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Discussion Starter #1
I bought a Fulton receiver when they were on sale. I figured I'd use this receiver for my first build.

I tried two barrels, a chicom and a LRB marked Criterion, and both of them will not come anywhere close to closing on a Clymer GO gauge (1.630") using two different new TRW bolts.

I'm thinking that in this particular receiver a chrome lined barrel will not headspace, and it will need a short chambered barrel that can be reamed to use it.

Is this issue common?

Thanks
SS
 

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Do you think lapping the lugs would work? Not that you want to take that too far. How close to closed do you get?
 

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Discussion Starter #3
I'm guessing how far based on my limited experience and the photos of the right bolt lug position in the "Complete Assembly Guide" but I expect I've got about 0.006" to go to close on the 1.630" GO gauge with the Fulton receiver/Criterion barrel/TRW bolt combo.

I took the Fulton receiver off the Criterion barrel and started to install a LRB receiver on it. I inserted the 1.630" gauge into the chamber, closed a new TRW bolt on it and hand tightened it until it stopped on the gauge. I know it was not the receiver getting too tight to turn by hand because I had to back it off a bit to get the bolt to open.

So, with the LRB receiver turned on the Criterion barrel at the place where it just closes on the 1.630" gauge it's indexing at about 7:30, maybe 7:15. Referencing the "Assembly Guide", it should be about 0.004" shorter when I get the barrel indexed properly.

"Assembly Guide" says don't lap more than 0.003" to get headspace BUT that is in a receiver/bolt combo that has already been lapped to each other to get good lug contact areas, and mine have not yet been lapped at all (new receivers and new TRW bolts).

I know it would depend on the receiver and bolt specs but how much should I expect to gain by lapping the lugs for good contact only .

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Timing and Headspace

I generally see .006-.007 crush when the hand timed barrel is at 7:30 with commercial barrels and less with USGI barrels. I've even had as much as .009. On a new receiver, new bolt combo I look for a headspace of 1.637-1.640 when hand tightened above 7:00. If it's between 6:30-7:00 I prefer 1.635-1.636 and below 6:30 I'll settle for 1.633-1.634. This is prior to lapping.

You must account for the parkerizing when lapping a bolt/receiver and add that to the minimum. That is, if you feel comfortable lapping off .003, then you must add a tolerance range for parkerizing; for both the receiver and bolt. If the bolt is previously lapped don't. I see many used bolts that are .005-.007 below initial specification. These bolts don't show any further setback after a year of heavy use.

You indicated you were using a new bolt and receiver, so I can see you safely lapping .003-.006 collectively from both the bolt and receiver and being perfectly safe.



I'm guessing how far based on my limited experience and the photos of the right bolt lug position in the "Complete Assembly Guide" but I expect I've got about 0.006" to go to close on the 1.630" GO gauge with the Fulton receiver/Criterion barrel/TRW bolt combo.

I took the Fulton receiver off the Criterion barrel and started to install a LRB receiver on it. I inserted the 1.630" gauge into the chamber, closed a new TRW bolt on it and hand tightened it until it stopped on the gauge. I know it was not the receiver getting too tight to turn by hand because I had to back it off a bit to get the bolt to open.

So, with the LRB receiver turned on the Criterion barrel at the place where it just closes on the 1.630" gauge it's indexing at about 7:30, maybe 7:15. Referencing the "Assembly Guide", it should be about 0.004" shorter when I get the barrel indexed properly.

"Assembly Guide" says don't lap more than 0.003" to get headspace BUT that is in a receiver/bolt combo that has already been lapped to each other to get good lug contact areas, and mine have not yet been lapped at all (new receivers and new TRW bolts).

I know it would depend on the receiver and bolt specs but how much should I expect to gain by lapping the lugs for good contact only .

SS
 

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Discussion Starter #5
OK, here's my LRB/Criterion, indexed with the Clymer 1.630" Go gauge in it. Does this look like I can lap in the bolt/receiver and be good? New bolt, new receiver, with park intact.

 

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OK, here's my LRB/Criterion, indexed with the Clymer 1.630" Go gauge in it. Does this look like I can lap in the bolt/receiver and be good? New bolt, new receiver, with park intact.

Yeah, looks like you got about 0.003" on each face to go. Maybe a hair more... I dont know. Im usually still reaming the chamber down at that point. (I realize you cant do that in this case)... Definitely wait and see if J. Wolfe answers...
 

