Ive heard this is similar to blueprinting? What exactly does it consist of and is it a good idea? I apologize if my question is dumb but my ignorance on the subject is genuine.

It is part of blueprinting and involves a little bit of lapping compound (abrasive paste, same as "valve grinding compound") on the back side of the lugs.

The purpose is to ensure full and equal engagement of the lugs so they hold the bolt head square under the tremendous force of the shot.

It can be done by hand without special tooling, but keeping it as straight as possible is not guaranteed. Several jigs are available to screw into the action and maintain pressure on the bolt face while the bolt is worked. Obviously they require pulling the barrel.
art

It is a process where you put lapping compound on your bolt lugs and put it in the receiver and work it up and down repeatedly. You use Dykem on the bolt lugs to check your progress.
It works best if you can apply steady pressure to the front of the bolt. They make tools to apply that pressure.
The idea is that you are slowly wearing away the metal in the receiver until you get close to full contact between the bolt lugs and their abutments in the receiver.

It is nothing like blueprinting but it has some value.

If I understand your question correctly, the concept is to coat the rear of the lugs with an abrasive compound and cycle the bolt, i.e., turning the lugs in and out of the rececesses in order to wear in their surfaces for more complete contact. This will change the headspace.

The other possibility in your question is to simply coat the bolt with an abrasive compound, and cycle it in and out of the action repeatedly in order to have a smoother bolt travel. If the lugs are not coated, and you don't turn the bolt down with each cycle, this will not affect the headspace.

Lapping the lugs should be approached with some caution.

Yep, what Sitka said!

Ok there are two meanings, thats what had me confused. I had heard both and didnt have any credible experience on either I appreciate the info guys. If i am pretty handy and familiar with some minor gunsmithing is this a DIY kinda thing?

You bet it is doable yourself, especially if you have the barrel out of the receiver.
Consistent pressure on the face of the bolt will help alot and check your progress often with Dykem.

What I did to lap the lugs on my model 70 was to disassmble the bolt so that there was no issues with the cocking action throwing the bolt off center. I then made a fixture using a cartridge case. I used a tubing cutter to split the case and made another cut so that the case body was missing about 3/8". I found some springs at the hardware store that would fit into the case and with the spring loaded case in the chamber it provided enough pressure on the bolt to lap the lugs in and do a very nice job without having to remove the barrel.

fish head

Good work!!!

Which is fine until you reassemble the bolt.

You might want to check for how much contact you have on the lugs before you do any lapping. Sometimes it is pretty good as it comes from the factory.

I'll admit it was a shade tree gunsmith solution to the problem, but with the bolt assembled and using a magic marker to see what kind of contact the lugs are getting, I went from having only one lug with less than 50% contact to two lugs having 70% or better contact. Besides, when a round is fired the cocking action is irrelevant.

I'm reluctant to post the other part of how I got there, but I'll give it a go anyway.

The bolt lugs were horribly machined and rounded off with less than 50% contact on only one lug. What I did to square up the lugs was to use a fine white creamic stone to flatten the lugs. The bolt face is flat so I stood it on a flat suface and used a phone book as a honing fixture. I peeled back the right amount of pages to provide a perfectly flat and square fixture for the stone to ride on. With some very careful stoning I was able to flatten the lug sufaces just enough and it was so precise that once I achieved contact on both lugs just the slightest amount of stoning one one lug would show a loss of contact using the magic marker. That's when I lapped the lugs for final finish.

As far as the headspace issue, it's a .300 win mag that the cases stretch about .017" before contacting the shoulder. I PFLR so that the cases to headspace on the shoulder instead of the belt. No issues with headspace.

It was a very shadetree solution and I probably should not have even posted this in case somebody gets the wrong ideas and screws up their rifle, but I have total confidence in my work. If you think I'm an idiot that's fine too. Obviously I'm not a gunsmith.

