M60 GPMG Owner's Guide (Pt 4)

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 4

 Operating System Group

The operating system group is considered to be the heart of the M60 machinegun's basic design, and it is here in these few parts that the potential for the most common and expensive wear and damage events within the whole weapon can occur. The operating system group is comprised of the operating rod, bolt and its internal components, and the recoil/counter recoil parts. If problems with these parts, or improper operation of this group, are allowed to occur they can also greatly affect several other attendant systems within the weapon, primarily the barrel and the feed system and its components.

The basic operational kinetic motion of the weapon centers on the reciprocating movement of the operating rod, and the rotational action of the bolt assembly as it is driven forward and backward by the operating rod throughout the operational cycle. The bolt and operating rod are thusly involved in a seemingly very simplistic relationship. But it is the inherently violent action of their mutually intertwined movements that cause a host of wear related problems to arise that must be dealt with for the sake of continued smooth, reliable, operation and also as a primary safety concern.

Both the bolt and the operating rod, as basic component parts, are manufactured from high grade ordnance steels, close tolerance machined, stress relieved and heat treated to give them exceptional strength to weight ratios and resistance to wear and failure. Bolt FP AssemDespite this auspicious beginning, both parts will evidence clearly visible wear and deformations of certain original, as manufactured, contours almost from the very time they are first put into use. Due to the inherent design of the weapon, this is completely normal! While it is nearly always unsettling for a new owner or operator of an M60 to discover these wear patterns, this is a very normal and natural occurrence and while it cannot be avoided, it can most certainly be mitigated. However, the fact that the gun "wears-in" new parts in this group, does not mean these parts can be ignored! If left unattended, these naturally occurring areas of galling and peening can produce dangerous stress concentrations, possibly leading to premature failure, injurious both to the weapon and likely the operator. For this reason, all military manuals covering both operational field use and normal maintenance delve into this topic in great detail.

There are known locations within this system that generate wear. Together, the cam cut on the bolt body underside for the operating rod, the bolt locking lugs, and the cam ways in the barrel extension work together to force the bolt body to rotate, both in locking and unlocking actions. The points of contact along these ‘two’ camming surfaces are subject to greatly localized stresses, and will seek to accommodate any stress concentrations formed by forcing the offending areas to spread this stress over a greater surface area. This action is what produces the visible areas of galling or peening visible on the bolt lugs, barrel extension cam ways (rare), and operating rod yoke (or yoke tower). This phenomenon was recognized early on, and several steps were taken to help alleviate the problem. The most noticeable modification was the addition of a roller bearing on the operating rod yoke, though more subtle changes were made in the angular diversion of the cam paths used, and a generalized increase in the contact areas of the separate parts. Even still, these contact points will most certainly "wear-in" with use and if the formation, even if small in surface area, of galled or peened edges is observed immediate remedial and corrective action is required to prevent continuing damages. The continued structural integrity of the various parts is dependent upon the removal of areas of concentrated stress. Any evidence of galling or peened edges present from any initial displacement actions must be removed to restore normal passage of lines of stress throughout the part(s). There are three main locations within this system where this type of wear is observed.

Firstly, the yoke tower of the operating rod along its frontal face, receiver guide ways, and the firing pin bearing channel. These surfaces essentially serve as forward and rearward travel limit stops for the bolt body at the end of its camming slot travel. Conversely, the large mating surface area of the bolt camming groove located along the bolt body underside effectively spreads out any stresses within the bolt body there, and wearing of the sharp edges on this camming cut is unusual. Normally, a smoothly polished interior travel surface will be seen, with occasional evidence of slight surface deformation(s). In accordance with service publications such as TM 9-1005-224-23&P (or –24), visible surface burrs, gouges, or galling on the yoke tower and attendant firing pin bearing channel and receiver guide ways do not render the operating rod unserviceable, provided the damages are corrected, returning the surface(s) to a smoothly contoured surfaced to eliminate stress concentrations.

