Gunsmithing Lapping Lugs... Ever done it?

[blurry6]

I was testing out the engagement of the lugs on my factory .338 RUM - degreasing the bolt/action, covering the rear surfaces of the lugs with black magic-marker, inserting a fired case and working the bolt up and down. Then pulling the bolt and seeing how much of the lugs engaged (scraped off the magic marker). I was suprised to see that only a tiny little streak of metal on the outside rim of both lugs showed engagement. I know this isn't ideal, and I've been doing some research on the net regarding the process of lapping lugs.

Ideally, it's done when rebarrelling and if trying it on a completed rifle should be done with great care so as not to screw with the headspace, but it looks like if done conservatively (<0.003" removal of the lugface) and approached slowly (600+ grit lapping compound and frequent remeasuring), it can be quite successful.

Anyone ever done this type of work on their rifle? If so, did it improve it's function/accuracy for you? Let me know your thoughts on it. It looks pretty straightforward to do, but am I just playing with fire,... Literally! Thanks!

Ryan

 

[Grand]

Re: Lapping Lugs... Ever done it?

I have lapped several 700's using the NECO kit. I generally start with 800 to see how much lapping is going to be required. If it looks like mimimal contact, I'll drop down to 600. Most of the time I start at 800 and finish with 1200. I can't express an opinion on whether it helps, but it sure can't hurt unless to screw up the headspace. Take out the firing pin assembly and you will reduce the effort. When your finished, use an aerosol bore scrubber to douche everything.

 

[GasLight]

Re: Lapping Lugs... Ever done it?

I have not lapped any lugs, but I will say this, the difference between a go gauge and a no-go gauge is between .005" and .006" IIRC. So that gives you an idea what kind of room you have to work with. Use a dial caliper to see how much material can be removed, not much. I am not saying don't try it, just saying go slow and don't take much if you plan to do it. Also, I don't think you want to lap the lugs if you can't take the barrel off, or if you don't have some other way to ensure consistent and even pressure on the face of the bolt. Taking the firing pin out is a necessity if you want to do it properly.

Dave

 

[Clark]

Re: Lapping Lugs... Ever done it?

I worry that the bolt body has too much clearance inside the bolt bore of the receiver. This clearance makes lapping problematic. The lugs could be lapped with flat contact when the bolt body is at one angle inside the bolt bore, but not at another. The Mauser bolt body is too busy with features to sleeve.

I am just an amateur gunsmith, but for 98 Mausers, I have:

I drill and tap a hole in the bottom of the receiver near the recoil lug.
I screw a 5 pound spring ball detent into the hole.
http://www.brownells.com/aspx/...0BALL%20SPRING%20KIT
http://metalworking.mscdirect....CTLG=54&PMT4TP=*LTIP
I notch the bottom lug on the bolt so the detent can get started.
This pushes up on the front of the bolt body.

I put the trigger assembly on the receiver. The sear will push up on the cocking piece if the bolt is cocked. This pushes up on the rear of the bolt body.

I cut a cartridge case in half and put a coil spring inside.
I put the cartridge with spring in the chamber.
I put Dykem blue mark up on the lug contact surfaces.
http://www.use-enco.com/CGI/IN...943768&PARTPG=INLMK3
I put valve grind compound on bolt lugs.
http://www.cylinderheadsupply.com/clovercompound.html
I turn the bolt a few hundred times, until the blue is gone on enough area.
I clean out the action and clean off the bolt.
The bolt is now at the top of the receiver bolt bore when cocked and and the bolt lugs fit is flat over some area, so they won't dent and change headspace with hot loads.
When the rifle fires, the bolt should not rattle around in the receiver bolt bore.

Bill Leeper, a real gunsmith over on the benchrest forum, tells me that old gunsmiths a long time ago used to use the spring balls like I am now.

 

[GasLight]

Re: Lapping Lugs... Ever done it?

That is a very interesting approach, and while I won't dispute it, it does raise some questions for me.

What keeps the front of the bolt pressed up at the top of the action when firing it? Do you leave the ball-spring in the front of the action when using it after lapping?

Without truing the race way, how do you ensure that pressing the bolt up to the top of the action isn't inducing an angle into the lugs during this procedure?

What keeps the rear of the bolt on top of the action during recoil pulse when the sear is released and the bolt isn't being pressed upward during recoil (the critical phase of lug contact)?

What makes a 'real gunsmith' vs some of the guys on this board?

Dave

 

[Keith Johns]

Re: Lapping Lugs... Ever done it?

I cant say I've done either, but I say dont fuck around with something like this. Have everything trued and be done with it. It makes no sense to try to lap your lugs with a barrel already headspaced.
You think rubbing you lugs together is going to make them square? It might mate them up a little but they could "mate up" in the untrue.
Couldnt you even fuck up cam trying to hand lap lugs.

