checking the diameter... till you are done.
The sanding will self center as it moves as you are using free hand. It is important you do this with a hand held drill to make certain the material taken off is uniform!
Now that the trunnion and the barrel are within limits, we lube them up
Now you may start to press the barrel in - Have your extractor cut positioned right, eye blinking later will not help. You will have to press out and start over..
The smoothness of the barrel pressing will depend upon the tonnage of your press and how tight the fit between the barrel and the trunion/receiver is.
If you are using a 12T press and the barrel "bounces" forward instead of creeping (despite the use of lube), the fit is a little too tight. Or you may just use a heavier press. If your barrel-trunion fit is too tight (should you decide to use a heavier press say a 40T), you risk cracking your your trunion.
Cryogenic cooling and then pressing does not help. Also it may cause rust over time, as the moisture in the atmosphere freezes on the cold surface right away.
Headspacing is adjusted in fractions of a MM. If, with each bounce you move .5mm or more, it is no good.
Last edited by fredieusa; 07-04-2009 at 9:24 AM..