I would not recommend that anybody should do what I did.

fish head

One other thing is to push forward on the bolt as you close it with just a little lapping compound on the lugs. Don't fully open the bolt each time you work the handle back and forth, just about 3/4 of the way to letting you pull the bolt back is fine. Use swabs, brushes, brake cleaner and HOT water to remove all the lapping compound from the action when you are done.

I don't know this to be fact, but I have read it in some gunsmith books I have.

The authors state not to use a spring loaded plunger type device to hold pressure on the lugs.

The reason given is that the spring will give, letting the abrasive coated bolt lugs bypass the high spots. You want to wear down the high spots, not the entire surface of the lugs.

The authors of the books recommend some type of solid plunger so that there is constant pressure on the lugs and causing the entire surface to abrade evenly.

As stated, I do not know this for fact. I seems like if a very strong spring loaded plunger was used, there would be enough pressure to wear down the high spots evenly until everything was a close fit.

How can you wear down the entire surface without hitting the
high spots. The problem is you dont know where the locking
surfaces are touching during firing. The case isn't centered,and
the action is flexing among other things.
Good luck!

Had the same questions. How does this compare to fully blueprinting an action? I am thinking of hunting rifles only as all target and varmint rifles should get the full treatment.

I am hoping to Bubba the next one and only square the bolt face and receiver after the lug polishing. Am I setting myself up for problems or disappointment?

How many rifles have you seen FUBAR'ed by doing this?

I have lapped several. For the person not owning a lathe, it is a good enough way to better consistent accuracy. I have never lapped a bolt face. As with anything else, just enough and not to much is the key. You are ever so slightly increasing headspace, or worse, depending on how poorly the lugs are fitted before you start.

"How can you wear down the entire surface without hitting the
high spots."

Actually, you couldn't. I think I described it wrong. With the spring loaded plunger, you are going to wear down both the high and low spots equally, with the end result being the same configuration, but with some metal removed.

With the spring tension, the high spots might wear a bit faster than the low spots, but probably not enough to get an even fit.

For proper lapping, you have to have a constant load on the lapped surface. A spring loaded plunger would not allow this.

A rigid plunger would wear down the high spots until they were gone, then the parts would be bearing evenly all over. As the high spots wear down, you might have to replace the plunger to regain the pressure against the high spots.

Plunger is not a good name for it, since it is ridgid. I just don't know what else to call it.

you can do it by hand and I like that way of doing it..inspect the rear surface of the lug and you can tell when you have gone for enough as the compound will show the polished area..the secret is to not go too far..It is but one small process in blueprinting...

A lot of blueprinting is hype to get your dollar, much of the process done may or may not be necessary as some fo the high tech equipment used at factorys is more accurate than the gunsmith doing the blueprintin...A good blue printer knows when not to perform an operation and does as little as possible to get everyhting within certain specs without overdoing it IMO...

I believe lapping is normally done to a bolt and lug recesses that have already been machined.It is like lappping valves that have already been ground. It would take a long long time to lap lugs that were very far out of square if not machined first.. The lapping process geneally changes the headspace an insignificant amount.

I certainly do not follow your " rigid pluger" versus a "Spring-loaded plunger" argument. The standard fixtures that screw into the action face are spring-loaded and work just fine... A rigid pluger would not stay on the bolt face as it travels because the lugs are ramps. The spring compensates for that travel during lock-up.

The factory does not cut both lugs and recesses on the same machinery and there are tolerances they work with in. In many cases bolt and lugs match neatly, but not all. It is easy to see how well they mate and easy to improve bad examples...

And it is about the cheapest part of the entire blueprinting job.
art

Looks like you are correct. I just went to the Brownell's site and it shows their bolt lug lapping tools to be spring loaded.

I read somewhere that spring loaded would't work. I'll try to find it later today, and why it doesn work as well as a solid plunger, if it won't.

It stands to reason that a very stiff spring would hold the lugs solidly against reciever lugs.

Okay, I found it. It is in a book called 'Do it Yourself Gunsmithing' by Jim Carmichel.

