• Bolt will not fit
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Bolt will not fit

Posted: Mon May 14, 2018 12:55 pm
by makebang
I am bout half done with a 4 cut weld up M53 build. Everything measures in spec from all the drawings i have found, camming piece fits perfectly, top cover fits perfectly, so far so good..... until i put the rails in and the bolt had to be hammered in to fit.
not good. A little filing, massaging here and there and now I can push it about half of the way forward before it stops and binds. The rails are just held in loosely with a couple temporary screws.

I am using new rails that seem to measure the same as the old ones except for being .010" wider so I filed them a bit to match.
Im kind of at a loss.... (and i have a lot of experience doing stuff like this, and a lot of gun builds under my belt)

Could I just have a slightly wider bolt? The receiver seems to measure but i cannot find all the measurements I want. (width, inside with of installed rails, bolt dimensions)
If I open the receiver any wider, my top cover isnt going to fit.
Any ideas?
Mu next option is to flatten out the rivet dimples just a bit but i really wanna know what's going on. The internals are not matching numbers to the receiver.

Re: Bolt will not fit

Posted: Mon May 14, 2018 1:07 pm
by JBaum
The bolt is factory, so it is obviously within spec. The problem is how the rails fit in the receiver. Maybe some built up weld is holding the rails closer together. That's up to you to determine, as we can't see the receiver. It may require slightly flattening the receiver dimples.
Making the bolt fit an out of spec receiver isn't an option. Is there weld built up near the cam piece? That would hold the rails together toward the front. Make sure the rails aren't bowed/bent. If you're using bolts to hold the rails in, make sure they're tight and not in the way. It's better to flatten the rivet dimples a bit than grind on the rails.

Re: Bolt will not fit

Posted: Mon May 14, 2018 1:34 pm
by makebang
Nah, there is no weld buildup. (i wish that was the issue lol!)
I won't go any more on the rails as now they match the original so no more should be needed. The caming piece fits perfect but the bolt won't even make it up that far.

The dimples on the receiver are very deep which is why i considered flattening them a touch. Im betting .020" or .030" on each side will get me where i need to be.

I don't know how much play those bolts normally have as i have never handled one before.

Re: Bolt will not fit

Posted: Mon May 14, 2018 8:09 pm
by Abominog
If rails are .01 thicker each that’s .02 which can be a lot. Filing isn’t very accurate. Make sure they aren’t bowed too. A twist to the rewear will also cause binding.

Coat the bolt and carrier in dychem to see where the wear is.

The 42 is far more forgiving than the 34. That receiver takes hours of fitting and polishing.

Re: Bolt will not fit

Posted: Mon May 14, 2018 9:24 pm
by robertmcw
Did you use a MG Rail Rivet Jig? You can spread the receiver with one. If you have one, measure the bolt and set the jig to the width of the bolt and see it the jig slips the track from the end to the camming piece. If you don’t have one, RTG has them pretty cheap.

Re: Bolt will not fit

Posted: Tue May 15, 2018 7:45 pm
Paint some Prussian Blue on the bolt contact surfaces and see where the contact points are. A narrowing of the gap between the rails, or a twist in the rails could cause serious binding. It is important that the rails be straight or the bolt will not move smoothly, causing all sorts of feeding problems.Be PRECISE.

Re: Bolt will not fit

Posted: Tue May 15, 2018 8:21 pm
by 42rocker
Wish I could say that I've not seen this before. But here are my thoughts if this was a 4 cut then a lot of stuff is correct, that's good news. What does happen a lot is in the cutting up of the receiver the back area closes just enough to cause this problem. Stop forcing the bolt in. Measure the slot that the top of the bolt runs in. In several areas then check against the drawings on this sight. You should find that slot closed to much. Open slot up causing the receiver to open which should allow the bolt to move better.
Also look for the rivet backups that you will use to set your rivets, make a pair or more. You can also use them to open up the receiver.

Good Luck

Later 42rocker