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Discussion Starter #1
Started lapping a TRW bolt to a LRB receiver last night. I started with 400 silicon carbide abrasive from Brownell's. My lapping tool consists of a polished & cut empty case with nylon screw, hammerspring and a barrel hand tightened to secure it.

Before proceeding this had good up-down "wiggle" at the rear of the bolt when closed, but barely precievable "wiggle" left-right. When lapping I could see where the sides of the bolt behind the lugs were rubbing a bit. Knowing that my left-right rear bolt play was minimal, I put a bit of lapping compound on these side areas too for a session of lapping. Now I have better clearance there and the left-right movement is satisfactory.

Right lug:



Right Receiver Locking Recess:


Left Lug:



Left Receiver Locking Recess:


Bolt lugs now measure about 0.0015" - 0.002" shorter than they did pre-lapping. Based on visuals of park removal, I'd assume that removal of material has been fairly even between bolt & receiver, perhaps a bit quicker on the receiver.

I have a few GI barrels, a Criterion and a few Chinese barrels (all chrome lined) to choose from for this build. The TRW I'd like to use for this build is indexed in this photo, with a Clymer 1.630" Go Gauge in it. This photo is before any lapping.


We'll see where I'm at after I finish intial lapping for near perfect contact between lugs and recesses and confirm or deny that I can use the intended barrel and get desired chamber headspace without lapping too far into the carburization. I'm switching to 600 grit now.

SS
 

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Looks good, ...Art Luppino mentioned that 60% contact is sufficient, and it looks as though you are almost there,....easy does it from here on in....Very nice work, might be a future in it for you GI1
 

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Discussion Starter #3
After a few minutes of additional lapping with the 600 grit I reinstalled the TRW barrel I want to use. I had already used 0000 steel wool on a dowel in my drill to polish the chamber.

The bolt closed on the 1.630" gauge with no feel. I went to the 1.632" gauge and it will not quite close on it.

I polished the bolt face with some Dremel polish on a Dremel 414 felt polishing wheel. It is the perfect size for the bolt face. I didn't do anything but slick it up.

I measured the bolt lugs and I'm 0.0025" shorter than I was before I started.

I should be ok lapping to get another thousandth or two of chamber headspace. I'd like to get it around 1.633". That should be at least 0.002" longer than the pile of M80 ball I have waiting for it.

More later
SS
 

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I know your not lapping for contact but you may want to check the lugs with dykem. You will be surprised what you will find. The shiny area may just be worn off parkerizing. The only way to really check for sure is with the dye. Sounds like your having a great time with this project. I have not installed any chrome lined barrels and this is a great example of the process. Thanks for posting it.
 

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Discussion Starter #6
Got chamber headspace so it closes on the 1.632" gauge but will just not close on the 1.634" gauge. That's just what I was looking for. According to my calipers (and the operator) the lugs are 0.004" - 0.0045" shorter than they were when I started, and that includes the thickness of the parkerizing.

Just to introduce her to the rest of the family I dressed her a bit with a CMP TRW parts kit.


For those that are interested here's what the lugs & recesses looked like before I installed & indexed the barrel last time.

Right lug:


Right recess:


Left lug:


Left recess:
 

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Discussion Starter #7 (Edited)
Bolt closed on empty for reference:


This is the bolt closed on the 1.632" Manson 762NATO Go gauge. It's all the way closed so it'll look like this with the Clymer 1.630" 308WIN Go gauge too.


This is the bolt on the 1.634" Clymer No-Go gauge. If you look closely you'll see that it's not fully closed (compare with the empty photo as the 1.6320" photo is not in good focus). I was aiming for chamber headspace of about 1.6335" as I have a good pile of various NATO surplus.


This is the bolt on the 1.638" Manson 762NATO No-Go Gauge. So this bolt lug position would be about 0.0045" from closing.
 

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Discussion Starter #9
Thank you. It will probably be a while before she's done. I have several things to do to her which require more tools, labor and parts.

I'll start building the Fulton receiver I have before finishing this one, so I'll be completing two at the same time. I will share the process for that one as well.

A word about the headspace thing that might make things less confusing for others. My "go-to" SAI rifle has chamber headspace at around 1.631" and I have shot nothing but 762NATO mil-surp out of it. I've not witnessed signs of excessive pressure or had any malfunctions or anything. I have measured (with a RCBS Precision Mic) the cartridge headspace of the three types of surp I have been shooting in her and one of the types is right at or under 0.002" shorter than my chamber headspace. This is walking the line as you need at least 0.002" longer chamber headspace than cartridge headspace for the cartridge neck to have room to expand and seal properly.

