Paint in Bolt Hole -- I Think

[Hunter308 6]

It appears the reason I can't get the upper-to-lower receiver bolt all the way through the upper receiver (when trying to secure my FB lower) is because there is paint in the last 3/16" of thread. The hole is threaded all the way through; at first I thought it was drilled/threaded part of the way from the outside, then part of the way from the inside, and that things weren't lining up, and that's why I couldn't get the new bolt all the way through. (By the way... the threaded portion of the ORIGINAL bolt doesn't even go all the way through -- both bolts stop at the same point.) However, I'm now reasonably sure it was drilled/threaded from one direction.

 

So... my problem is, I need to "clean up" the threads. Do I get a purpose-specific tap (matching threads, of course) to "drive on through the paint" -- or do I try using paint remover? I can get the gun together but, when I slam back my Magpul CTR stock to open up the length of pull all the way, the upper and lower receivers separate by 3/16"... and, interestingly enough, that appears to be the same distance of painted threads.

 

In the very beginning, when I first discovered I couldn't tighten the bolt all the way down and that I had 3/16" of slop, I thought I needed some washers to take up the slop... but that's not the problem, paint is. (I assure you, I've tightened things down *tight," repeatedly, but the bolt always stops at the same point.) I can see where the threads in the upper receiver are shiny and where they're all black, never touched whatsoever by the bolt's threads... and that's the 3/16" I need to bolt things together completely.

 

How do I clean up the threads, guys?

Edited June 29, 2013 by Hunter308

[duggr 13]

Hunter,

I had to clean my threads so the F.B. lower would mount up flush to the upper....

Heres what you need....1/4 X 20 thread tap below....If By chance you can't find it, I'm in Illinois and I could send

you the tools needed and after just send them back to me when your finished....Doug

 

Edited June 29, 2013 by duggr

[Hunter308 6]

Thanks for the information! I'll check around here first and see what I can turn up!

 

 

Thank you very, very much, Duggr, for the information and offer!

Edited June 29, 2013 by Hunter308

[Rebel Katt 6]

Better make sure thats not an M8x1.25 screw.

[Hunter308 6]

Better make sure thats not an M8x1.25 screw.

I drove the hardware store to buy a 1/4" - 20 tap and the turning tool, and drove back home, and tried it... well, well, well... how about that? Didn't fit. Practically dropped right through the hole. Went back to the hardware store with both bolts this time... the nice lady who works there checked the threading at my request... M8x1.25 -- and, they didn't have metric taps. <sigh> So, I drove over to the nearest El Cheapo (Chinese) tool joint, "Discount Tools," and got a complete (metric) kit... that did have an M8x1.25 tap (and die, too, for that matter). Came home and went to work. The end of the story is -- all is well and the gun is together. (Yay!) I do want to add, though, that who ever did the threading/painting of the upper receiver hole SHOULD BE SHOT TO DEATH WITHOUT A BLINDFOLD!!! It wasn't just paint... in fact, though there was paint coating the threads, that was NOT the problem. The actual problem is the threaded part got smaller than the rest of the threaded hole that was a true M8x1.25 configuration. I can't possibly describe how much METAL came out when I ****FINALLY*** "broke through" to the inside of the upper receiver!!! If I'd have threaded holes like this back when I had metal shop when I was in junior high school, Mr. Irwin (long gone... good guy) would have SKINNED ME ALIVE!!!

 

Anyway... both bolts work, going completely through the hole. The short one Nanci sent is installed (I added a dab of thread locker [blue]), everything is together, and I'm a happy camper.

Edited June 29, 2013 by Hunter308

[Rebel Katt 6]

Sorry I didn't see the thread before you went through all that.

But sounds like all is good.

[duggr 13]

Man, I don't understand that.... I had to actually clean the aluminium shavings off this tap before I took the photos so I know this is the tap that I used for my MKA Upper.. Measured the bolt with the thread gage and tried the tap in the hole....

I apologize hunter for the wrong info....

[Hunter308 6]

Man, I don't understand that.... I had to actually clean the aluminium shavings off this tap before I took the photos so I know this is the tap that I used for my MKA Upper.. Measured the bolt with the thread gage and tried the tap in the hole....

I apologize hunter for the wrong info....

Hey, it's cool. I finally got to use some thread-cutting skills my Dad taught me a gazillion years ago, along with what I learned in metal shop with Mr. Irwin.

 

I've been wondering if there were different "runs" of these guns, where some had 1/4-20 holes and others had M8x1.25 holes. I wouldn't be surprised in the least.

 

The *ODDEST* thing, I think, is how the threads were at the beginning of this mess. In the original configuration, the longer bolt only had to go just so far, holding the receiver group together. It did that job just fine with only so many of the threads in the hole. However, if there was some reason that original bolt HAD to go any deeper... let alone, all the way through... it would have been impossible.

 

I don't know if there are taps shaped like a spitzer bullet... but that's what I think could have caused this problem. If such a tap was used to cut the threads when the gun was made, the "bullet tap" only had to go so deep; when the sides of the "bullet tap" cut enough threads... the tapping was stopped at that point because the threads for the original bolt were deep enough. However, the front part of the "bullet tap" (the ogive, in bullet terminology) threaded the hole that way -- and that part of the threads in the hole weren't a concern at the factory, since the original bolt never had to go that deep. I'm completely guessing on all of this, but it DEFINITELY wasn't an issue with some paint screwing up the tolerances so I couldn't seat the new bolt all the way.

Edited June 29, 2013 by Hunter308