View Full Version : AR-15 Thread Patterens
07-12-2010, 03:07 PM
While searching for Blueprints recently, I ran across drawings for the M16 Government Profile barrel. I was ecstatic and instantly opened my CAD program and started to draw it to scale. However, when I got to the threads on the front and rear I quickly concluded that my system for drawing thread patterns must be wrong. The Muzzle threads called for 28 T.P.I. , which I did and when I was done the angle of the threads seamed to be incorrect. According to the American Machinist Handbook these threads should be 60 degreeís apart as in a V pattern. Instead they ended up being 114 degreeís apart. What the Hell am I doing wrong? I drew up the Major Diameter as being .4989 as the prints showed and the Pitch Diameter as being .4757 as shown. I divided 1íí by 28 and it came out to .03571428 and made that my standard between threads. How is this wrong when drawing out to a full inch? If I could get this down right it would benefit the Gunco community greatly for my drawings are spot-on except for this. Thanks for any help provided!!!
07-12-2010, 05:07 PM
I can't help you with the cad stuff, but I've only seen a couple AR barrels that came close to having barrel threads for a 1" length and they were AWB models with a pin on muzzle bread that had a 3/8" thick washer. A quick measurement of the barrels I have handy showed measurements from .548"-.598" for barrel thread lengths. An A1 type birdcage flash hider when screwed on without a washer would bottem out on the step for the shorter distance, and on the threads leaving a gap on the longer distance.
07-12-2010, 07:19 PM
What I probably should have asked is, How many threads are there from the muzzle to the face? (Where the threads end) My Colt is from the Clinton era so, I don’t know how many there should be between the two points. From the prints I have it shows it as being .630 and the reference drawing I showed was what 28 T.P.I. would look like if drawn to a full inch. What I have done is taken the 28 T.P.I. and pasted it to the muzzle end, then omitted the remaining threads. This being that the thread is not an inch in length, but rather coving the area given with 28 T.P.I. (Threads Per Inch) Sounds confusing but that’s the way it is.
By the way Moleman love your SGN-9, it has character. I seen in your last post on it’s page you said, “Ugly isn't it? I think its Awesome and have already drawn up plans for my own!!!
07-12-2010, 10:17 PM
AHHH! My buddies with neutered lug and pinned muzzle brake was a dpms gun and the threaded part was close to an inch. The spare barrels I have now that I measured are a sgw-oly and I believe a J&G or an M&A. My colt barrels have flash hiders on them at the moment. I've noticed that some barrels have a relief cut down to the minor diameter next to the shoulder and others don't. Over the years I've owned colt, sgw&oly, armalite, dpms, m&a and j&g barrels and all have been a little different in the muzzle threads. Without a releif cut .630x28= 17.64 threads.
I got a chance to really shoot the crap out of the sgn-9 build over the 4th weekend and it is really to heavy to be comfortable while shooting for any length of time. It was very accurate though and I didn't have any ftf or fte until it got dirty when I was down to my last few rounds anyway. Once it was real dirty the bolt started hanging up about 1/4" out. I built it with only .005" bolt clearance like my suomi build, but unlike my suomi build it has a bolt slot that allows dirt to enter the receiver. Upon cleaning it I found some dirt/sand had gotten inside and was causing it to stick. I think I'll turn the bolt down .002"-.005" more for better reliability when dirty. If I were to build it over I'd go with a thinner walled receiver for better weight savings, and put a car type stock on it. (its still ugly though).
07-13-2010, 02:19 PM
Itís funny you said, ďneuteredĒ because I think an Assault Rifle without a flash hider/muzzle brake is like a dick without a head. It just seems like its not all there. I never seen the logic behind the whole 94 ban on semi autos, other than to be a pain in the ass. Anyway, I greatly appreciate your help with the threads and will send pictures of my SGN-9 build when done. However this may take a while to complete.
07-13-2010, 04:03 PM
I would have to dig out my old book for drawing threads, but I rmember it was a pain in the ass to draw threads correctlly.