Understanding the 0.002" longer chamber headspace concept, I could have headspaced this LRB @ 1.632" and been ok with the ammo I have, but I wanted at least one rifle that would safely shoot whatever 308win or 762NATO I found to put in it. If the chamber headspace is 0.005" longer than the cartridge the accuracy may not be as good as if it was closer to 0.002" longer. But this LRB I'm building is meant to be a field grade rifle (thus chrome barrel, 1.6335" headspace, etc) and as long as I can deliver 4 MOA with her, I'm happy.

So if someone wanted to build a match rifle they would probably not choose the same headspace measurements that I did, as they would not be shooting mil-surp ball but tighter spec'd, shorter cartridge headspace ammunition like match 308win or handloads.

So remember that rifles don't shoot headspace gauges and don't just read this and think YOU HAVE TO set your headspace at 1.6335".

Thanks specifically to Gus Fisher & Art Luppino in helping me wrap my brain around all of this and more.

Gus has a great thread on headspace which helped my learning curve here:
http://www.m14tfl.com/upload/showthread.php?t=66601

SS
 

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Nice work!! Thanks for posting. Especially all the great pics. This thread has shown what you can do if you have the patience to work thru issues.
You mentioned that you can't finish it yet. What other projects do we have to look forward to?
 

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I have a question about your comment -
"This is walking the line as you need at least 0.002" longer chamber headspace than cartridge headspace for the cartridge neck to have room to expand and seal properly."

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I thought the 0.002 HS clearance was needed for reliable chambering and bolt closing & opening. Basically so there is room in the chamber from the shoulder to the boltface to accept the cartridge. I don't think that HS is involved with the neck measurement.

Neck clearance is determined by how that area of the chamber is cut - the chamber HS is rearward of the neck, from the chamber shoulder area to the boltface.
And typical 7.62 & 308Win chambers are cut with plenty of room for neck expansion.

Jay Kosta
Endwell NY USA
 

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Masterful job with Pictures.

The Post and pictures are very well done, as is the narrative. Everybody that goes into this sort of modification should have a copy of your contribution.

There is little to be added to your Post other than to remind all that the primary thing to keep in mind is: Modify your rifle to fit the purpose of how you intend to use it..Do not come away from this Post assuming the lapping mod is for the purpose of setting Head Space to chamber a certain type of ammunition, that's a Sizing Die job..

The purpose of Lapping is to ensure both Bolts Lug contact receive recoil surfaces in an even manner to prevent undue stress on one Lug. Bolt Lug lapping is a "Safety" procedure first.. If you buy a Commercial M1A receiver this is something you should consider doing as the building process begins... Doing the Lapping on an assembled M1A requires knowing the existing numbers of the Head Space. There is no reason why you should not Bolt lap to get the benefits. primary and secondary, except the possibility of excessive Head Space resulting or cutting through the surface hardening.. Know your numbers before getting into Lapping.

This Gentleman has earned the second "Big Red" Award this week. Congratulations. Art
 

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Great job, thanks for sharing and providing pics.
 

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Discussion Starter #14
I have a question about your comment -
"This is walking the line as you need at least 0.002" longer chamber headspace than cartridge headspace for the cartridge neck to have room to expand and seal properly."

--------------------
I thought the 0.002 HS clearance was needed for reliable chambering and bolt closing & opening. Basically so there is room in the chamber from the shoulder to the boltface to accept the cartridge. I don't think that HS is involved with the neck measurement.

Neck clearance is determined by how that area of the chamber is cut - the chamber HS is rearward of the neck, from the chamber shoulder area to the boltface.
And typical 7.62 & 308Win chambers are cut with plenty of room for neck expansion.

Jay Kosta
Endwell NY USA
Hey Jay, you are absolutely correct that headspace (for this cartridge) is measured from the bolt face to the datum on the shoulder and the 0.002” clearance does aid in feeding and extracting.

I probably worded that wrong and should have left "neck" out of that sentence. Let me try and explain how I envision it for clarification and so someone can step in and correct me if I misunderstand the situation.

Let's consider the bolt action for a moment. In a bolt gun, handloaders will often have the case sized as close to the dimensions of the chamber as possible for accuracy and longevity of the case. Brass that has been fired in a particular rifle will often just be neck-sized as the case is fire-formed to the dimensions of that rifle’s chamber (minus maybe a thousandth where the brass retracts when cooling). I know I like to “feel” the tightness of my handloaded 308win rounds in my rem700 when I close the bolt. In this example the cartridge headspace is not 0.002” shorter than the chamber headspace and the case’s shoulder is tight against the chamber shoulder when the bolt is fully closed. When firing, the case becomes so hot its basically molten and most of the “flow” of the brass will show up as increased length in the neck area because the cartridge headspace can't grow any longer. This is not a dangerous condition in a bolt gun because the bolt is locked shut until you move it. No matter how fast you are on the cycling of the bolt the fired brass will be much cooler than the automatically ejected brass from a gas gun because it had significantly more time to cool in the chamber (and retract from its largest size). Since you are the operator of the bolt and cause extraction and feeding it does not matter if it takes 2 ozs or 20 lbs of torque to operate the bolt, it still can be operated without fail.

