Micro-Lapping Winchester Model 70 Locking Lugs

In this report, I’ll discuss lapping the bolt-locking lugs on a Winchester Model 70 rifle, although the basic principle used here can also be applied to most other bolt-action rifles. This procedure can be accomplished with the barrel on the receiver. I prefer to remove the barrel from the action, however, since this makes it easier to observe the results and do the cleanup later.

The only items required area lapping compound, an empty cartridge, and a shim to be placed between the chambered cartridge head and the bolt face. With the shim between the cartridge and the bolt face, the chambered cartridge will apply controlled axial preload against the bolt face, which is transferred to the locking lugs. If the barrel is loosened and freely turned, the bolt-face shim can be oversized in thickness for the lapping process.

Pressure between the boltface and chambered cartridge can be increased or decreased by simply turning the receiver against the bolt face while lapping the lugs. This is best done with the barrel in a padded vise. In this way, the left hand can turn the action as the right hand works the bolt. If the barrel is to remain fixed during the lapping, take shim stock and cut it to the size and shape of the cartridge head. This may require several thousandths of shim stock.

When a firm feel is attained on bolt lift, it’s time to apply lapping compound to the locking-lug surfaces. I B. Bore Paste serves as a very good medium for lapping the locking lugs on a bolt-action rifle. The shim between the cartridge and the bolt face serves to create pressure during the lapping process. use J. B. Bore Cleaning Compound (from Brownells) for this. This product is a mild abrasive and does an excellent job of lapping.

During the lapping process, you’re not trying to remove an appreciable amount of metal from the lugs or the receiver’s mating surfaces; in fact, you want to stop as soon as uniform contact is made between locking lugs. In most cases, this will remove less than .002 inch of metal from the high contact lug and will not alter cartridge headspace to any appreciable degree. As a light lap to the bolt face, I sometimes apply a small amount of

Above left and center: These before (left) and after (center) photos of the locking-lug surfaces show the evenly lapped surfaces on this bolt. Prussian blue can be applied to the lug surfaces to show how well the lapping process is going. Half the lug is lapped where bolt-lug contact is made, shown by the worn-away bluing on half of the lug’s mating surface. the J. B. Compound to the bolt face as well. I do this as I see the locking lugs becoming polished.

This lapping process is a very effective means of getting the locking lugs uniform, which is likely to improve accuracy in any bolt-action target or varmint rifle. There’s virtually nothing in materials cost that you can pass on to your customer, but you can easily charge for an hour’s labor, which is usually more than enough to perform a complete lapping job. Thus, this project is often a good "add-on" that can increase your profit when performing other accuracy work on a customer’s rifle. I