Frankford XM177 uppers not fitting

[jwehrle]

I have a Frankford XM177, came all original. Factory upper pins right on no problem.
Every other other upper I try (Spikes .22, Colt and AAC) the front pin fits fine but it won't fully close to push the back pin in.
I can press it almost closed by hand but to pin it I have to use a clamp to get it the last .020 or so down.
Once it's closed and pinned it works fine but something is not right.
It sort of appears that the forward pin hole it too low relative to the top, causing a pinch when closing but when I measure it against an M16 print I found online it appears to be within .005 of nominal position. Anybody have any input or ideas how (or who) to go about fixing this?

 

[yzfchet]

Typical Frankford Arsenal/Olympic out-of-spec fitment issue. I had the same thing on mine. Three ways to fix for new uppers:

1) Send the lower to a professional (M60Joe or U.S. Anodizing) and they will adjust your lower so that standard in-spec uppers fit. Your original XM-177 upper will then be loose but so what. Usually between $200 and $500 depending on what needs to be done. Your mag well may too tight, also, and they can correct that.

2) Mount the desired upper to the XM-177 lower with the takedown (rear) pin and close the upper onto the lower. You will not be able to insert the pivot (front) pin. With a sharp pencil or thin marker, repeatedly draw the circle through the lower pivot hole onto the upper pivot lug. It will almost be the same as the existing hole but not quite; there will be some upper lug material that gets marked. Remove the upper and place it in a drill press vise/jig with the appropriate drill (usually 1/4 inch but check for fit prior to drilling) in the drill head. Drill out additional material around the existing lug hole; you'll end up with an oblong-looking oval shaped hole in the upper pivot lug. At this point you should be able to mount the upper firmly and tightly to the lower with ease.

3) Sand the bottom of the upper with a sanding block and fine sandpaper. The area above the magazine well will require more sanding than the area near the rear takedown pin. you may need to sand the rear radius area of the upper to match the radius of the lower. Check for fit, repeat sanding fit check, etc. until the upper closes and pins smoothly onto the lower. This method destroys the anodizing on the portion of the upper that gets sanded.

Solution #1 is the best and most permanent. Also most expensive and time-consuming.
Solution #2 is the quickest and least effort, but also may move the sear timing a slight amount; it may work OK or present problems. Each gun is different.
Solution #3 is cheap and not too much effort, and should function just fine. Upper's lower edges get de-anodized but there it is.

 

[jwehrle]

Thanks for the info. I actually had started to lightly sand the lower just behind the front pin where the pinch appeared to be but stopped because I didn't want to mess something up, maybe took off .005-.007. Wish I hadn't done that now.
Your first suggestion the direction I'm moving. I went to m60joe's site tonight and sent him an email, sounds like he's a good start. I was hoping to find somebody recommended who knew their stuff and could be trusted with a registered lower. Hopefully I'll hear back from him soon and he can help me out.
Thanks again for the detailed reply, that was great help.

 

[DINK]

If it was my expensive lower, I would first send it to M60joe to be "blueprinted", then to
U S Anodizing for a milspec hard anodize coating. Both vendors are top-notch and have long track records of excellent work.

 

[jwehrle]

10-4 that's the direction I'm going. I got a quick reply back from m60joe, I replied back with a couple of questions, sounds like he's got a good reputation.......thanks again for the feedback and suggestions

 

[y2k87k2000]

m60 joe is definitely the way to go. I have similar fitting issues with frankford arsenal xm177. what I did with 1 upper is file the back and inside of the front pins of a 9mm upper with fitting issues. I wouldn't touch the lower myself!