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Discussion Starter · #1 ·
Seems as if I've rain into my first "real" problem. After drilling all of my holes for the receiver, I temporarily dry fitted the rails by sliding the center support bushing and rivet through. I intend on screwing the rails in since I don't have a welder. After dry fitting, I noticed a very small but obvious gap in the heigth of the lower rail in relationship to the plane of the notch in the front trunnion. I would estimate the drop in the lower rail to be about .05'' below the trunnion notch. Unfortunately, the rails I got from TAPCO have predrilled holes for the center support and hammer axis pin. Since I drilled both of these holes before I fit the rails, this means that my holes line up fine, but my rails sit about .05'' below what they need to. What kind of tolerances am I dealing with here? What would be the best approach to resolving this problem? Buying a different set of rails that didn't have predrilled holes? If this were the case, I could just move the rails to the correct position and use the existing holes in the receiver as a guide.
 

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Josh-- are your lower rails perpendicular with the upper ones? Did you position them using the "c" bit?

My thought is to never pay any attention to the predrilled holes-- why not position the rails in the right place and drill another hole through them?

 

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Discussion Starter · #3 ·
edit:yes they are perpendicular with the upper rails, but the lower rails do not line up with the notch in the trunnion....so the bolt wont travel from the lower rail and engage the trunnion. sorry no pics, i'm out of batteries.

I tried to use a letter "C" bit, but it is obviously too small in diameter if I use the existing holes. But, yes if I did use a letter "C" drill bit and didn't use the existing holes, I would be right on the money. This is what I need, but the existing hole is so close to being correct, that when I drill the new hole in the rail, it would actually go into the old hole. But, you've just got me thinking!! The holes that come in the rails are for the center support and the hammer axis pin. It really doesn't matter if I egg the holes out or drill new ones. The pins will fit through to the other side. Once they are in the right position, all I have to do is drill my holes for the (4-40) screws, tap the holes and then the rails are secure! I was actually worried about my existing rail holes being egged out (center support and hammer axis pin) and then my rails would move around because of the play in the holes. But, this won't be the case because I'm screwing the rails in with other screws(4-40).

I guess the only other alternative would be to order some new rails that didn't have the holes already drilled. This would enable me to position my rails and then drill through them with my existing receiver holes as templates. This would only keep me from having egged out holes or multiple holes for the center support and hammer axis pin on my rails. If I'm not cosmetically concerned about this, I can go ahead with my build.

What do you think? Would you order new rails so that you didn't have to egg out the existing ones in the rails. Or, would you order a new set and drill the holes exactly where you need them through the rails so that they would look cosmetically perfect?

From now on, I will wait to drill my center support hole until last. I will also buy the rails that don't have holes through them too. I have learned a very important lesson here. Do you guys understand what I'm talking about?
 

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I call the extra holes lightening cuts. It's one of those things custom guns get.

The holes through the receiver will stabilize the axis pins.
 

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Josh,

As Winn mentioned the holes in the receiver will locate your pin holes.

Locate and install your lower rails in the correct position (they're most important) then just "clearance" the holes in the rails enough to allow the pins to pass through them.

Open up the rail hole with your Dremel, do not try to open the rail hole by trying to drill through the receiver hole.
The rail is much harder than the receiver, your drill bit will walk, and you will end up egg shaping your receiver hole.
Please don't ask me how I know this :wink:
 

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Discussion Starter · #6 ·
Winn R said:
The holes through the receiver will stabilize the axis pins.
Exactly! And it took me a while thinking about this to realize that. My screws/weld will hold the rails in place not the pins.

762, Thanks for the advice! Perhaps I'll use some clamping pliers or vice grips and hold the rails in place and then mark where I need to drill out the holes. Then remove the rails and dremel them out.

Thanks guys!
 

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Discussion Starter · #7 ·
I eyeballed the rail with the ejector by lining up with the trunnion and tried the "C" and "D" bits. The "D" seemed to fit about right for me, although I think 762 and I are using the same trunnion and flat. Moved to the opposite side and tried the "D" bit and I'll be damned if the rail appears to be higher than the trunnion. I don't know how this could possibly happen! I checked the "upper" rails for a 90 degree angle by taking the lower rail with the ejector and using it as a guide. Basically all I did was move the rail from where it's permanent position should be, up under the upper rail. The 90 in the lower rail should match up with the 90 in the upper perfect. I slid it all the way down and it matched up perfect.Did the same thing to the other side too.

