Lapping Locking Lugs

[joeb33050]

Mausery kinds of bolt action rifles have a pair of locking lugs that can be lapped for intimate contact with the receiver.
Some gun wizards claim that lapping locking lugs keeps the bolt from tipping at firing, and thus accuracy is enhanced.
I have lapped locking lugs on a number of rifles, ~ 10 if I had to guess. The process was explained to me many years ago, so I've done it, but never yet has my life been changed or serious improvements to accuracy noted. But that's just me.
The caveats:
First, lapping involves putting grinding compound, abrasive stuff, into the workings of your rifle. This gives me the heebie-jeebies. Careful and thorough cleaning after lapping is mandatory! Malt beverages serve to still the heebie-jeebies.
Second, lapping those lugs WILL increase headspace. Normally in the tenths of thousandths of an inch range, which hurts nothing.. Do not lap a lot off a lug on a complete rifle. I've always been able to lap the lugs removing very little material.

Here's a picture of a bolt from a mausery rifle. The locking lugs are the two things sticking out on the right hand or front end.


Here's a closeup of those lugs. The back surfaces of the lugs, the bottom surfaces perpendicular to the bolt are what we're going to attack.
I take the barreled action out of the stock, and take the bolt apart as much as I'm able.
Then I slather magic marker on the back surfaces of the lugs, put the bolt in the rifle, pull back and move the bolt handle up and down at the same time.
Then I take the bolt out of the rifle and look at those lugs. I want the magic marker to be rubbed off the both lugs, completely.
If the magic marker is rubbed of MOST of BOTH lugs, I'm done.
Generally I find that at first, most of the magic marker is rubbed off of one lug, little is rubbed off the other lug.
In this case, I put some abrasive on rear surface of the lug that had the most magic marker rubbed off-this is the lug/receiver lug thingy that needs to be ground down a bit. Put the bolt in the gun, and pulling back, move the bolt handle up and down-operate the bolt.
Stop, clean everything as well as you can, put on some more magic marker and operate the bolt again.
More abrasive if needed, do it again.
Now, I've never got ALL the locking lug surface to have the magic marker wear off, but I've been able to get MOST of the magic marker to rub off both lugs pretty quickly.
I've used various kinds of abrasives, left them when I moved down here, and now use NAPA valve grinding paste. When I'm done, I use some JB bore cleaner and get things cleaned up fairly easily on both lug rear surfaces.
I used to clean with oil or bore cleaner, but now clean with water-attempting to get all the abrasive out of the workings.
I don't think that you should do this locking lug lapping a lot, if you're not done in five or six laps, there may be something wrong. It takes me an hour or so to do the entire business. I don't want to take much material off a lug/the lugs.

[Char-Gar]

The laping of locking lugs won't by it self produce pronounced change in accuracy. It is one a number of small things, when we combined together can produce an accuracy improvment. I am talking about such things as squaring the bolt face and such things.

Tis true, extra headspace can be induced, but seldom is it enough to be a problem.

The whole idea is introduce the bullet into the barrel dead straight. This requires the threads of the action to be striaght with the action and the face of the bolt to be at a 90% angle to the changer/case head. A chamber which is not dead straight with the bore can also introduce less than best accuracy.

You cannot take a rifle with a bad barrel and turn into a good rifle. But this kind of tweaking can make an accurate rifle a little more accurate. There is no magic bullet but attention to the detail will add up on the target.

[44man]

I have lapped many in exactly the same way. Never found a great increase in accuracy but the important thing is to distribute the pressure force evenly to both lugs. 55,000 to 65,000# to one lug, stretching it until the other touches does not appeal to me.
Make sure the lugs are lightly lubed at all times too. No sense wearing them and increasing head space.
Instead of pulling back on the bolt, I put a fired case in the chamber. If there is still play, I put a piece or two of tape on the back of the case so there is slight pressure on the offending lug. Some bolts have too much slop and you can tip it when pulling back.

[joeb33050]

I have lapped many in exactly the same way. Never found a great increase in accuracy but the important thing is to distribute the pressure force evenly to both lugs. 55,000 to 65,000# to one lug, stretching it until the other touches does not appeal to me.
Make sure the lugs are lightly lubed at all times too. No sense wearing them and increasing head space.
Instead of pulling back on the bolt, I put a fired case in the chamber. If there is still play, I put a piece or two of tape on the back of the case so there is slight pressure on the offending lug. Some bolts have too much slop and you can tip it when pulling back.

I like that second paragraph, May I use it?
joe b.-looking for mistakes and additions.

