Checking receiver thread on Rem 700

[trg-s338]

Hey folks, need some advise on how to check receiver thread parallelism to bolt axis on my brand new Remington 700 donor (7mm Rem Mag). This is for a budget entry level long range rifle build involving rebarelling of a new 700 receiver with a 6.5mm Remington Magnum barrel (still needs chambering/crowning but already fully profiled.) Trying to save some cash by not fully blueprinting the receiver and limiting the receiver work to facing the receiver barell abutment and bolt lugs recesses. Would like to retain the stock thread if possible without single point rethreading if not necessary, keeping in mind this is somewhat budget entry level. How does one check the factory receiver barrel threading for parallelism? Is the thread, receiver ring/bolt lug recess facing enough to achieve some level of accuracy that makes it worth doing? Is it necessary to get the bolt face machined also? The money I've spent so far is just that, spent! I'd like to limit further expenses to that which will give value for the finish of this build. Your information and advise will help me save/appropriate the necessary funds to complete my project. Or maybe I should just take the receiver to Randal for inspection and recommendations. Thanks in advance for your constructive input, it will be appreciated.

P.S. I've got some dial indicators and other instruments I can use to do the checking.

[ar15barrels]

Setup the action in the lathe.
Indicate within 0.0001" with a fitted mandrel at two points at least 3" apart.
Remove the mandrel.

Single-point chase the threads.
When you first start cutting, make note of the number on the dial.
When you finish cleaning out the threads to full depth, make note of the dial again.
The difference in those two numbers will be your total indicated runout of the threads BEFORE you fixed them.
While the action is there, skim the lugs and reciever face to minimum cleanup +0.001" depth.

[mofugly13]

That assumes that the bolt raceway is perfectly concentric, from one end to the other, with the outside of the receiver. Steve Acker has a great DVD that outlines a good way to setup a receiver in a lathe so that the receiver rotates around the bolt centrline, and all cuts taken are perfectly concentric, or at a perfect 90 degrees to the bolt raceway centerline.

Steve's method involves turning a mandrel between centers until it is a tight fit in the receiver. The mandrel is inserted through the receiver and locked down with a setscrew through the 1/4-28 hole in the front of the receiver. Then he puts a collar around the outside. The collar is about 1 1/2" OD and about 1 1/2"-2" long, and a slip fit over the outside of the receiver. He uses a little Bondo to secure the collar to the receiver, so that about 1/16" of the receiver sticks out past the collar. Once the collar is secured to the receiver, and the receiver is mounted between centers, he turns the outside of the collar true to the ceterline of the receiver. At this point he has a receiver with a collar whose surface is true and perfectly concentric to the bolt raceway. I think he also takes a facing cut across the front of the collar to roduce a surface perfectly perpendicular to the bolt raceway. I think this would be for indicating if needed later.

At this time a facing cut can be made to the barrel mating area of the receiver. Next he sets up a steady rest so that the fingers of the rest ride on the outside of the collar. Then he loosens the set screw on the mandrel and slides the mandrel back into the receiver until it's just clear of the bolt lug mating surfaces, and tightens the set screw back up.

Now I get a little fuzzy on how his next setup is. I believe he removes the center from the 4 jaw and grabs the mandrel directly in the chuck in order to drive it, with the outboard end of the receiver supported by the steady. You would need to make sure to indicate the setup when switching to the 4 jaw to maintain concentricity. Once you are all dialed in and reading minimal runout at the chuck end of the mandrel and at the collar, you have a setup where you can single point the threads, if needed, and true the locking lug recesses.

However, that is a lot of work, and all you want to do is check the threads for runout. I am in the same boat as you and have a Model Seven receiver I am doing a "not quite blueprinting" proceedure with. What I did is turn a mandrel between centers until it was a very snug push fit in the bolt raceway. With the receiver mounted to the mandrel I made a facing cut to true the barrel mounting surface to the bolt centerline.



Then I made a guage to check if the threads were cut parallel to the bolt centerline. Its just a chunk of colled rolled I had that I cut 1 1/16"-16 threads to a shoulder, with a facing cut across the shoulder to ensure a surface perfectly square to the threads, and a thread relief groove to ensure the guage would screw in all the way up to the shoulder. The threads are cut until they just screw into the receiver, not any sort of wiggle as it bottoms out in the receiver. When the guage was done, I coated the trued face of the receiver with prussian blue and screwed it into the receiver until it bottomed out. When the guage was removed, it had a perfect ring of blue on it transferred from the receiver. Any gaps in the blue transfer would mean that the threads are not parallel to the bolt centerline. I think I lucked out, because mine were.



I also decided to see if the bolt lugs were making even contact with the lug recesses. I made a tool to put rearward pressure on the bolt in the receiver and used fine valve grinding compound to lap the lugs in.



