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Discussion Starter #1
Howdy all, new owner of a previously-used and slightly abused SCAR-17 here. I picked this thing up on a bit of a whim at an estate auction over the weekend. I’d actually been contemplating the SCAR but couldn’t justify the cost; then Fate intervened and I ended up as the winning bidder at a very attractive price.

I’ve got some questions though, about cleaning it up and making it ready to shoot. I’m retired military so pretty familiar with the M16-series direct-impingement system but the gas piston system is totally new to me. This SCAR apparently was “rode hard and put up wet” as they say. When I first picked it up I’d noticed a green streak at the muzzle, and dried green gunk on the threads where the birdcage is attached. Not knowing its history or the previous owner, I have to assume that someone must have dumped solvent down the bore and not cleaned it or wiped it down properly. The chamber too, was absolutely filthy dirty. I think that I can clean this up without too much trouble, but need to know the torque specs for the birdcage / compensator.

What I did not count on though, was the condition of the gas regulator and piston. The piston was seized in the bore, and was removed only with a great deal of difficulty. Once finally out, it revealed caked-on carbon and dried green gunk (which I assume is copper fouling) as well as a few spots of rust. I need to remove this from the piston, the regulator, and in the cylinder bore in which the piston travels. Normally I’d just soak these parts in some Kroil but the owner’s manual was pretty emphatic about not using any sort of lubrication on these parts. I’m going to try to attach some photos so you can see what I’m talking about here.

I’d appreciate any advice you can provide regarding the clean-up of this system. I’ve already cleaned up as much as I can with a bronze brush and a dental pick. My next thought is to use a piece of stainless steel wool soaked in Kroil to clean the piston and cylinder, followed by a degreaser. The gas regulator is another matter entirely. The carbon build-up inside the regulator is very bad, almost like slag, and the dental pick isn’t even making a dent in it. Maybe I can get FNH to sell me a new one.

Is there anything else critical which I need to look at (besides what’s covered in the owner’s manual)? Anyway, I appreciate whatever help you can provide.
 

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Discussion Starter #2
I'm going to try attaching some photos. Here's the gas regulator, piston, and firing pin after I disassembled the carbine:
Piston_port_firing pin.JPG

Here they are after some scrubbing with a bronze brush. The crud remaining inside the gas regulator is hardened and I'm not able to scrape it off with a dental pick:
Gas piston and port (closeup).JPG

Here is the gas cylinder in which the piston moves. What you cannot see is the rust at the very back of the cylinder.
Piston chamber.JPG
 

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you normally can find whole new scar 17 16inch barrel assembly for approx 700-800$ . I don't believe you can buy the gas block /piston assembly by itself. I dont think carbon in itself is a problem as long as the piston can move freely in the gas block. If it is that used I would worry that the barrel could be toast depending on firing schedule. Swapping the recoil spring assembly is never a bad idea. I thing the receiver and bolt are rated to around 100k rounds. Torque on the muzzle device is 20-30 ft/lbs normally but a nice snug against the jam nut should be fine
 

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It won't hurt to soak the parts in Kroil to loosen up the fouling's. The gas block will really need it to free up your gas screw that is down in that hole, it will need to be removed (unscrewed) for a good cleaning on the small hole in it, the green scotch bright pads do a pretty good job to clean these parts. Might want to stay away from the brass or s.s. brushes so you don't scratch the Al. FN recommends to reassemble dry, personally I use Hornady dry lube for reassembly. High Desert Dog sells aftermarket gas blocks and pistons if yours are bad, but I think they will clean up for you. I think your biggest challenge might be the gas screw, just let it soak for some time. Good luck !
 

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Discussion Starter #6
Thanks, gents. Boomslang, I didn't even see a screw down in there; wonder whether it's missing or just caked in crud.
But while looking around the forum, I noticed that one of the sponsors carries parts. I've placed a new gas regulator on order (for $9 it's worth it), as well as a firing pin "just in case" it's ever needed. Apparently the "gas control screw" comes in different sizes -- how do I know which to order?
 

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Discussion Starter #7 (Edited)
you normally can find whole new scar 17 16inch barrel assembly for approx 700-800$ . I don't believe you can buy the gas block /piston assembly by itself. I dont think carbon in itself is a problem as long as the piston can move freely in the gas block. If it is that used I would worry that the barrel could be toast depending on firing schedule. Swapping the recoil spring assembly is never a bad idea. I thing the receiver and bolt are rated to around 100k rounds. Torque on the muzzle device is 20-30 ft/lbs normally but a nice snug against the jam nut should be fine
Vertigosol, I bought it at an estate sale, so the previous owner was not available for questions or comments. No idea whether he had shot it a lot without maintenance, or if it was a NOK who did this before turning it over to the auctioneer, or what the story is. The piston was froze-up solid when I got it home, but I've cleaned enough gunk off of it that it will move freely now. Appreciate the information on the torque specs. Will give it another thorough cleaning and see what happens.
 

