M60 GPMG Owner's Guide (Pt 6)

The following article(s) are republished with permission of the author, Thomas T. Hoel of Tactical Advantage.  The articles were originally published as a 5-part series beginning in the November 2003 issue of Small Arms Review.  All content © 2003-2017 by Thomas T. Hoel.

 The Civilian M60 Machinegun Owners Guide
Part 6

Practical Repair Procedures

This final installment of the series will entail detailed indigenous inspection and repair methods for the most commonly seen problem areas using techniques and information that are more appropriate to the Civilian realm than the “official” military envelope. These are essentially progressive maintenance protocols, to be performed by the owner/operator with commonly available shop tools, and designed to alleviate serious damages that will occur from neglect of the few particularly susceptible components during normal operation of the weapon.

We will assume here that the current owner-operator has thoroughly familiarized himself with the relevant military manuals and TM’s to the extant that the terminology and nomenclature of components and assemblies is clear, and that familiarity with individual components is sufficient enough to permit a rapid discussion of detailed operations and/or inspections.

The M60 system has several known, and recognized as critical, areas within the design and manufacturing parameters that need close attention and careful observation over the operational lifespan of the weapon in order to maintain serviceability and assure operational safety. The following discussion is by no means a complete rendering of all important problem areas, only the three most relevant to our discussion as civilian owner-operators, and the most cost efficient in regards the diminishing supply and advancing costs of available critical spare parts. These three areas are the Operating Rod assembly, the Bolt Body, and the Fire Control Group.

The interactions between the bolt body, Op-rod, and fire control mechanism are the primary wear areas of the gun. The contact surfaces between the bolt body, Op-rod, and sear sustain considerable forces at the contact margins, and are prone to unavoidable wear as a result. Limiting to the maximum extant possible the inherent induced wear should be the prime concern of any operator of these weapons. These parts account for the greatest costs associated with maintaining, or repairing, the entire weapon on a wear/damages basis.

There are several owner/operator actions and checks that can be done to limit the accrual of damages to these areas of concern. The basic action of the gun’s mechanism relies primarily upon the movement of a few key parts along various inclined surfaces under high pressures due to small contact surface areas. The stresses concentrated along these small surfaces produce strong angular force vectors, and these extreme stresses are the method through which accelerated wear of even the most carefully hardened parts occurs so easily. Improper lubrication protocol, using excessive amounts of lubricants, cannot prevent these problems from occurring and the addition of excess amounts of lubrication will not counteract the damages once they have begun.

Almost immediately after being placed into service, the individual camming surfaces on the bolt locking lugs, bolt body camming surfaces (and to a certain lesser degree the barrel extension locking surfaces), the Op-rod tower yoke channel (firing pin camming channel), the Op-rod sear notch, and the fire control sear bent will all start to wear and show signs of peening, burring, galling, and even chipping of the contours and edges of all contact zones. Allowing any of these deformations, with there attendant rises and sharp edges, to remain unchecked will permit stress concentrations to focus along these small areas, and in extreme cases the metal may begin to recrystalize due to these stressed focal points. Such wear is “normal” to the extant that it cannot be avoided totally, but the most severe long-term damages occur from not tending to these initial surface deformations on a preventative maintenance basis, as the altered surfaces if not treated properly with remedial and progressive maintenance actions will cause greatly accelerated wear patterns to form. Amongst these parts the operating rod is the most easily damaged, and the bolt body has the most potential for dangerous damages to develop.

Operating Rod:  Most common damages to the operating rod can be generally avoided by studious operator care and preventative maintenance. The Op-rod is prone to damages in two distinct areas, the yoke tower/firing pin interface area, and the sear bent area(s).

The yoke tower is most often times damaged by careless insertion or removal of the tower channel into the bolt body camming cut when the yoke is inserted or removed from between the firing pin spools. All exposed surfaces of the tower channel and both machined ridges should always be maintained free from burring, galling, and peened deformations. These surfaces must always allow smooth contact with the firing pin assembly and bolt body camming areas. Any burred or peened deformations that develop should be stoned and polished as directed below.