[Bass Ackward]

I have lapped many in exactly the same way. Never found a great increase in accuracy but the important thing is to distribute the pressure force evenly to both lugs. 55,000 to 65,000# to one lug, stretching it until the other touches does not appeal to me.
Make sure the lugs are lightly lubed at all times too. No sense wearing them and increasing head space.
Instead of pulling back on the bolt, I put a fired case in the chamber. If there is still play, I put a piece or two of tape on the back of the case so there is slight pressure on the offending lug. Some bolts have too much slop and you can tip it when pulling back.


Jim,

Ya, but iffin you do that, who says that your bolt face is true. If the case is setting on any angle, then you will be grinding that angle into the lugs. And iffin it get's up in the chamber, you won't be real happy either.

Nope, there is a proper time and place for everything and action prep is done before barreling. Otherwise, the action will break in and line up on it's own as you use it. Which if you don't want to pay for a blueprinted action job, build on an older action. That's my choice anyway.

[HORNET]

IIRC, they used to recommend using an old cleaning rod (like that brass one that Joe told you to throw away) to apply the force to the bolt face to avoid tipping the bolt when lapping the lugs. Check the bearing surfaces on a Rem 788 sometime (and they'll usually shoot very well indeed)....But, as Bass says, it still won't fix a bolt face thats out of square with the universe.

[joeb33050]

Jim,

Ya, but iffin you do that, who says that your bolt face is true. If the case is setting on any angle, then you will be grinding that angle into the lugs. And iffin it get's up in the chamber, you won't be real happy either.

Nope, there is a proper time and place for everything and action prep is done before barreling. Otherwise, the action will break in and line up on it's own as you use it. Which if you don't want to pay for a blueprinted action job, build on an older action. That's my choice anyway.

Well, Bass, let's say that you've got the rifle, it's together, and you've checked and only one lug bears. You can't/won't have the barrel taken off or any other gunsmith work done. You aren't building a rifle, you have the rifle.
Is it better to leave it alone or lap the lugs so both bear?
joe b.

[Wayne Smith]

And, once you've done it, assuming your answer is positive, I see how you get the grinding compound off the bolt lugs, but how do you get it out of the recesses without removing the barrel?

[Bass Ackward]

Well, Bass, let's say that you've got the rifle, it's together, and you've checked and only one lug bears. You can't/won't have the barrel taken off or any other gunsmith work done. You aren't building a rifle, you have the rifle.
Is it better to leave it alone or lap the lugs so both bear?
joe b.


Joe,

Toughie, that you have to sort through yourself. Question is , are you sure?

There can be many things wrong that you can't correct by lapping. Or .... there could be nothing! The action can have a little play in the bolt that the cocking cam and spring are holding the bolt to one side and giving you a false impression that you need to lap. The only way to test that is to disassemble the bolt and remove the spring. Use a fired case and repeat your test.

I had a 700 Remington that was false just like that. If it is, it is most likely the lug on the side the bolt closes will be the one showing heavy contact. But you can change this by whether you push forward more on closing or pull back before you clear on the open. So how you work your bolt can give you this impression too. You would certainly want to eliminate the spring from this process. Then I would want a No Go gauge just for peace of mind.

Plus, some newer actions and bolts aren't heat treated through and through. Just a flash or surface hardening to alter metal hardness so you don't get excessive friction / wear. (Two like metal harnesses will wear quicker than two different) If this is the case and it is removed, then you can get greatly increased friction cocking assuming your action is cocking on the open.

Real toughie. Your rifle, your confidence level, but I would watch how my bolt attempted to close on the no go gauge and lock that point into my mind as I went through the process you describe. Even if it is, realize that many rifles are made this way and wear in naturally. By the time your barrel is broken in, probably the action is too. And since 98% of people don't clean or lubricate their lugs anyway, the grit that builds up there will be lapping right along. If you wanted to help this process, I would use JB on the bolt for lubrication and just shoot if it were me.

Sorry, I just saw it's not new. Use the fired case without the spring and see if you get the same results, then make your own decision. If it is like that, you need to ask yourself why it wore in the fashion. Everything tells a story.

[joeb33050]

And, once you've done it, assuming your answer is positive, I see how you get the grinding compound off the bolt lugs, but how do you get it out of the recesses without removing the barrel?

I can and do get my little finger into the locking lug area of bolt action rifles. I try to clean with a big patch on a brush, and generally get most of the stuff out. Then I check with my little finger, and frequently get black stuff out.
Working with water and patches and my little finger I can get most of the abrasive out, probably never ALL. That's how I do it, remember those heebie-jeebies though.
joe b.