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I woudn't be too worried, you can bring her back to life. Scrub, scrub, soak. Then just get out and shoot it......If it shoots good, who the heck cares what it looks like.

Edit: to clean, scotch brite pads and flitz metal polish. Drill bit with the pad and flitz to get into tight places.
This. You can get that clean and if you don't, it doesn't matter, it will run anyway. That's the beauty of the scar.

Nothing wrong with spare parts though, so if you've already ordered them just clean and keep the old.

I actually do lube the piston area. The key is to get all(every bit) of the excess lube out of the piston area. It makes the next cleaning a little bit easier


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Those parts aren't actually in that bad of shape. Just a bunch of carbon and copper that weren't cleaned off. Spray everything down with a good solvent (CLP or whatever you have. I have used many different ones and they all work well for cleaning. Ballistoll is one of my go-to's for removing heavy buildup). Let it soak for 30 minutes or so. Get a brass bristle brush and nylon bristle brush and go to town! You can also use picks, abrasive scotch bright pads, etc. Then wipe it down clean. I also have a little Hornady Sonic Cleaner (only costs like $60 or something) and you can get one of those and buy their "Gun Parts Cleaning Solution" to use. Drop all the parts in there after you scrub off as much as you can and let the sonic cleaner try to get some more off. When you reassemble, just make sure to wipe off any excess lube on the parts that FN recommends keeping dry. The reason for this I believe is that if you leave oil on these parts they will attract lots of carbon and build up sludge very quickly. But the Scar is a very well engineered weapon system and functions great even when dirty. Lucky us! :D

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Tache, nice find! I bet she goes bang the first time you pull the trigger. If the rifle was that dirty on the gas piston and chamber I would fully remove the barrel assembly and give the upper receiver a good cleaning as well. Double check those interior rails, wouldn't want the BCG riding over crusty rails.

as for the piston, the manual does emphatically say to not use any kind of lubricant on it but I have treated mine with fire clean with no issues. It really helps with cleaning especially when firing it suppressed. I just make sure to fully wipe any excess off before replacing into the rifle. As some of the other guys have stated, chuck the end of the gas piston into a drill and use a brass brush against it to get all the hard core gunk off.

im interested in hearing how it all works out for you. I'm always amazed at some of these AK stories where they have been stored in a cache for 20 years or so and they fire without any treatment all.
 

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Sorry but I'm not sure as to stock factory part # for gas screw, give FN customer a call or forum sponsor MGW a call, he will fix you up.
 

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Discussion Starter #13
Sorry but I'm not sure as to stock factory part # for gas screw, give FN customer a call or forum sponsor MGW a call, he will fix you up.
Thanks, I found the part number at MGW but it asks for a size. Maybe best for me to just send them an email.
FNH SCAR Gas Control Screw
Now that I know what it looks like, there is one in that hole but it's pretty crudded-up. I put some Kroil on it and will clean it up tomorrow after work. Need to get some of those Scotch pads, but I already have the Flitz and Ballistol.

The rest of it is cleaning up pretty well except for the gas regulator. Two of the gas ports are completely blocked with carbon, and on the inside there is what looks like molten metal but is probably heavy carbon buildup. That's soaking in Kroil too; dental picks won't even dent the stuff. I do have some of that foam copper remover, but will have to wait until I'm home in the daytime and can work on it outside in the sunshine.

There are also some wear marks on the aluminum rails of the upper. Any preference for lubing the rails? I usually use Lubriplate for this sort of thing, but CLP may be sufficient.
 

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^^^ Yes, I think either will do you good, I been using CLP but as you suggested I am going to switch over to a lite grease also on my rails. I'm not sure at all by for some reason 2mm is in my head, but I could be completely wrong. Do a search on gas screws, one fellow had a very informative thread on them on hear awhile back and also had info on screws for shorty's too. If I find it I'll PM you.
 

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Just wait a a gosh dern minute!.................













Piston_port_firing pin.JPG large.jpg

There be Liberty in that there SCAR, I always knew there was.:mrgreen::mrgreen::mrgreen:
 
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Discussion Starter #18
Thanks, gents. I did get in touch with MGW and they told me that each carbine is tested at the factory and an appropriate size screw used based on that test. Quiet Warrior, those recommended sizes will be a good start. At the moment, I'm still unable to get it out, but at least it appears clean. The piston has cleaned up well, as has the rest of the gun. I was able to get some of the carbon off the gas regulator, but by no means all of it. But the replacement part showed up today, so I can clean up the old one later and keep it as a spare.

Boomslang, the reason that I'm going to use some Lubriplate on the rails is that the cleaned-up gun showed light wear on the aluminum rails. I've reassembled it, and hopefully will get out to test-fire it soon.
 

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My piston and regulator had almost that much blue carbon fouling last time I opened it up and at most only had 400 rounds between cleaning. No rust though. Wonder if the federal ammo I got a deal on is running really dirty.
 
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