07-13-2010, 08:20 PM
Just checked my prints and the thread length for the muzzle is .620 +.002 -.000 , these are from the actual Colt prints.......I'd post if I could...but they were shared with me on the condition they were for my personal use......
07-14-2010, 02:49 PM
I feel rather stupid, not thinking before asking. I have an e-drawings of the barrel in a solid model I downloaded long ago. I removed the flash hider on the barrel and counted the threads from the muzzle to the face. Just as Moleman had said, it came out to 17 threads, coving the .630 area given. This said, how long is the thread relief cut between the threads and the face? Does this matter or is it pre-determined so the flash hider slots come out top dead center? Sorry about all the questions. However, Iím very meticulous in nature and try to be as precise as possible. The AR15 Barrel, Barrel Extension, Barrel Nut, Front Sight, and Magazine Blueprints can be found at www.firearmfiles.com (http://www.firearmfiles.com/andromeda.php?q=f&f=%2Ffiles%2FAR15). Thanks again, for any help given.
00redZX-6R- I asked an experienced machinist a while back about thread patterns and he did concur that they can be quite confusing when dealing with the variations in terms. Iím guessing that the UNEF fine threads have a larger V pitch angle between threads.
Mad-Machinist- Are the drawings you have for the M16 Government Profile or M4? You might want to look at the prints I have and see if there are much of a difference between the two. I understand about not being able to share the Blueprints you have. If someone confided in me I wouldnít share either, as I try to be a man of my word. I just wish are politicians felt the same.
07-14-2010, 06:10 PM
Last I checked, all American style threads have a 60 degree V between them and that is the cutter I use when cutting 1/2-28 on the lathe. Matches up to the flash suppressors quite well. The relief cut is there so the threading tool can stop in a groove instead of against the shoulder at a dead end, easier to cut and less breakage of tools.
The original flash suppressors used a lock washer as they didn't have to be "timed" like the closed bottom A2 style uses. Those use what is called a "peel washer", essentially a stack of compressed shim washers that can be individually peeled off the stack to make thinner as needed. You tighten the suppressor up to get a reference point then remove a layer or two of washer at a time until the suppressor tightens up where you want it to. The actual threads aren't timed.
07-14-2010, 10:29 PM
HF...be happy to compare...I have both the M16 and the M4....and as far as the colt prints no relief is specified.....the note on the print specifies .100 max to first full thread.
and a little more info on gas ports....
The gas port diameters are as follows:
*GAU2A (10" barrel): 0.073"
*(No longer authorized for use)
07-15-2010, 04:51 PM
Kernel Krink- Your post confirmed my suspicions. Iíll probably try and find a barrel and take measurements off of it, just for peace of mind. Thanks for the heads up.
Mad-Machinist- Thanks for the gas port info, this will come in handy when finding the proper placement for it. I plan on drawing up prints for the front sight base and moving it onto the barrel to find itís position. I figure the gas portís top dead center is dependent on the fitment of the barrel extension to the barrel, or are the threads timed on the barrel? If there not, why do you have to fit the front sight base yourself? Bushmaster will not sell barrels without this already installed, from what I read in their downloadable catalog. Iím not trying to make Stone Soup from asking a lot of questions, if you remember the old fairy-tail. I just want to be as accurate as possible and hope to benefit others who maybe wondering the same thing.
07-15-2010, 07:29 PM
Threads are not timed......so after barrel extension is fitted and chamber is cut and headspaced...then the gas port is drilled and the FSB is installed......and if you buy a new FSB it has no holes drilled...just got a couple in today for customer guns....no holes...makes it a LOT easier to drill ream and pin .......if you want to email me your drawings...I'll be glad to amend them with corrections for dimensions from the prints I have..............
07-16-2010, 02:33 PM
Thanks Mad-Machinist, Iíll finish up drawing the other barrel profiles I have on hand and send them to you A.S.A.P. These should be nice for you to have as reference prints.