[44man]

They way I look at it, when a shell is chambered, that is how the gun will be shot. If lapped that way, every chambered shell will let the lugs touch evenly, headspace is there, yes, but when the case expands it will do it the same every time anyway. The force is the same on the bolt face even if it is not perfectly true. I would not like to try lapping lugs to get the bolt face to true up to the bore. How much closer can you get to the actual firing mode then with a case in the chamber and enough shim to take care of the headspace? The small amount of material removed doesn't change things enough to worry about and I usually get it done with just a few bolt movements and 600 paste. Slow and easy, clean and check. Very few guns need course treatment. Then I go to 280 paste. Valve compound is too course and cuts too fast for my preferences. I would rather spend a little time then be in a hurry.
The old military rifle with excess headspace should be left alone since high pressure loads are not shot anyway. The headspace was created by worn lugs or a replaced bolt.
I made a brass rod that takes a thick felt flap to fit the lug recesses. I clean with several and they can be washed. With little effort, all the abrasive can be removed. I am sure all of you are creative enough to get them clean! [smilie=1: It is a good idea to clean them everytime the gun is cleaned anyway and new lube put on the lugs. How many of you actually lube these things? Most of you wear them in without lapping needed.

[DonH]

I AM NOT a gunsmith, just a tinkerer but have given this topic (bolt lug lapping) much thought. I have read and considered the method(s) described above. Then I read "RIFLE ACCURACY FACTS" by the late Harold Vaughn. On this topic he talked about lug lapping with a stripped bolt as is generally advocated. Then he talked about something which caused me to utter a loud "Duh!". With the bolt and trigger mechanism fully assembled, close the bolt, cocking the striker. In a Mauser action, at least (and maybe most others), when the sear engages the striker the rear of the bolt may be observed moving upward. THIS IS THE POSITION OF THE BOLT PRIOR TO FIRING! If the lugs are lapped to bear evenly with the bolt stripped, the bolt reassembled, placed in the action and cocked with the rear of the bolt rising as cocked, the bolt is now bearing on the bottom lug only! Vaughn advocated, and I now agree, lapping the bolt lugs with the bolt assembled so as to lap the recesses/lugs as they will be when in ready-to-fire condition.

[44man]

Have you asked yourself what happens to the bolt when the striker is released and before the pin hits the primer????? Does it stay up???? Besides, once the upward force is released, the case head is going to push back once the headspace is filled. That will sure force the lugs into contact and drop the bolt in the rear.
I can tell you this, once the trigger is pulled, there is nothing to hold the rear of the bolt up.
If lapped with a cocked striker, every time the gun is fired, the force will snap the rear of the bolt UP.

[Bass Ackward]

I don't disagree with what is said. There are many things to look at and some can be gun specific.

Where the bolt needs contact is not in the cocked position, but the fired position. If there is enough play in the bolt, pressure will pick the front of the bolt up and change the angle away from what the cocking mechanism was doing. Thus you could remove the contact on the bottom lug all together. If that was what you were lapping to, then all you did was enlarge headspacee cause it will still have to even out under pressure.

Bottom line, I would rather trust indicators when you can do it up right. The gun will seat itself in to cover minor misalignment.

[Mk42gunner]

Most of the older militay surplus Mausers were case hardened. I remember reading or hearing somewhere- that it was not a good idea to lap the bolt on mausers due to possibly cutting though the case; then having the lugs bear on the softer core material. Kuhnhausen maybe? Or it could have been while attending armorers class at Susanville.

So far the rifles that I have built for myself have been okay, so I haven't had to lap the lugs. If I were to build a newer rifle that is though hardened I would lap the bolt in and square the bolt face on a lathe.

Robert

[44man]

Bass is right, that is the reason bolts are not fit super tight in the action. They have to move a little the same as a good revolver needs a tiny amount of cylinder side play. Lapping just removes too much movement and the vibration it causes.
Old military rifles with excess headspace already have the case hardening worn away. Some have triple the headspace they were made with. Not only are the lugs worn but also the lug ways. If you are fortunate enough to have one that is not worn, make sure you keep the lugs lubricated. Even neck sizing and having bolt resistance is hard on them.

[felix]

After the action is right, then use moly powder on the lugs and where the trigger release meets the bolt. The idea is to keep those areas dry in a dusty environment so the dirt and grime can be blown out with a high pressure air hose, and the moly dust re-applied at random. Do not put moly powder down into the actual trigger mechanism on a 2 ounce BR trigger. Use lighter fluid every once in a while to flush the mechanism for these kinds of triggers. Moly will increase the creep in a highly tuned trigger over time. However, moly is fine everywhere internally for revolters. I personally do not use moly within the barrel, as for boolit lube. Gets my clothes dirty forever. ... felix

[44man]

Yeah, stuff is nasty! :roll: I have a big can of it and it even penetrates my plastic bench top. I mix some with alcohol to paint on stuff. What a mess.