Discussion in 'Gun Project Questions & Gunsmithing' started by skeeljc, Dec 8, 2019.
It is a tight slip fit. The opposite end is against the chuck jaws.
I might as well continue my fitting experience. First I used a .250" diameter reamer and hand reamed the front takedown pin bores (upper and lower together). The lower was previously reamed to .250". Very little material was removed from the front upper pin bore. Now my .2495" diameter shoulder bolt fits nicely.
The back end of the DPMS upper required removing .012" off of the right side to match the left side and fit the Aero Precision lower. I machined it in the lathe by using a 1" x 1" piece of .001" ss shim stock between the 1" mandrel and the upper bore and sliding the front of the upper against the chuck. I then machined the back end until it matched the the other side using light .003" cuts. Sorry no pic.
The upper and lower now mate together at the bottom of the upper and there is .006" gap at the back end between the two receivers.
Actually there will be more than .001" clearance all the way around the fit. The DPMS upper measures 1.0018" diameter and I am sure my barrel is under 1.000" dia. (I have not taken it off of the Aero Precision upper yet).
I am following Joe Carlos procedure. He said a combination of shim stock and 620 Loctite made the best improvement in accuracy. According to his article I have around 30 minutes to do the assembly before the Loctite begins to set.
It's not the clearance, it's the fact that 0.001" shim stock is easy to fold and crumple.
To ream the rear takedown pin holes I took a piece of 1.000" TG&P 1045 stock and put it through both receivers from the back and then slipped the buffer tube over the stock sticking out and tried to screw the buffer tube into the lower. It would only engage about a half a thread. I removed more material from the right side of the upper just below the flat area I machined in the lathe until I could screw the buffer tube in all the way. This resulted in contact between the two receivers just below the rear pin bore on each side. Then I used a .251" diameter reamer to ream the back hole. The lower had been previously reamed with the .251" reamer so I only removed material from the upper. Now my .250" diameter rear takedown pin fits perfectly.
I did true up the barrel seating face.
Well I removed the White Oak barrel from the Aero Precision upper. The barrel extension measures 0.9994" to 0.9990" in diameter. The DPMS upper measures 1.0016 with a bore micrometer. So the diametrical clearance is 0.0022" to 0.0026". I cut a piece of .001" ss shim stock 0.80" wide and 3" long. I wrapped the shim stock around the barrel extension leaving the ends on either side of the locating pin. I positioned the shim an eighth of an inch from the barrel flange thinking the shim might slip forward as I installed the receiver over it. I got the upper started over the shim o.k. and worked it half way on by hand by rotating it some in each direction. I was amazed that the shim remained stationary on the barrel extension as I worked the upper over it. When I got it half way on I could no longer move it so I heated the upper with a hair dryer. I was then able to push it on up to the locating pin. I then seated it the last 1/8" with a dead blow hammer.
It went together so well I may just go with the shim stock (no Loctite).
BTW: the shim stock measures .00101 thick.
Good job. I hate Locktite. I had to completely destroy the delta ring and spring on a PSA upper to get the barrel off. Pretty sure they red-Locktited the threads and there was a thin film of something decomposed on the extension too once I got it off. Eugene Stoner didn't specify Locktite; neither did the Army Ordnance Corps.
How do you measure to 10 millionths?
I have to heat all the PSA uppers I have taken apart. They must use red Loctite.
Here is the finished result. I blackened the machined surface with Aluminum Black.
I run BCM uppers. They are interference fit with the barrel. True up the face, heat the upper, slide the barrel in with a dose of loctite 609 and enjoy
Wow 250 degC ( 482 F ) seems like a high disassembly temps at my 1st thought but I may me overly concerned. dunno
How difficult has it been for you to get a worn out barrel out of the upper receiver to put a fresh barrel on ?
My oven goes to 500
It can be done. Not as difficult as you think.
Just out of my own curiosity, how much runout are you getting with that fixture on the chuck side. Are you using an adjust tru 3 jaw? I would think if you are just chucked up in a standard 3 jaw, you would have some runout on that end. Unless you have a dead tru running 3 jaw, I could see you getting an inaccurate runout reading on the face of the reciever.
My 3 jaw runs within .001 TIR. I marked the shaft to maintain the same relationship to the chuck both when I cut the center and mounted the shaft in the chuck for the facing operation. I could have used my collet chuck which is adjustable but I did not think I needed to be that accurate.
Or I could have used Joe Pieczynski's trick for getting a part to run true in a non adjustable chuck but that takes a lot of time.
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