Just true it, professionally. And know that your gunsmith with go the whole nine yards truing it. If he's good, you'll have an action on the same level as custom actions IMO.

 

[GasLight]

Re: Lapping Lugs... Ever done it?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AZPrecision</div><div class="ubbcode-body">
Couldnt you even fuck up cam trying to hand lap lugs.
</div></div>

Yes, the timing of the bolt will absolutely change, how much will depend on how much material is removed from the lugs.

Dave

 

[Keith Johns]

Re: Lapping Lugs... Ever done it?

See, all the more reason hand lapping lugs seems stupid to me


Juuust have it trued

 

[sandwarrior]

Re: Lapping Lugs... Ever done it?

blurry6,

Lapping a bolt is only going to make the action closing a little smoother. That will eventually happen if you shoot, and thus work, the bolt enough. It won't square anything up, nor improve upon accuracy.

The best thing is if you are not getting consistent accuracy then I would take it to a gunsmith who knows how to first; true the action, and second; finish that out by lapping the action-if necessary.

Truing is the machining process that squares the bolt up with the chamber/bore while in the closed position. Lapping is really just polishing it down to a fine finish. Both will change the timing somewhat of the bolt. Ordinarily, lapping won't change it enough to matter. Neither does truing if you don't have to take it to the extreme.

Your description of your lug engagement sounds like you might want to take it in and have it trued. That is, if your current accuracy isn't consistent to your liking. It isn't necessary if you are getting the accuracy you want. I would certainly include that in the cost of rebarreling that rifle if you do.

-good luck

 

[BOLTRIPPER]

Re: Lapping Lugs... Ever done it?

i have witnessed the need for a lug job due to a mauser action making brass.....right before a match....the whole gun had to come apart...barrel off the action and lapping compound on the back-faced surface of the lugs.....very tedious ...it seems there is some jack-ass in around alberta that thinks one can screw on a barrel to an action and call it a custom rifle.....

 

[blurry6]

Re: Lapping Lugs... Ever done it?

I think you guys are right,... I ain't gonna screw with anything that effects the headspace so much, especially with a .338 RUM - can you say BOOOM!! I'm not unhappy with the accuracy (about 0.75-1.25 MOA conservatively),...

It needs a new trigger for sure, the factory one is set at a brutal 7+ lbs, and I've already got a Jewell trigger on another Rem 700 I plan on swapping out once its' trigger arrives - that will probably improve its' accuracy considerably. Once I'm done testing that out, I'll see if I think it needs action work. Thanks for the advice.

Ryan

 

[Clark]

Re: Lapping Lugs... Ever done it?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: DebosDave</div><div class="ubbcode-body">That is a very interesting approach, and while I won't dispute it, it does raise some questions for me.

What keeps the front of the bolt pressed up at the top of the action when firing it? Do you leave the ball-spring in the front of the action when using it after lapping?

Without truing the race way, how do you ensure that pressing the bolt up to the top of the action isn't inducing an angle into the lugs during this procedure?

What keeps the rear of the bolt on top of the action during recoil pulse when the sear is released and the bolt isn't being pressed upward during recoil (the critical phase of lug contact)?

What makes a 'real gunsmith' vs some of the guys on this board?

Dave </div></div>

Target actions have tight bolt body to receiver bolt bore fits, while sporting and military actions have a looser fit for reliability.

The Mauser sear connected to the receiver pushing up on the cocking piece connected to the bolt body pushed up the bolt body in the rear to the top of the receiver bolt bore.

The spring ball screwed into the bottom of the receiver pushes up on the bottom bolt lug, when keeps the front of the bolt body pushed up to the top of the receiver bolt bore.

After the lugs are lapped, the trigger assembly and spring ball are left in place, assuring the bolt body is pushed to the top of the receiver bolt bore, when used in the field.

The position of the bolt body in the top of the bolt bore, when pushed by a point in the rear and a point in the front, has some possible error, but that is a secondary effect and small error.

Bill Leeper is a real gunsmith relative to me, and not others on this forum.
I have searched for and read every word on the internet written by Bart Bobbit, Gale Mcmillan, and Bill Leeper.
I have also exchanged email with them.
The internet is mostly regurgitation, opinion, or incorrect, but those guys have posted first hand knowledge.

 

[GasLight]

Re: Lapping Lugs... Ever done it?

Clark,

What keeps the rear of the bolt at the top of the action when the rifle is fired? Once the trigger is pulled, the sear releases the firing pin, and there is no longer upward pressure on the rear of the bolt. Maybe this doesn't matter at that point, as the lugs have been lapped to accomoddate this? I am just asking questions.

Thanks,

Dave

 

[Clark]

Re: Lapping Lugs... Ever done it?

I think the firing is faster than the bolt body drop.