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Discussion Starter · #1 ·
I’m about to do my first GI bolt to Chicom receiver conversion.

The only headspace gauge I have on hand is a SAAMI 308 WIN FIELD gauge. I can borrow a GO & NO-GO from a buddy this week, but I’d like to go ahead and start the conversion so the GI bolt will close. I can wait to lap until I get his gauges.

With an empty chamber this is what my TRW bolt looks like.


I have “The M14 Complete Assembly Guide” and I’m confident that I can properly relieve the receiver ring to make this bolt close.

Trying to get an idea of what I’m looking at for headspace I put my FIELD gauge in and try to close the bolt. This is what I get.


Based on the photo at the bottom of page 101 in Kuleck & McKee’s book, I’d say I have about 0.012” - 0.014" to go before the bolt would close on this FIELD gauge. Knowing that the SAAMI GO gauge is 0.008” shorter than my FIELD gauge, I expect to have to lap my bolt lugs about 0.004” - 0.006" to get this bolt to just close on the SAAMI GO gauge.

Does my estimation seem correct to you gentlemen?

What headspace do you try to get on your builds? I want the chamber as tight as I can get without sacrificing reliability. I will shoot mil-surp ball almost exclusively.

Also, how many thousandths can one safely lap a bolt without compromising heat treat, or lug geometry?

TIA!

SS
 

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MGySgt USMC (ret)
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Please read this thread also as it contains a very serious warning about making sure the FRONT of the bolt lugs are not
keeping the bolt from going down properly. The first thing you must do is ensure the front of the bolt lugs are not hitting the rear of the barrel or the receiver and THEN continue fitting the bolt.

http://www.m14tfl.com/upload/showthread.php?t=86960
 

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Discussion Starter · #4 ·
Thanks for sharing your knowledge and experiences with others. There are some good guys on this board.

I put off doing the conversion until I get some adequate gauges in. The one FIELD gauge I have is a Forster. GI8

I ordered a set of Clymer 308WIN gauges from Brownell's, and a set of 762NATO gauges from Manson today. Both had everything in stock and ready to ship.

I was going to spend that $$ on another bolt, but anything worth doing is worth doing right, and you gotta have the right tools. Especially with 55,000 lbs of pressure at your face....

I'll update the thread once I proceed.

Thanks guys.
 

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First thing you need to do is get the bolt to close on an empty chamber. Your better off pulling the barrel to do that though so you ensure that it is not the barrel that is keeping it from closing it.

1.)Pull the barrel
2)modify reciever to fit the bolt. Do not lap the bolt yet.
3)Spin the barrel back on and check headspace.
4)Report back after headspace is checked with a fitted bolt and poly barrel.

A good way to get estimates of headspace is to use scotch tape and put it on the back of the headspace gauges.
Scotch tape is .002" so you can take a 308win go gauge and make it a 1.632" gauge or a No go gauge would be 1.636" with scotch tape on the back of it. Just make sure you trim the excess tape off the edges.
 

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Discussion Starter · #8 · (Edited)
Forrest is still a hometown hero round these parts. He chased Streight through here and caught up with him at which time he illicited Streight's surrender. Streight had three times the men that Forrest did, but didn't realize it until after he had surrendered.

Forrest had a very sure and confident way about him. Not a man I would want after me.

I have used various types of tape to shim my other gauges before. I am careful to make sure the tape is only on the back and not on the rounded edges.

What do most people consider the max removal when lapping the lugs? I read the carburization is about 0.012"- 0.018" deep. How far into that can you go?

Seems to me that when you lap lugs you're removing material from both the lug and the receiver where the lugs lock. So, if you needed to lengthen the headspace 0.008", you would only get into the carburization about 0.004" on each surface.

Is that how it works in practice?

SS
 

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Not sure on the case hardening of poly recievers. As far as lapping and material removal, I would assume the softer material would be removed at a faster rate. I wouldn't want to do to much lapping. I guess fit the bolt and check headspace and see what happens.
 

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I have done a few Chicom M14 USGI bolt conversions and based on what I am seeing in your pictures, I would see if a USGI or commercial barrel would work out better with your particular bolt/receiver combo.

What you will likely find with your combo is that excessive lapping will have to be done to get proper headspace with your combo. Most Chinese barrels are actually short chambered when used with USGI bolts.

It is usually a good idea to pull the barrel and use the proper lapping tools to get proper lug contact with a minimal amount of lapping unless the bolt is an almost drop in fit.
 