Mr. Carmichel recommends using a discarded barrel, and a solid piece of metal called a "Spud," instead of the plunger as I described it. The spud is slightly longer than the chamber of the discarded barrel.

The barrel is screwed into the action, hand tight, then the spud dropped into the chamber and the bolt installed. If the bolt will not close, unscrew the barrel until it will. Take out the bolt and apply abrasive to the locking lug (the long one) and re-install the bolt.

Screw the barrel in slightly, until the spud causes the locking lugs to make contact. Raise and lower the bolt. As the lug is lapped, the barrel can be turned in to compensate for wear and to maintain pressure on the spud and the bolt face.

Mr. Carmichal also says that you can use a fired case and shims to compensate for abrasive wear, adding shims as work progresses.

He said that you could also use a headspace gauge, but that after several lap jobs, a No-go gauge might turn into a GO gauge because of the abrasive.

Mr. Carmichel also states that he does not recommend a spring loaded dummy cartridge or spud. He says that such a device would aid in polishing the lugs, but the spring load would allow the lugs to ride over the high spots and apply pressure to both the low and high spots, whereas the solid spud only allows pressure to be applied to the high spots.

It has been several years since I lapped the lugs of an action. I did not have a discarded barrel, so I turned and threaded a short stub to fit the action, and drilled a hole part way through the stub. I placed a piece of rod (I think brass, but I don't remember) into the hole and applied pressure by slightly tightening the stud to take up for wear as the lapping progressed.

This information is put here only to pass along some information from a person that knows much more about what he is talking about than I do. I do not endorse either method, but if I ever lap another bolt lug, I will use the spud method.

I read that he recently won some type of high profile bench rest match, so I suppose he knows how to lap lugs and build accurate rifles.

With all due respect to Mr Carmihael's achievements on the bench rest circuit and his writings... On this one he is full of schit.

I am not saying his spud will not work, but rather it is a ridiculous route compared to a simple spring-loaded plunger. I understand his intent in letting the lugs seperate with each turn so the abrasive will not be wiped off the high spot, and frequent redistributions of grit are required when lapping, but I do not see the spud changing that...
art

Sitka deer,
I don't want to argue with you, but Carmichel raises good points with his method.
Butch

Butch
I am all ears, but is his method; 1) demonstrably better at the range? 2) easier or faster?

There are more questions one could ask, but perfectly mating surfaces need not be perfect surfaces. If the bolt face is true(d) after lapping could it possibly make it shoot better?
art

My pea brain tells me, that the solid plunger/spud is the only way you are going to truly come close to mating the lugs to the lug recess. Using a spring loaded plunger will allow the lug ride up and over a "hump" in the recess. It will polish it for sure, but true it up...I doubt it. Just because it wears away the blueing, it doesn't mean you have "100% contact". It took a few explanations by an older gunsmith friend, but I finally "got it", and believe he, like Mr. Carmichael, are correct.

Then again, for a hunting rifle, the whole exercise, along with trueing the boltface, is likely a waste of time!

Jeff

My opinion on this has been stated before on this forum. If you don't have a truly blueprinted receiver that is fitted with an oversize bolt body in a reamed raceway you are wasting time. A factory bolt will be loose enough that the cocking piece will push the back of the bolt up when cocked. That keeps the upper lug from contacting the lug abbuttment. Why lap if it doesn't touch when the rifle is cocked. Tell me what it does.
Butch

None of my rifles fire when cocked, only when the firing pin is forward.

My question is, in a loose-bolt sporting rifle, does the rear of the bolt fall to resting position after the firing pin is released, but before the cartridge fires and applies thrust to the bolt face? The sear is no longer holding it up as soon as the release happens.

Bruce

Yep, and if it has a plunger ejector, you have an off center spring loaded plunger that likely pushes the bolt kittywompus as well. Unless, as Butch stated, you have a reamed raceway, and a bolt that fits it like a thumb in the butt. For the average hunting rifle, there's likely way more to worry about, unless the action is grossly out of whack. Fun to obsess over, and dink around with yes; vital to having a pretty damn accurate deer rifle....I kinda doubt it.