On the rear of the tower channel (firing pin channel), directly above the bolt roller bearing, the upper end of the right hand machined ridge (as viewed from the rear) has two distinct surface cuts (on all Op-rods mfgr’d since the late 1960’s) designed to limit the amount of burring and galling occurring during insertion/removal. These are both 45 degree angled grindings (chamfers) that permit easier slipping of the yoke tower end into the firing pin spools (Top cut downward 45 deg x .020”, to + .010” length, side cut 45 deg x .080”, to +.010” length). These surfaces are usually the first to exhibit wear patterns. They must be kept smoothly contoured to avoid burrs forming that will damage the firing pin and inhibit smooth operation during firing pin retraction, possibly jamming the bolt mechanism in extreme cases. The most severe burrs will form in the tower channel toward the inside of the upper, downward vertically angled, 45-degree cut on top of the right side ridge. As soon as the initial burring and galling begins developing, these surfaces must be treated with a fine grade abrasive tool, or cloth, to restore the original contour, angularity, and surface smoothness. An 800-1000 grit wet-stone medium is ideal, though files of similar grade are acceptable too if used correctly, finishing to a final polish with a 2000 or finer grade medium. No metal on the original surface contour is to be removed! Only shape off the formed burrs and galled rises.

The yoke channel front ridge vertical faces will also develop significant impact deformations (galling/peening) from induced contact with the firing pin front spool inner face as the bolt slams closed during final locking dwell movement and firing of the cartridge primer. (Similar to above, both inside forward edges of the ridges are also originally chamfer ground at a 45 deg x .020”, + .010” length, angling inward toward the firing pin channel from a line perpendicular to the ridge centerlines.) These upper corners will tend to peen with an outward spreading contour, flattening the tips of the machined ridges outward and backward. Again, remedial treatment is as above, stoning/filing to remove raised surface deformations and restoring original contours of the inward chamfers.

Care must be taken to regularly inspect the amount of material that has been peened on both ends of the ridges on top of the tower, excessive material loss will increase the free end play of the yoke inside the firing pin spools, drastically increasing the amount of hammering and pounding the yoke will incur during bolt movement. If this material has been lost to the extent that the front and back column of the yoke tower is contacting the firing pin spool surfaces, battering of the bolt roller bearing located on the top rear surface of the yoke tower will occur, quickly damaging the bolt roller and likely the bolt body itself. If evidence of contact of this nature is present, the Op-rod must be discarded as unusable.

The bolt roller itself is critical to smooth operation of the bolt mechanism and must remain undamaged and properly positioned to allow smooth rotation of the bolt body along the camming surfaces contacting the Op-rod yoke during locking/un-locking actions. The roller is commonly neglected during servicing, and can quickly become fouled with firing residues, or commonly rust, which will prevent normal rotation. Care should be exercised during maintenance periods to clean and lubricate the roller assembly; any flat spots that have developed on the roller wheel surface should be stoned as smoothly round again as possible. If a damaged roller is observed, it should be replaced before further operation of the weapon. The roller can be removed by driving out the lower roller shaft retaining pin, then driving downward the top of the roller shaft far enough to allow the roller to be pulled forward from the yoke tower. A bolt roller frozen in place by rusting may be extremely difficult to remove, but under no circumstances should an Op-rod with a frozen bolt roller ever be used, as severe damages will result to both the Op-rod and bolt body! A rust-seized roller may usually be loosened to the point of allowing removal by a liberal application of a capillary action type of solvent such as Kano Aero-Kroil®, allowed to sit for several days, then gently tapped out using the above sequence. It should be noted that replacement rollers are not always easy to find, as such the roller outer surface may be reground if no more than .003” is removed (all such work must be done on a turning center to preclude flat spotting which will peen the bolt body).

The last area for concern in the Op-rod yoke tower area is for operator actions taken during insertion or removal of the rod from the bolt body assembly. Improper actions here are responsible for serious damages to all the above areas. The protocol to follow to avoid damaging actions is as follows: To insert the Op-rod, grasp the bolt body in the left palm, bolt lugs facing toward you with the camming cut behind the extractor centered vertically. Grasping the Op-rod in the right hand and at an approximate 45-degree angle to the firing pin, engage the rear yoke tower edges against the rear firing pin spool, gently push the spool backward in a straight line without rotating the Op-rod in the channel until it snaps over the firing pin shaft. If you push too fast, or rotate the Op-rod, you may allow the firing pin to snap forward as it slips the yoke channel, and there is enough spring force in the firing pin spring to easily peen the yoke tower surfaces!! Safe removal is not the reverse procedure!! To remove an assembled Op-rod, push the Op-rod rearward against the firing pin spring tension and rotate it to the left side fully (the “bolt unlocked” position), then allow the front of the yoke tower to come forward against the front camming cut, then simply lever the nose of the Op-rod downward and the bolt body will gently snap out without hitting the yoke tower. Done smoothly, you will not damage the yoke tower surfaces.