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Discussion Starter · #11 ·
I tried the Forster FIELD gauge in my shooter M1A (all GI parts) which is not known to have ever malfunctioned and is so far, reliably a 2-3 MOA rifle in field positions, w/ M80 ball (762NATO mil-surp is all that she's ever seen) and based on what I'm seeing...

....my gauge has got to be longer than the 1.638" that's listed on it. I estimate the bolt is about 0.008" from closing (in the SAI rifle) with the gauge in, which would put my headspace at about 1.630", which just doesn't jive with my experience using the rifle.

I'll update once I have my new gauges in hand.

Thanks to all.

SS
 

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Discussion Starter · #12 ·
Ok, I got gauges now. I bought a set of Clymer 308win gauges and a set of Manson 762NATO gauges. Specs are as follows:

Clymer 308win:
Go = 1.630"
NoGo = 1.634"
Field = 1.640"

Manson 762nato:
Go = 1.632"
NoGo = 1.638"
Field = 1.642"

Both Clymer & Manson state a 0.4000" datum, so it should be close to comparing apples with apples.

I still have not thrown away the Forster 308win Field 1.638" gauge, so I could compare with the Manson NoGo at the same stated length. This is presuming that Forster uses the SAAMI 308win datum of 0.4000" also.

I have been researching the wealth of info here on the forum (and God bless you guys who contribute), online, and in the books I have, and some of the info can seem contradictory. I decided to start with what I do know.

I know I have two shooter M14 types that are proven to shoot M80 ball reasonably accurately. Neither rifle has given me any malfunctions, nor signs of excessive pressure on the brass.

I decide to start by determining what the headspace is on my rifles, with my gauges, and note the position of the right bolt lug with each gauge to get an idea of how the lug position translates into headspace (sort of like pages 98-102 in Kuleck & McKee's book).

The first rifle is a 79,*** SAI M1A with all GI parts. Barrel is an H&R. After I brought her home I did a tear down, clean up and lubrication and checked the muzzle with my CMP gauge and it measured "0". She showed little to no use. The only mod I made was to shim the gas cylinder. It has NM sights.

I have only shot German & Lithuanian M80 through this rifle. 460 rounds so far. I scored Expert on a full distance AQT for my Senior Instructor test with this rifle using the Lithuanian ball. I see no evidence of high pressure when looking at the cases. I have kept all the Lithuanian as it's boxer primed, and went through it to doublecheck before posting this.

Here's the bolt closed on the empty chamber:


Here's the Clymer 308win Go gauge of 1.630"


The Manson 762nato Go gauge of 1.632"


OK, if I'm correct, according to my gauges, my rifle headspaces somewhere around 1.631"? So my previous post's estimate of 1.630" was not too far off after all.

For reference I'll continue through the gauges.

The Clymer 308win No-Go of 1.634"


The Forster 308win Field of 1.638"


The Manson 762nato No-Go of 1.638"


The Clymer 308win Field of 1.640"


The Manson 762nato Field of 1.642"


So, based on this should I consider it dangerous or at least threatening premature wear on the bolt and receiver to shoot M80 ball?

Like I said, no signs of excessive pressure on brass or primer, no malfunctions, good accuracy, not hard kicking, etc.

Next we'll look at a Crazy Horse built by SEI on a Polytech, and see what she looks like. That'll be another night, though.

SS
 

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Is it me or does it look like the bolt is not closing all the way down on the empty chamber? This rifle may have the bolt roller defect were the bolt roller touches the rail when fully locked up. This can give you a false headspace reading. Check and see that the roller can spin while holding the bolt in the full locked position.
 

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Discussion Starter · #14 · (Edited)
Is it me or does it look like the bolt is not closing all the way down on the empty chamber? This rifle may have the bolt roller defect were the bolt roller touches the rail when fully locked up. This can give you a false headspace reading. Check and see that the roller can spin while holding the bolt in the full locked position.
No sir, it does not spin freely when I hold the bolt in the closed position. I wipe away the grease to get a look at the recess and you can see where the finish has been removed by contact.



Thank you for pointing that out, bigblock. I'll have to remove material and do this again. I'll also check out the Crazy Horse for clearance before I post photos of it.