Jeff

This pretty much sums up my decision on attempting to lap my 700

I agree with all of the above. When I lapped the lugs on my model 70 I didn't see any magical improvement in accuracy. It's not a rifle that has stellar accuracy either. I did it mainly because of the horrible lack of contact on both lugs. As far as I see it, once the firing pin hits the primer, the bolt is no longer out of whack and having both lugs making even contact will prevent, to some degree, the lug or lugs setting back into the action. As far a a safety or strength issue, the factories don't seem to be too concerned with minimal lug contact. I'm sure that under the tremendous forces generated that everything "flexes" into contact.

I've traced my accuracy problem to the barrel itself. After slugging the barrel I found out the bore is very uneven with a tight constriction about 7" in front of the chamber. That's next on the list.

akjeff,

I agree with all you said except for the use of the term "kittywompus" - "caddywhompus" is the proper technical term. I'm an expert on that one. Not a gunsmith though.

fish head

"I agree with all you said except for the use of the term "kittywompus" - "caddywhompus" is the proper technical term. I'm an expert on that one. Not a gunsmith though.

fish head"

Duly noted!

Jeff

When the bolt thrusts back, if in fact it does so, it will impart force to the receiver. That force may be directly back, or if one lug is contacted, also torsional. Direct, repeatable,non torsional force is better for accuracy.

I didn't mention it in my last post, but as Fish head stated, and also Mr. Carmichel stated in his book, one of the primary reasons for lapping the lugs was to get both lugs bearing evenly. If only one is touching, then that one lug is taking the entire load when the rifle fires, so a safety factor enters into it, also.

But, also, I have never read of a bolt lug shearing off.

If there is only .001 or so clearance on a single lug, it might be that there is enough stretch or distortion that both lugs will make contact upon firing.

Mr. Carmichel also said that he has seen rifles that neither lug touched, and the bolt was supported by only the safety lug.

If nothing else, lapping the lugs from a safty standpoint should be a good enough reason.

I did a Google on Bolt Lug Lapping. I got several hits, but I did not read all of them. Some contributers were in favor of the Brownell's spring loaded plunger and thought it worked very well.

Others stated that no matter what you used, use a small enough spud that it does not touch the spring loaded ejecter. This could cause the bolt lugs to bear unevenly against the receiver lugs.

Another contributer said that he thought, IIRC, that uneven bearing surfaces on the lug, one lug supporting and one not touching, could be what caused an occasional (or regular) unexplained flyer 1 or 2 inches from the otherwise small cluster.

"I've traced my accuracy problem to the barrel itself. After slugging the barrel I found out the bore is very uneven with a tight constriction about 7" in front of the chamber. That's next on the list."

Fish Head:

If you are planning on replacing the barrel to repair this, you might want to consider lapping the barrel first. You would not be any worse off if it didn't work.

I have never lapped a barrel, so I won't offer any advice on how to do it, but information is available on the net.

That's the plan. I've already lapped a barrel on a .22 I have just to gain a little experience before I do this one. I feel confident enough to go ahead and give it a try. I'm up to speed on casting a lead lap on have the right lapping compouds to do it with. Nothing will be lost if it doesn't work out. Right now with handloads it shoots about an 1 1/4", somtimes less, somtimes more, a lot of unexplained flyers though, decent, but not the accuracy I'm looking for. It will be interesting to see the results afterwards.

fish head

I agree with all you said except for the use of the term "kittywompus" - "caddywhompus" is the proper technical term. I'm an expert on that one. Not a gunsmith though.

fish head

I disagree, in my neck of the woods the correct term is "Kattywompus" To the untrained it may sound like caddywhompus

Around the here, in the bar scene, it is "cat-is-whompass"
Many possible inferences,
1. Hot kitty,
2. That one will whomp your ass"

As opposed to: "cats-a-swampass"
1. a totally different meaning.