At this point, I'm pretty frustrated with these freakin rails and getting them lined up. Let me run through the logic again, and you guys tell me if I'm totally missing something here....If the trunnion is identical on both sides, and the lower rail is to align with the grooves in the trunnion, then if I fit one side with a "D" bit, then the other side should also use a "D" bit. They should use the same size bit or distance when measuring the distance from the upper rails to the lower rails.

.......frustrated as hell in GA!
 

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Hey, it's ok.

This stuff is sheet metal. If you don't like where something is at-- bend it.
 

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Winn is correct (as usual)

Line your rails up with the trunnion, the upper rail can always be tweaked if necessary.

Clamp your rails in place, check fit with your bolt and bolt carrier.
They should slide smoothly into the trunnion, without getting hung up.
 

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Discussion Starter · #10 ·
yosuthnmasa said:
At this point, I'm pretty frustrated with these freakin rails and getting them lined up. Let me run through the logic again, and you guys tell me if I'm totally missing something here....If the trunnion is identical on both sides, and the lower rail is to align with the grooves in the trunnion, then if I fit one side with a "D" bit, then the other side should also use a "D" bit. They should use the same size bit or distance when measuring the distance from the upper rails to the lower rails.
What would I bend here guys? My uppers are at a 90. What do you guys think about the above?

Edit: also, do you just take the bolt and sit it on the rails and run it down, or take it and the bolt carrier and run it down to see if everything fits right?
 

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Josh-- we are talking about four thousands of an inch.

I cannot tell when a receiver is out of square that amount.

 

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Discussion Starter · #12 ·
Winn,

I ended up putting a few bolts through the rails, prior to permanently attaching them, and ran the bolt and bolt carrier down the receiver a bit. When I get to the rail without the ejector, the piece of the bolt that rides on top of the lower rail actually hit the lower rail as it approaches the start of the lower rail and wouldn't countinue because of this. This is the one that looks like it should be moved down further.

The part that doesn't make a bit of sense to me is that I used a letter "D" bit on both of the distances for the lower rails. Having said this, you would think that both of them are at the same heigth. This is quite confusing. The left side seems to be just perfect while the right is higher but I used the same bit to measure both of their distances from the top rails. The only thing I can think that may be causing this is because my center support isn't in and pulling the sides together all the way. What is a good way to determine if my top rails are at a true 90? Any suggestions here?
 

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Josh--My suggestion would be to put it together first, fit it second and harden third.

The difference between the "c" and "d" is .004". I can't look into the receiver and see that small an amount between the rail height and trunnion tab. The sheet metal thickness is 10 times that amount.

Do like Max said--

The object is to get a gun that works not that's to Nasa tolerences.

I drilled for a set of rails today.
  1. Jammed in a trunnion so the mag was tight
  2. Painted the outside with dycum
  3. Lined the rails with the tab
  4. Leveled em with a bit
  5. Used dividers to measure the depth on the inside
  6. Scribed the outside and marked 5 x's a side
  7. After they're welded I bend, file, scrape, and fit.
  8. Then harden
 

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I like that adolescent look.
 

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yosuthnmasa said:
yosuthnmasa said:
At this point, I'm pretty frustrated with these freakin rails and getting them lined up. Let me run through the logic again, and you guys tell me if I'm totally missing something here....If the trunnion is identical on both sides, and the lower rail is to align with the grooves in the trunnion, then if I fit one side with a "D" bit, then the other side should also use a "D" bit. They should use the same size bit or distance when measuring the distance from the upper rails to the lower rails.
What would I bend here guys? My uppers are at a 90. What do you guys think about the above?

Edit: also, do you just take the bolt and sit it on the rails and run it down, or take it and the bolt carrier and run it down to see if everything fits right?
I tired the letter c bit like I read to use but it was off a bit. I have had better luck using a 1/4 inch drill bit. ak'sr4me
 

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Discussion Starter · #18 ·
ak'sr4me said:
I tired the letter c bit like I read to use but it was off a bit. I have had better luck using a 1/4 inch drill bit. ak'sr4me

I really don't know what is wrong with my receiver. I ended up using a letter "d" and a letter "h". Doesn't make a bit of sense that it should be this way, but this is they way they seem to line up. I guess we'll see when I get it all put together whether or not the bolt will go through to the front trunnion or not smoothly.
 
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