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Discussion Starter #7
And here's the Fulton/Chicom combo, indexed with the 1.630" Clymer Go Gauge in it.


I believe both of my bolts are new. You said "I see many used bolts that are .005-.007 below initial specification." Can you talk me through how to measure my bolts to determine that they have not been used or lapped and then reparked?

Much thanks,
SS
 

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I dont know about the Fulton/Chicom combo (looks more than .006" - but could be the light), but with the LRB and Criterion Id probably go for it, lapping that is.

Bolt lug face on the Fulton/Chicom picture looks very flat, so I might be inclined to think it was lapped before. (See further comment below), but again could be the angle or the light.

Now, that said, I dont buy chromium lined barrels, so Ive never done it for that purpose (or ever plan too), but if I wanted one, Id realize either Im gonna have to do it or or just pay someone else to do it so I dont have look...

I dont have any special way of telling if a bolt is new or reparked or whatever other than it looks new, comes in the original wrapper and believe when people such as the nice Mr. Jon Wolfe sell it to me and tell me it is....MCORPS1

I just haven't handled enough to know for sure...
 

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Headspace

Based on the picture your headspace is about approximately 1.625-1.626 if you only applied light pressure to close it. You can lap that in, but unless you have a low grit compound it might take a while.

You may want to start with a 300 grit and get it to close before you move to a finishing grit of 600 and then 800.

Use an unfired case and hammer spring with 4-5 coils cut off for pressure. If you have extra hammer springs you can cut one longer than the other and start with more pressure for a more rapid process.

You may also bear in mind that some receivers are tight and the fact that the bolt will not close could be due to other factors not related to headspace. You should always try to pre-lap a bolt before installing the barrel to rule out those other factors. You can also close the bolt on an empty chamber and see if there is any wiggle room front to back and side to side. It could be binding on the sides and even on the front left lug (as viewing from the top of the bolt) as the front left lug could bind on the barrel face or the receiver.

What I would do at this point is put an unfired round in the chamber and see how far the bolt will close. If you have a precision mic gauge you can better estimate your actual headspace. Most commercial rounds have a length from the face to the datum of 1.627-1.628.

If you don't have this tool, you can lap the rib cage of the bolt; the area right behind the lugs and the left front lug to make sure it's not binding in the receiver. After this you can get a better feel for actual headspace if the bolt was binding. If it's not binding, your picture indicates the headspace is still very short and will need lapping to bring it in to a safe tolerance. The picture indicates you have quite a bit of lapping in front of you and if I had one that tight, I probably would try lapping but if it doesn't move much after your first round (10 minutes) I might look at other options.


OK, here's my LRB/Criterion, indexed with the Clymer 1.630" Go gauge in it. Does this look like I can lap in the bolt/receiver and be good? New bolt, new receiver, with park intact.

 

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Discussion Starter #10
Based on the picture your headspace is about approximately 1.625-1.626 if you only applied light pressure to close it.
I went back and closed the bolts again after reading this, and yes it's with light pressure. My Clymer gauge doesn't seem to flex. Once it stops with light pressure, that's where it will be even after increasing pressure. I didn't take a punch and try to cam it further shut with the extractor hole or anything to force it.

You can lap that in, but unless you have a low grit compound it might take a while.

You may want to start with a 300 grit and get it to close before you move to a finishing grit of 600 and then 800.
I have some Permatex Grease Mix Valve Grinding Compound which lists silicon carbide on the can. It doesn't have a number rating on it but I would guess it's 200-300 grit. Can I start with that?

Use an unfired case and hammer spring with 4-5 coils cut off for pressure. If you have extra hammer springs you can cut one longer than the other and start with more pressure for a more rapid process.
I have a cut case, polished down, with a hamerspring in it using a nylon flathead countersunk head type screw. The head matches the 308win/762nato head size perfectly.

I don't have any extra hammersprings. I could order some, but, do you see any issues with me just backing out the barrel to decrease the pressure as needed, using the full size spring?

