using dry ice to replace FSB

**MicroMonkey** · October 19th, 2011, 06:04 PM

has anyone ever tried it? I used to work on heavy equipment (cat, komatsu, etc) and we would use dry ice to replace certain bearings rather than beat them in or press them into place. We would pack the bearings and races into dry ice for about 15 minutes, then just drop them into place......carefully. You only have about 10 seconds or so to set the bearing before it would expand and seat, then you could use minor effort to finish the job.
I wonder if you could use this same method to replace a FSB on an AK 47. Use dry ice to contract the barrel and slide the FSB into place with little or no adjustments after it sets.

Its a curious thought that I had. taking the FSB off would be no problem, but putting one on would need a press it seems.

**cdi** · October 19th, 2011, 07:04 PM

If the front sight base and the barrel are the proper dimensions, nothing special is needed. Those parts are closely fitted but not close enough to need dry ice.

**PA Rifleman** · October 19th, 2011, 09:07 PM

Originally Posted by MicroMonkey

has anyone ever tried it?...You only have about 10 seconds or so to set the bearing before it would expand and seat, ...
I wonder if you could use this same method to replace a FSB on an AK 47. Use dry ice to contract the barrel and slide the FSB into place with little or no adjustments after it sets.

Its a curious thought that I had. taking the FSB off would be no problem, but putting one on would need a press it seems.

Yes, it should work. You're right, you'll have a narrow window of opportunity for final adjustments by hand. After that you'll need a lead/brass hammer or some other non-marking persuasion to adjust it. I applied automotive grade anti-seize to help things along during assembly.

I have done this also in an automotive environment. I haven't tried it on an AK front sight base (FSB) replacement*, just be aware that you have less displacement "going cold" than hot. I recall dry ice is about -110 °F, so obviously this will work better in a warm work area than a cold one. Consider boiling the FSB in water for 10-15 mins after you get a rolling boil. This will give you an additional 120-150 °F of temperature differential.

My last experience:

Years ago I was working on a old Cadillac (472" V8) that had a timing chain issue. The crankshaft gear was pressed on the end of the crankshaft, and it was an outdoor field repair in sub-freezing temperatures. I boiled the new crank gear and slopped the crankshaft with anti-seize. After 20 mins or so, I took the pan & boiling water with the gear still in it over to the car. The gear slipped on but made an eerie creaking/tinkling sound and got tight *fast*. I don't recall why I didn't do the obvious (heat the gear with a propane torch), maybe I was curious whether it was cold enough to pull it off with just hot water. The point is, it was so cold outside that the gear only needed 210°F and it wasn't an interference fit anymore.

That said, give it a try and let us know how you fare. One advantage of your technique is no danger of fire, and losing the temper in any metal parts.

*I have replaced several front sight bases, though. And yes, they needed heat.

**ham385** · October 20th, 2011, 12:22 AM

I've never done a FSB on an ak, but it seems to me it would be a lot easier to expand the ring than it would to contract the barrel given it's mass and ability to act as a heat sink. Why not use a torch on the FSB?

**MicroMonkey** · October 20th, 2011, 06:12 PM

Maybe I will try it this winter when I dont have anything else to do. I would rather use dry ice than fire, because I worry about the temper as was mentioned in a previous post. Im not sure if its REALLY that big of a deal, but I just dont know for sure. I DO know that dry ice worked for me in the past on other things.
I guess the thing to do would be take the FSB off, and see exactly how hard it will be to put on the new one with some brass punches or whatnot, and then decide if hot or cold would be the way to go. maybe just a very moderate heat would work. I guess I wont know until I see how close the tolerance actually is

**red dog** · October 20th, 2011, 08:38 PM

i froze a fn fal barrel once before installing, it seemed to help. work fast you do not have much time.

**knight0334** · October 23rd, 2011, 06:26 PM

I use my chest freezer and my oven when I have such situations, depending on which part it is and what it is made of.

**Asmodeus6** · October 23rd, 2011, 11:44 PM

I just hose it down with a penetrating lubricant and whack it a few times with a rubber mallet. (Side to side) Then drive it right off. In my experience, ON, is easier than off. A lot easier.

YMMV of course. Ice might be a neat idea. But I'd just hit it with a torch "lightly". While the tip of the barrel was in 1/2" of water. That should keep the barrel from gaining temperature as fast as the FSB.

Then hit it a few times.