I had seen this "sticky" by Art on the bolt roller impact defect but never got around to checking my rifles. For those who may follow this saga here's his link:
http://www.m14tfl.com/upload/showthread.php?t=50747

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I AM NOT A GUNSMITH SO DO NOT DO WHAT I'M PROPOSING UNLESS A SMITH SAYS IT'S OK.

If the Chinese combo is so short that you'd be concerned about lapping too deep, that would seem to suggest the bolt face is too far forward with respect to the bolt lug engagement surfaces (as compared to a USGI bolt). So is there any reason one could not lap the lugs for good engagement then lap the front of the bolt face with a pin gage the size of the cartridge head? Firing pin protrusion would increase, though it would be interesting to know if the ChiCom FP and USGI FP gave similar protrusion.

The ultimate goal would be to modify the bolt lugs AND face to approximate the GI bolt so you aren't lapping the receiver excessively.
 

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Discussion Starter · #16 ·
Ok, bolt roller impact fixed.


Empty chamber


Clymer 308win GO gauge of 1.630"


Manson 762nato GO gauge of 1.632"


So it reduced my headspace just a bit (based on visual comparison with the Manson 762nato Go gauge before the bolt roller impact mod) but is still between 1.630" & 1.632". I will get some 0.001" shim stock in the next week or two so I can maybe get a bit more precise.

I may look at lapping the bolt a bit to increase headspace. I need to closely inspect the bolt lugs and recesses to see if I think SAI lapped the bolt during assembly (doubtful) but initial peek at the lug recesses suggests not.







Funny how I started out wanting to do a GI bolt conversion to a Polytech and now I've taken a Dremel tool to the SAI receiver and looking at lapping the bolt in. GI8

More later on how the Crazy Horse headspaces and probably on the process of lapping in the bolt on the SAI M1A, as I could use better lock up and a little longer headspace when shooting M80 ball.

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Discussion Starter · #17 ·
I AM NOT A GUNSMITH SO DO NOT DO WHAT I'M PROPOSING UNLESS A SMITH SAYS IT'S OK.

If the Chinese combo is so short that you'd be concerned about lapping too deep, that would seem to suggest the bolt face is too far forward with respect to the bolt lug engagement surfaces (as compared to a USGI bolt). So is there any reason one could not lap the lugs for good engagement then lap the front of the bolt face with a pin gage the size of the cartridge head? Firing pin protrusion would increase, though it would be interesting to know if the ChiCom FP and USGI FP gave similar protrusion.

The ultimate goal would be to modify the bolt lugs AND face to approximate the GI bolt so you aren't lapping the receiver excessively.
I had thought of that too, but sort of poo poo'd it because of the affect it might have on firing pin protrusion. Plus, at the time I only had one suspect gauge.

To keep from being overwhelmed I'll deal with the SAI rifle issues first, document the conditions of the Crazy Horse, and then go back to the GI bolt conversion in the Poly with that which I learned.

SS
 

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Proper 7.62mm head space

Chamber HS and how it relates to case HS is a subjective issue. There are three situations when taking about case to chamber fit, Safety and Accuracy or both.

There is no question people fire 7.62mm ammo in M1A,s that have WIn. .308 HS readings, I am avoiding the numbers on purpose because that has been covered many times. When you use 7.62mm ammo in a Win..308 chamber you are possibly putting yourself and the rifle at risk, I have read comments too many times on this Forum stating: "I shoot 7.62mm in my M1A all the time and never had a problem". Getting away with it is not the same as avoiding it, next time you may not be so lucky, it would be wiser to be certain the ammo you use is ** case HS** at least .002" shorter than the chamber HS regardless of the chamber HS numbers.

Accuracy is related to Case HS vs Chamber HS. art
 

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art it would be wiser to be certain the ammo you use is ** case HS** at least .002" shorter than the chamber HS regardless of the chamber HS numbers. [/QUOTE said:
1st I am not a reloader but I do know how to check the headspace on my rifles chamber with my match headspace guage set. Is there a way to check the case HS on surplus ammo?
 

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Discussion Starter · #20 ·
Roger that, Art. Gus' thread on headspace suggested (IIRC) 1.6335" to be safe with 762NATO surplus and 308win.

I see the advantage in knowing what the chamber HS is, and then the case HS to make sure you're good. I know that there is significant variations in specs between NATO countries.

I am ordering the RCBS precision mic for 308.

bigblock- This is what it looks like:


RCBS Precision Mic Instructions here:
http://www.rcbs.com/downloads/instructions/PrecisionMic.pdf
 
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