You may also bear in mind that some receivers are tight and the fact that the bolt will not close could be due to other factors not related to headspace. You should always try to pre-lap a bolt before installing the barrel to rule out those other factors. You can also close the bolt on an empty chamber and see if there is any wiggle room front to back and side to side. It could be binding on the sides and even on the front left lug (as viewing from the top of the bolt) as the front left lug could bind on the barrel face or the receiver.
I want my builds to have chrome-lined barrels. So I figured instead of lapping the bolt to receiver first, I would install the barrels I have and see if the HS could be lapped in. That way if it looked like one of my receivers would not HS with any of the chrome lined barrels I have, I could resell the receiver. I guess it doesn't really matter as once lapped I could just sell the bolt with the receiver as a matted pair.

On the Fulton/chicom, once the barrel was indexed the bolt would not close on an empty chamber because of the barrel face. I used Blue Dykem (you can probably see some in the photo) in an attempt to find the offending areas, and carefully and slowly, used a Dremel with a right angle attachment and a couple different grinding/sanding attachments to relieve the offending areas. Now the bolt closes in that one. I may have to relieve just a bit more once the bolt is assembled as I think the extractor will stick forward a bit more than the front ring of the bolt itself.

On the Fulton/chicom combo I have good wiggle left-right and up-down at the rear. I do not have forward-rearward wiggle, which I thought you don't want much of anyway. I would expect that you'd get the little bit of forward-rearward travel you're looking for once the bolt/receiver recesses are lapped in.

On the LRB/Criterion combo I have some (not much though) wiggle left-right at the rear. It has good wiggle up-down at the rear.

This LRB/Criterion also has no forward-rearward wiggle which I think will change once lapped.

To check whether the left lug is binding at all on the receiver or barrel face I've been using a really thin sheet of receipt paper that I cut down to a wide strip, sort of like what a shredder kicks out, so I can stick it where I want, close the bolt and see if it binds the paper. On the Fulton/Chicom it just slides out without much feel. On the LRB/Criterion it will bind the paper in the full closed position, but if I open the bolt at all, I mean any percievable amount, the paper slides out easily. So, I think the LRB will be fine once I start lapping and both future rifles are ok as far as bolt fit at this stage. Do correct me as necessary.


What I would do at this point is put an unfired round in the chamber and see how far the bolt will close. If you have a precision mic gauge you can better estimate your actual headspace. Most commercial rounds have a length from the face to the datum of 1.627-1.628.
Here's a photo of the Fulton/chicom with a Winchester round in it.


And the LRB/Criterion with the same Winchester round:



I bought the RCBS Precision Mic for 308win a few weeks back. Measuring a couple of loaded rounds and using the "0" on the RCBS gauge as the reference point I get -0.0015" from Winchester SP and a DAG mil-surp. I get -0.0035" from another Hornady hunting load.

Now, my Clymer 1.630" gauge reads -0.0015" also. I can get different readings +/- a thousandths or so with the Precision Mic depending on amount pressure, direction of gauge when using, etc. But these readings are all done the same way, gauge upright, spinning the head with light pressure until it stops, each round done multiple times to get a consitant repeatable measurement.

When I put my 1.632" (Manson 762NATO GO) gauge in the Precision Mic it reads about "0", and so on as I try different length gauges, so I think the index marks on the RCBS gauge itself is just a bit off and the HS gauges I have are fine as the increase in gauge lengths seem to be what they should be.


If you don't have this tool, you can lap the rib cage of the bolt; the area right behind the lugs and the left front lug to make sure it's not binding in the receiver. After this you can get a better feel for actual headspace if the bolt was binding. If it's not binding, your picture indicates the headspace is still very short and will need lapping to bring it in to a safe tolerance. The picture indicates you have quite a bit of lapping in front of you and if I had one that tight, I probably would try lapping but if it doesn't move much after your first round (10 minutes) I might look at other options.
I presume that you lap the "rib cage" and "left front lug" areas with the bolt in the receiver just like you do with the rear of the lugs? Not that I think these need it, but I'm trying to learn everything I can, saving what mistakes I can by picking others' brains.

So, basically it's "happy lapping", eh? As long as it will be safe to do so I don't mind taking the time to lap enough to make it work.

Talking with a buddy, we thought of other things that might help reach headspace, too.

I could polish the bolt face itself. Jerry's book says you can do this once, removing as much as 0.002" in material. I would just polish smooth to take the high spots of the parkerizing off. I have Dremel felt polishing wheels that are the perfect size to do this. Also thought polishing the chamber might help also. It shouldn't hurt reliability of the rifle, that's for certain. These two items may give me 1-2 thousandths right there.

