I know the subject of lapping bolt lugs has been discussed here (I've reviewed all the prior posting I could find using the search engine) but there is one question I haven't found an answer for: How much contact is achievable through lapping?

I'm using 240 grit silicone carbide lapping compound and I've rigged a device using a barrel stub to put constant pressure on the bolt-face as I work the bolt handle. After about an hour's worth of work I've got about 90% contact on the solid lug, but the split lug still only contacts on the leading (right side viewed from rear).

It seems intuitive that we're after 100% contact on all lugs but since I've never done it before I'm not sure as progress has been very slow. Any tips on performing this procedure would be most appreciated.

TooManyTools,

I think I read somewhere that 80% is acceptable--don't remember where I read it though. Maybe someone who knows for sure will weigh in.

Good Hunting,

Toomany Tools

The question I have always had with lug lapping is....to what position are they being lapped?

I have often thought where full lenght sized brass is used, that is where there is some headspace, then the best lapping would be done with the bolt be opened closed in the rifle and with firing pin in place.

Mike

Too many,

After your done with that 240, you can polish if you want, email me your address. For $6.00 I will send you one vial each of 600 and 1000 grit silicon carbide powder. Mix with some cutting oil and make those lugs shiny. I know not necessary but when you sit in front of the tv you could do this just as well. The vials are full, 40mL vials. I have a bunch. Goes a long way.

Toomany,

The whole purpose of lapping lugs it to ensure the uniform contact and transfer of thrust to the bolt's locking lugs during detonation of the round. This reduces the likelyhood of the bolt shifting, which helps accuracy.

One of the big mistakes people make when lapping in their bolt lugs is failing to make certain that the bolt's face is square and true to the firing pin axis or bolt tunnel prior to the lapping process.

If you apply direct pressure against an uneven bolt face when lapping, then this results in uneven pressure to the lugs which will result in poor contact, frustration and poor results.

Gun writers and some builders expound on the fact that bolt lugs need to be lapped for top accuracy, this is mostly true. But what they don't tell you, probably out of ignorance, is that in order for this to be successful, there has to first be a square and true surface for which to apply the pressure against.

In short, it does little good to lap in the lugs if the bolts face is out of whack.

Regards,

Malm

Toomany,

Sorry about that, I had meant to give you an answer to your question. Full contact is 100% achievable if the lugs are square. Though 100% is not absolutely necessary, the closer you can get to achieving this degree of contact, the better.

I have seen bolt lugs out as much as .015 and more. I am sure other gunsmiths have seen similar variations in lug tolerances. It would take seemingly forever to close that much distance by lapping alone and depending on the action, it could destroy the heat treating in these critical areas and create an unsafe condition.

Check this out, with the barrel off the action and the bolt locked in battery, take your finger and move the bolt head from side to side. This lateral movement (present in all guns) together with an out of square bolt face is responsible for a lot of accuracy trouble and could possibly be the source of trouble you are experiencing. The way to correct this, is to start with a square and true bolt face and then machine the lugs to match.

Malm

Sometimes the lugs will not ever achieve 100% contact simply due to the shape of the locking lug seats and /or the length of the camming surface. I have seen many cases where only 3/4 of the surface of the lug could contact.
I have seen a Winchester M70 that was lapped until the lugs and seats looked like those of a P17 Enfield (angled)because the "smith" didn't know when to quit. Often the approach on the locking lug seat is lapped so that contact by the other half of the split lug(on a mauser or Sako for instance) is impossible unless an excessive amount of lapping is done.
By the way, I think it is better that the bolt face be square to the outside of the bolt body rather than the firng pin axis. The firng pin may not be coaxial with the bolt. Regards, Bill.

Tools , on a M98 the left split lug only have contact the upper part the lower lug don't have contact , if you check an M98 from the barrel threads , you'll see the cam surfaces , measure where this cam ends and you'll notice the the lower lug it's in the "air".

Saludos

Daniel

The "firing pin axis" is the term used to describe the exact centerline of the weapon. It starts at the bolt tunnel (hence, the term "firing pin axis") and continues uninterupted through the crown. It is from this point of reference, that all machine operations are indicated from.

While the bolt's body is assumed to be concentric and straight, the actual firing pin tunnel through the bolt seldom is. This is why as Bill stated, the bolt's face and lugs are squared to the bolt's body.

Malm

Too quickly people tend to want to jump into lapping lugs without first examining the degree of problem that they face. 9 times out of 10, I have to remachine the bolt and locking abutments, as well as the threads and receiver face before any lapping should take place. Lapping is intended to be the final step in a long careful process of getting all the pertinent parts running in relationship with one another. It is not to be used as the first step, and if the machine work is done correctly, a lot of times it is not necessary to do it at all.
You should always be able to achieve 100% contact. If you don't, you've done something wrong.

I agree that before you lap lugs, the receiver locking surfaces and the bolt lugs need to be machined parralel first. lapping following machining works great on Rem 700's and other modern actions made with modern steels. On older Mausers in particular, recognize that by either machining or lapping you are potentially wearing through the case hardening. This will need to be re-done after the lapping process. Otherwise get ready for some potentially serious bolt lug set back.-Rob

Matt and Ron said it best. First ensure that everything in the receiver is square and straight, including lug recesses, receiver face and threads. Then you square and lap the lugs and then you finally square the boltface. If you suqare the boltface and then lap the lugs, the boltface will just go right back put of square by the amount of the lapping of the lugs. In other words, if you aren't going to do the whole job, a small part will not help that much.

In order to insure that uniform pressure is being applied equally to the lugs during lapping, the bolt face has to be square to begin with. It is the bolt face that you are applying the pressure against to lap the lugs...

Malm

I want to thank you all for your inputs to my question. I got out my micrometers and dial indicators and have dsetermined as best I can that my action is in-fact square and the contact I've achieved through lapping is as close now to 100% as I can get it.

Thanks again.

John