In the M14 there are a variety of parts which move at a particular time and speed with the pressure curve of the ammunition it was designed for in order for it to operate properly.

When any cartridge is fired the brass will “flow” towards the neck because that’s the path of least resistance (with the cartridge being supported in the rear by the boltface and on the sides by the chamber walls).

Imagine a bottleneck case a few thousandths shorter than the chamber being fired. The brass can expand ("flow") towards the neck before, at, or after the shoulder without much interference. Now imagine a case that has its shoulder tight against the chamber shoulder. It’s like the case is pinched at the shoulder, thus decreasing its ability to flow freely towards the neck. While this is desirable in a bolt gun it is not desirable in a M14.

In a boltgun the fired case has time to (relatively) cool down and retract from it's largest size and gas pressure is nonexistant by the time you can manually cycle the bolt. This is not the case in the M14, where it is rotated and begins to extract with gas pressures still present.

My understanding is that if there is not the clearance for the case to grow that 0.002” it may cause excessive wear on some of the rifle parts, possible extraction issues and may not make the gas seal properly. It may be that since the case wall is pinched at the shoulder and only the neck area can "flow", the brass may stretch from that point, get really thin there and even possibly break at that thinner point, as the hot case is violently twisted and yanked from the chamber. That's not good with the gas pressure present.

I hope that makes sense. Anyone should feel free to correct me or add to this. GI6

The Post and pictures are very well done, as is the narrative. Everybody that goes into this sort of modification should have a copy of your contribution.

There is little to be added to your Post other than to remind all that the primary thing to keep in mind is: Modify your rifle to fit the purpose of how you intend to use it..Do not come away from this Post assuming the lapping mod is for the purpose of setting Head Space to chamber a certain type of ammunition, that's a Sizing Die job..

The purpose of Lapping is to ensure both Bolts Lug contact receive recoil surfaces in an even manner to prevent undue stress on one Lug. Bolt Lug lapping is a "Safety" procedure first.. If you buy a Commercial M1A receiver this is something you should consider doing as the building process begins... Doing the Lapping on an assembled M1A requires knowing the existing numbers of the Head Space. There is no reason why you should not Bolt lap to get the benefits. primary and secondary, except the possibility of excessive Head Space resulting or cutting through the surface hardening.. Know your numbers before getting into Lapping.

This Gentleman has earned the second "Big Red" Award this week. Congratulations. Art
Thank you Sir! It is an honor to give a little something back as I have received much from those like yourself who are willing to volunteer time and effort to help bring others along. Besides, the documentation of my learning process helps solidify things in my own mind and gives opportunity to be corrected when wrong, or learn other, better ways of doing things.

Nice work!! Thanks for posting. Especially all the great pics. This thread has shown what you can do if you have the patience to work thru issues.
You mentioned that you can't finish it yet. What other projects do we have to look forward to?
Thank you. I have the Fulton receiver to build up also. I still have a Polytech that I need to do a GI bolt conversion on. Maybe eventually I’ll do a LRB M25 build.

On this LRB build I’ll need to get a knurling tool as the op-rod guide is loose. I’ll be trying my hands at bedding in the next months also. I’m sure other things will come up and I'm happy to document them for my own and for others' learning.

SS
 

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On this LRB build I’ll need to get a knurling tool as the op-rod guide is loose. SS
Lookin good, ...a little trick you might want to try if you don't have a high dollar barrel,...just use a punch and delicate like , tap the roll pin relief slot on the barrel a couple times from both directions toward the slot...not a lot, just enough to tighten up the pin,..this will help hold it still a good while, when you drive the pin back in.
 

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Discussion Starter #16
Lookin good, ...a little trick you might want to try if you don't have a high dollar barrel,...just use a punch and delicate like , tap the roll pin relief slot on the barrel a couple times from both directions toward the slot...not a lot, just enough to tighten up the pin,..this will help hold it still a good while, when you drive the pin back in.
Yea I thought about staking the end of the slot or dimpling the area of the bearing surface with a hammer and punch but neither helps me become equipped or experienced in the NM op-rod guide mod.

Since I've got shooters I just might be able to muster the paitence to wait for the knurling tool....

SS
 
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