Another thing I was wondering about. Does anyone ever shim the barrel? The LRB took much less torque to index than the Fulton. Could you put a thousandths or two shim on the barrel shoulder to increase draw and HS length? Seems like the threads are heavy enough that you could add a thousandths or two.

Thanks to all that make this website what it is, which is nothing short of awesome.

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You can add to the shoulder by rolling it or peening. I thought about shims for this but was discouraged from it from several people here. A shim that is only a few thousnds thick would surely gall and distort.
 

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Bolt measurements

And here's the Fulton/Chicom combo, indexed with the 1.630" Clymer Go Gauge in it.


I believe both of my bolts are new. You said "I see many used bolts that are .005-.007 below initial specification." Can you talk me through how to measure my bolts to determine that they have not been used or lapped and then reparked?

Much thanks,
SS
A new TRW bolt left lugs will be .461 and right will be .571
You will have to rock the gauge and check several times as the rounded area will give you phantom readings.

I received a N.O.S. Winchester Bolt that measured .465 left and .575 right....but it is not the norm.

On three occasions, a new TRW bolt with these measurements dropped in with G.I. barrels on three different manufacturer's receivers and headspaced at 1.6325 before lapping....just dumb luck ? who knows....probably living right.
 

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Discussion Starter #14
A new TRW bolt left lugs will be .461 and right will be .571
You will have to rock the gauge and check several times as the rounded area will give you phantom readings.
That's exactly what these bolts are reading. Thanks.

You can add to the shoulder by rolling it or peening. I thought about shims for this but was discouraged from it from several people here. A shim that is only a few thousnds thick would surely gall and distort.
Copy that! Makes sense to me.

I would not use valve lapping compound. It will gall both receiver and bolt. There are several suppliers of lapping compound. Brownells is a good place to start.
Thank you. Already have some on order from Brownells.

SS
 

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Discussion Starter #15
I've got a few USGI barrels on the way and in a week or two I'll try them out. I'd prefer to not have to lap as far as it appears I will have to in order to make decent HS.

I'll update then.

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Discussion Starter #16
LRB/NOS TRW Bolt/ Like New TRW Barrel

I have since obtained a few GI barrels to work with. I installed one of them in the LRB. It definitely closes further on the 1.630" gauge. Now I'm a bit less worried about lapping too far into the carburization. What do you think? Add a thousandth or two for park, plus another 3 for lapping and I should be around 1.633"????

With 1.630" Clymer GO gauge.


Before I installed the barrel the bolt seemed to close fine on empty. At the rear of the bolt it had movement up and down and minimal movement left & right when closed.

Now with the barrel in, the bolt will not close on empty. When closed as far as it will go with light finger pressure I still have up & down movement in the rear of the bolt, but I have no left & right movement.

Would it be safe to presume that lapping the bolt in to get HS will give me back my rear bolt movement? Is it ok to put some lapping compound on the left rear lug when lapping?

Here's as far as the bolt is closing on an empty chamber:
 

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couple of things, either your receiver lugs are rough machined ( should be visible, exhibiting a line ) or your bolt is binding at the barrel between it and the bolt face shroud.

Maybe pre-lap as Jon said....I wouldn't do anything to the guide ear on the rear of the bolt until getting the front end straightened out, then address it.
 

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Face it guys the chamber is short no matter how you slice it. It needs to be finish reamed to head space correctly.

Just because a barrel was finished reamed for one receiver, it does not mean that it's good to go on another receiver.

Just my 2 cents.
 

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Discussion Starter #19 (Edited)
ripsaw - It's probably the bolt/barrel face as it didn't bind before I installed the barrel, and the lug recesses look good, smooth.

So pull the barrel, lap the bolt to receiver to get good contact on each lug/recess, reinstall the barrel and then see where it's at? If after that it's still binding on empty, say at the front of the left lug against the barrel, how do you relieve that area? Lap the front of the lug too?

BrokenVet - It's a USGI chrome lined bore and chamber barrel. You have to use an expensive reamer to attempt it as the chrome's so hard, and then the chrome is likely to start pealing off from the throat thus ruining the barrel.

Ron Smith told me it (reaming a chrome lined barrel) could be done "but you better #@*%! know what the %$$#@& you're doing", which I don't. GI1

Thanks guys.
 
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