The Op-rod sear notch (or dual notches on the newer E3/E4 rods) is prone to both peening and gall forming burring actions in normal use. These areas are induction hardened, to provide protection against wear, but they will still peen even with normal use. A trigger pull gauge can be used to ascertain sear notch condition as follows: tested trigger pull to be between 6 lbs minimum, to 11.5 lbs maximum. Pull less than 6 lbs is indicative of a worn sear notch, or a worn sear lug (sear bent). Trigger pull greater than 11.5 lbs is indicative of a burred or peened Op-rod sear notch, or sear lug. (Heavy trigger pulls may also occur with a sear spring that is fouled/jammed or broken). Both sear lug and Op-rod sear notches deformations can be repaired by careful hand stoning, but absolute integrity of the perpendicular engagement angle must be maintained! This is one area of repair that should not include a final polish, simply dressing away the damages is sufficient. Again as mentioned is a previous installment, trigger manipulation should always include full and abrupt release of the trigger to allow full and immediate engagement of these surfaces to stop the firing sequence. Conversely, not fully pulling the trigger back to the stop while firing will allow the Op-rod to quickly batter the sear nose, peening these surfaces and causing the most common way for the gun to begin to “run away”, i.e., un-commanded firing. Battered and peened sears and sear notches will quickly cause this condition to arise. If a damaged Op-rod is encountered it can be often times be repaired by surface grinding, milling with a carbide cutter, or even plasma EDM machining to re-cut the proper engagement depth and contour. Re-heat treatment, such as carburizing or induction hardening is appropriate, though simple flame hardening followed by oil-quenched tempering may allow the rod to give a useful amount of continued service. A desired finish hardness Rockwell C 60-64 should be measured on the re-cut faces. If the notches begin to chip or flake, discard the rod as unusable.

Bolt Body:  The bolt body is best maintained by following a similar protocol as outlined above for the Op-rod, as it is the mirror image of induced wear patterning in almost all contact areas. The locking lugs are the single most safety critical area in the entire weapon design, and appropriate attention should be paid to their care! As with any weapon with a fixed headspace design, no combination of a bolt and barrel assembly should be placed into service without verifying the headspace of that combination are within safe limits. Proper procedures are outlined in the relevant TM’s, but once a combination is ascertained compatible, a common marking should be engraved or scribed on both the barrel assembly and the bolt body to allow easy identification of a safe pairing. If more than one barrel is commonly used with each bolt body, the same headspacing proof should be done and each marked accordingly. Far too many well used bolts and barrels are being sold as “new” after a fresh reparkerizing job these days and the only way to guarantee a safe combination is to headspace each combination. Always remember that these guns are intended to be measured using 7.62mm NATO headspace gauges, as opposed to commercial .308 Winchester gauges, and while close, commercial gauges may give erroneous belief in a safe condition if close to maximum limits. Using the intended bolt body the maximum allowable headspace measurement is 1.641 inches. If the bolt closes on this NATO maximum limit headspace gauge, use of an M60 Field Test Bolt (Bolt, Field test, minimum field service, NSN 11699750) will show which component, the bolt body or barrel, is beyond wear limits. M60 Field Test Bolts are a rare accessory today but can prevent a lot of problems with proper usage. Even without such a test bolt, certain individual combinations of used bolts and barrel extensions may allow safe headspace limits, so it is wise to procure as many of each as the budget will allow!