They lapped in nice and even, so I didn't feel the need to attempt to true them with cuts on the lathe, however, after doing more research on the subject, it seems that most precision gunsmiths just go ahead and skip lapping and make truing cuts to the bolt and receiver on the lathe.

At this point I am debating whether or not to true the bolt face. I think I will. The rifle I am biulding is only meant to be a very accurate hunting/metallic silhouette rifle, nothing more. But I do plan on picking up a M700 action in the future for a 1,000 yard rifle build, which I will give the full blueprint treatment to. This one I'm working on now is just to get my feet wet!

[ar15barrels]

That assumes that the bolt raceway is perfectly concentric, from one end to the other, with the outside of the receiver.

I'm not indicating the outside of the receiver.
I'm indicating the mandrel which is fitted to the receiver.

Here is my receiver holder that I also use to dial-in barrels for threading and chambering:



Any truing method that includes re-chucking the receiver in the middle of setup is fatally flawed.

[mofugly13]

Well, you did say

Setup the action in the lathe.

with no mention of using a jig or fixture of any sort. Probably would have helped the OP if you had mentioned it and perhaps posted the photo which you obviously had readily available.

As I said, my memory is fuzzy about Steve Acker's setup after truing the collar on the receiver, but when I get home, I'll pop the DVD in and watch it, and get back to you (the OP). Steve is known for building very accurate rifles, and while his methods may have been improved upon over time, you could hardly call any part of his method "fatally flawed." Seems a bit dramatic.

[ar15barrels]

As I said, my memory is fuzzy about Steve Acker's setup after truing the collar on the receiver, but when I get home, I'll pop the DVD in and watch it, and get back to you (the OP). Steve is known for building very accurate rifles, and while his methods may have been improved upon over time, you could hardly call any part of his method "fatally flawed." Seems a bit dramatic.

I have his book.
I know about his method.
It works acceptably for many people, but it still IS fatally flawed.

Let's say that his method can be used to get a reciever to within 80% of perfect concentricity.
The single-setup method can get the rest of the way to 100% perfect concentricity.

80% is a HUGE improvement over factory, but it's still NOT 100%.

Read what Greg Tannel has to say about it.

[mofugly13]

I hear you, and understand where you're coming from, but the OP obviously doesnt have a fixture truing jig, and stated that he is looking to do a "less than 100% blueprinting" on a budget.

Trying to save some cash by not fully blueprinting the receiver and limiting the receiver work to facing the receiver barell abutment and bolt lugs recesses. Would like to retain the stock thread if possible without single point rethreading if not necessary, keeping in mind this is somewhat budget entry level.

Almost every home shop machinist has enough scrap laying around to make the tools to use Acker's method. And it seems as if his method would be perfectly adequate for what the OP is looking for. I have tons of scrap in the garage, but I dont have a piece to make a receiver truing fixture from, yet. So, Steve's method would be an option. But I do agree, if all the operations can be completed in one setup, you are that much closer to perfection.

And I did go and read what Greg has to say about it:

With the once accepted method of turning a collar with the action between centers and then mounting in a 4 jaw and indicating only the collar in, two devastating accuracy robbing mistakes have been made. # 1 you have moved your setup form your collar cut and will be machining from a secondary setup. You have lost center line alignment. # 2 you dialed in the collar around the action in a four jaw chuck to 1/10,000 or less and believe you have achieved alignment. Here is the fly in the soup. Are the jaws on your chuck perfectly parallel with the rotating axis of the lathe to 1/10,000. If you put back or leave the mandrel in the action and check the runout at the end of the mandrel, you will with a sinking feeling see the action rotating in a progressively larger cone as you move away from the collar you dialed in on. The problem with the collar system is no way to take out this misalignment in this type setup. ALL MACHINING WILL BE INACCURATE & ASKEW FROM CENTER LINE OF THE BOLT BORE!

[trg-s338]

Thanks for the input from both of you. I suppose checking by creating a gauge and assessing contact at the facing would be the methodical way of finding out thread parallelism. I have a Monarch 10EE lathe but am still trying to figure out three phase power to it in my garage but until then, I'll have to put of finding the answer to my question until my lathe is operative. That is unless someone else comes up with another creative method of checking. Thanks all.

[ar15barrels]

I disagree with assessing the contact of the facing because the gauge will be pulled in tight all the way around even if the threads are not parallel.

If you made a gauge with a tight enough thread fit that there was no slop in the threads, perhaps it would work better.
What such a tight fitting gauge could really tell you is the runout from the centerline of a dialed-in reciever without actually cutting the threads.
Of course if you have the reciever dialed in to make such a measurement, you might as well go ahead and chase the threads too...