0.629 gb ok for 0.631 barrel?

[devilfish13 6]

Hey all, this may be more of a machine shop question but here goes. I had a machinist here where I work ream out my Krebs combo block to 0.629" since they didn't have any larger reamers that weren't too big. With a cheapo digital caliper it's between 0.629" and 0.630", should I have them hone it a bit or should I just be able to mallet that on at home? I don't have a torch so heat isn't an option... plenty of oil though.

Edited September 27, 2012 by SquidKitten

[mancat 2,368]

It's probably going to be too tight. You can try the old trick of freezing the barreled receiver for several hours, while heating the GB in the oven. This causes one to shrink, and the other to expand. Hopefully it will change the tolerance enough temporarily to mate the two parts.

 

Otherwise, you may be able to bring the GB up to .631 with little more than some polishing work. An appropriately sized wooden dowel wrapped in finishing sandpaper (600 grit or above) should be enough to do this slowly to avoid runout or oversizing.

 

Stripping any finish from and polishing the journal where the GB sits will also help things go together smoother.

 

Use anti-seize when assembling to make pressing easier and to allow the block position to be easily changed when aligning to center. Once the component is installed and pinned, make sure to either heat the rifle or give it a healthy test firing to make the excess anti-seize leech out before applying any sort of finish.

 

On my Saiga, the pin channel on the factory block (where sporter HG anchors) was identical to the pin channel on the AK-74 GB. I literally put on the new block, the existing channels lined up, ran a 3mm bit through the channel to ream it, and pinned it. I think I got lucky, and most won't be this easy. For this reason I suggest when you have the factory block off, to mark the gas port position on the underside of the barrel. The replacement block will have a hole on the bottom where the 90-deg gas channel is drilled into the block. Make sure to align this hole over your gas port mark in order to ensure that the port is not occluded, or you can create an under-gas condition.

Edited September 27, 2012 by mancat

[devilfish13 6]

Yeah, I was afraid of that. I already talked to the machinist again, and he handed me a tool of which I do not remember the name, despite having been trained as a machinist myself seven or eight years ago... I'll post a pic later so you know what I'm talking about. He said to just run it in a drill and keep it lubed and it will do all the work. More or less same effect as the dowel, just with spring tension to keep it even.

 

I'm going to pick up some antiseize on the way home. Thankfully it's payday

 

My AK-74 FSB lined up like that too; it looks like the Bulgarians may have actually used the same specs as the Russians. The new fate of it is a little different, I've already removed the sight from it and ground it down so it's nothing but the m24 threads and the bayonet lug. I still wanted the bayo lug, despite wanting a combo FSB/GB for future SBR if/when Michigan SBR law changes.

[mancat 2,368]

Bulgarian and Russian AK-74 were identical in the early 80s. Same blueprints and tooling. Kalashnikov himself was quoted at one point to have said the Bulgarians made a higher-quality AK than Russia themselves. Fortunately for us the Russians never changed a whole lot after that.

 

Oddly enough Bulgaria dropped the AK-74 entirely. Even the commercial stamped SLR series are oddballs, having many internal AKM-spec parts such as carrier, bolt, front trunnion, while using external AK-74 dimensions, parts and layout, all manufactured on the old early-80s Russian tooling.

Edited September 27, 2012 by mancat

[devilfish13 6]

Ok, stage 2. I ground out the block to between 0.630" and 0.631" with a polishing jig and got it on with antiseize about 1cm before it stopped flat. My 3lb sledge won't budge it now. I brought it in to the shop and the machinist I've been going to thinks I should have ground it out a bit more but is confident that we'll be able to get it on the rest of the way with a press. Wish me luck btw, I'm one of those lucky guys who works at a pro-gun employer with a no-ammo policy only, so I don't need to be discrete.

[devilfish13 6]

Front sight block is now on, but not straight (~10° off). It will be receiving some TLC later tonight complements me and my deadblow hammer. It looks pretty sweet right now regardless! Really kind of want a Krink rear sight now.

[devilfish13 6]

Sigh. Can't get it to budge, too tight. I also don't have a torch, otherwise I'd just heat it up and try again... looks like I'll either be taking it to a smith or back to work on Monday. Probably better off with a smith, more likely to get it straight; it'll just cost more than free.

 

Some things are worth a little more money than free.