The main reason to pay strict attention to proper headspace issues, from a purely wear stand point, is that improper headspace will always lead to excessive wear of both the bolt body locking lugs and the barrel extension locking recesses. Once damages from peening or galling begin to form burrs the bolt locking lugs must immediately be stoned smooth and re-polished or severe chipping and/or cracking of the locking lugs will begin. A properly matched combination will show evidence of smoothly polished wear surfaces on the locking lugs and locking recesses. There will be no sharp edges raised, or peened surfaces. Finish wear around the locking surfaces will be uniform and highly polished in appearance. Contrary evidence such as peening or galling of the locking lugs is prima fasciae evidence of improper headspace extant during operation; a barrel/bolt combination worn beyond safe limits will cause severe peening of the locking engagement contact surfaces. These surface damages can be removed by careful stoning, or use of similar fine grit abrasive tools, and a final polishing with a extra fine grit medium is recommended.
Bolt Chip 3x402

WARNING! Any cracks, chipping, or deep gouges on the rear engagement surface of the top stripping lug are cause for rejection of the bolt as unserviceable! Small burrs and peening in this location are repairable so long as they can be smoothed off with only minor stoning. The front left corner of the top stripping lug will quickly show peening wear, this can be stoned away repeatedly without being cause for rejection of the bolt. The bottom locking lug can be repaired by stoning/smoothing any damages repeatedly without being cause for rejection, so long as full locking engagement is obtained.

WARNING! Bolt bodies with any evidence of cracks or cracking on any visible surface must be rejected as unserviceable!!

Most of the above damages are the result of allowing the bolt to close on an empty chamber. Two effective remedies as outlined in an earlier segment of this series are to simply stop firing the gun before it runs dry, and never dry-fire the weapon, ever! (Always ride the charging handle slowly forward on an empty chamber if not actually firing the weapon!) One of the best firing protocols to protect the gun in this area is to adopt the practice of releasing the trigger when the last visible round in the belt enters the top cover feed opening, this will allow the gun to stop with 1-2 unfired rounds remaining with the bolt held back on the sear. Should you decide to employ this protocol, you must remain extremely vigilant to properly safe the gun by placing the safety lever on “Safe” then raising the top cover and immediately clearing the belt remnant from the feed tray. For those who wish to let the gun run “dry” another effective practice to adopt is to make up several dummy rounds and fill the last link position with one dummy round to cushion the fall of the bolt as the belt runs dry. (Any such dummy rounds should be constructed with highly visible configurations, such as drilled case bodies, to readily identify their status and function at a glance! Due to the continued abuse these will see in this role, the projectiles should be glued to the case mouths in addition to using a very heavy crimp) Alternately, one can employ any of the commercially available solid-construction dummy rounds for this purpose.

The firing pin bearing sleeve also plays an important role interacting with the op-rod tower yoke firing pin channel. To this end there exists a specially cut contour on the forward end where it contacts the rear firing pin spool face. In order to keep from binding the firing pin spools and yoke channel on assembly the front (closed) end of the bearing must be radiused to .063”R, and must remain smoothly contoured at all times. A grossly scarred edge, or one with deep gouges or damages, must be replaced with a new bearing.

Firing pin protrusion is critical to preventing an inadvertent early cartridge detonation, which could severely injure the operator and cause severe damages to the weapon. Firing pin protrusion is acceptable at:  Maximum 0.043” to Minimum 0.035” protrusion. Stoning of a high firing pin point is acceptable if end contours are maintained. Too short firing pins must be replaced.

Fire Control Group: The primary component that requires attention is the sear. The interaction of the sear contact face with the operating rod sear notch will cause similar wear patterns as observed for the operating rod. In use the sear bent will tend to wear at a slower rate than the op-rod notch. As opposed to the op-rod notch, the sear bent will actually tend to chip more often than burr, or gall. These chips, and all other damages, can be dressed out using the techniques illustrated above. Where the removal of base metal will not exceed .010” in restoring the contours, these damages can be surface ground mechanically with care being taken to not overheat the surface and possibly cause cracking of the hardened face material. The sear face is to be reground with a 5.0 degree, +/- 15 min, reverse angle on the front face. Re-heat treating should be as above for the op-rod, with a similar finish hardness of Rockwell 60-64 C. If the regrinding results in sear face to pivot pin hole center distance less than 1.020”, the sear must be discarded as it will not reliably hold control of the firing sequence.

The trigger spring must produce a minimum trigger pull weight of 6.0lbs to preclude damages to the sear bent and op-rod notch from incomplete engagement of these surfaces. Pull weights less than the minimum may not allow the sear to rise fully and will cause battering of the sear by the op-rod, and also likely causing un-commanded firing.


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