Experience has shown that on the yoke, the rear edges of the firing pin channel will evidence the most burring, directly above the firing pin roller bearing. (Note that the right hand top edge, as viewed from the rear, is manufactured with a forward raked cut to the top edge; this is normal and reduces wear at this corner)  Use of the proper method for disassembling the bolt body from the operating rod yoke will greatly reduce operator induced wear during maintenance at the firing pin channel location, a major cause of premature wear here. 

To correctly disassemble, invert the operating rod and bolt group, grasping the inverted top of the bolt body in the left palm, rear end facing away. While grasping the shank of the operating rod forward of the sear notch, apply pressure rearward against your left palm compressing the firing pin spring. Apply this pressure straight back until the yoke just starts to rotate…, do not allow yoke to rotate! While holding the yoke firmly against rotation and firing pin spring tension, lift the forward end of the operating rod gently upwards, carefully allowing the front edge of the firing pin channel to come up out of the bolt camming slot, and then slowly ease the firing pin forward to clear until it stops. DO NOT allow firing pin to snap forward! Done properly, neither the yoke tower or bolt body will slip out under pressure of the firing pin spring, which often times will cause gouges or burrs to form if allowed to happen. Reassembly is performed by reversal of these steps. (As additional incentive, operators who learn to perform this technique well will notice a marked reduction in the deposition of blood and skin fragments along the bolt body and yoke!)

The second area to observe is the bolt head, and it’s locking lugs (and cartridge feed lug). These areas, in conjunction with the cam ways in the barrel extension, are the most stressed in the group as they bear the full intensity of the firing pressures of the cartridge.New Bolt2 3x4 New Bolt3 3x4These areas encounter the brunt of the horizontal and rotational trauma during bolt locking and unlocking actions as contact along these points is what starts and stops the violent rotation, and subsequent linear movement, of the bolt assembly in each direction of travel. These lugs will usually show more signs of wear than the corresponding cam ways in the barrel extension, and will be where the most attention must be directed in terms of preventative maintenance actions taken by careful removal of burrs or peening which will occur with use. Even more so than with the operating rod yoke, extreme care must be taken here when correcting any wear to make absolutely sure no lug contours are changed or excess material removed as safety of the locking action can be compromised otherwise! The top locking lug also forms the cartridge feed lug (cartridge stripping lug), and as such has significantly less material to start with than the lower locking lug. The top lug should always be given the closest scrutiny for any possibility that the incurred damages might not be removable by normal corrective action(s), without altering the underlying strength of the lug. The common wear pattern observed on the lugs is for the frontal surfaces to evince galling or slight burring around the sharp edges of the lug(s) where they travel in the cam ways of the barrel extension. Again, in accordance with service publications such as TM 9-1005-224-23&P (or –24) these minor deformations of the frontal surfaces of a particular lug are not cause for rejection of the bolt body as long as these areas are corrected to return a smooth contour.

The most severe and detrimental damages that can occur to either the bolt lugs or barrel extension cam ways is for cracking or chipping to form. Heavy gouging or deep indentations formed in this area will almost always lead to incipient cracking, and evidence of this type of damage is cause for an immediate inspection for integrity. If any evidence of cracks, incipient cracking, or chipping,Bolt Chip 3x4 is determined to exist on the rear area contact locking surfaces of any lug, rejection of the bolt body is mandatory! The equivalent problem may be observed along the exterior edges of the barrel extension cam ways. If any evidence of cracks, incipient cracking, or chipping, is determined to exist on the surfaces of the cam ways, a full inspection of the barrel extension (cam ways) is also immediately indicated. The only safe method for proper determination of any cracking suspected of the above areas is to subject the area to a fluorescent dye-penetrant test (with equivalency to MIL-I-25135), or MagnaFlux®, type inspection procedure (Virtually all aircraft service and repair shops will be able to offer these inspection procedures). If such an inspection procedure is unavailable, the only safe recourse is to replace the suspect part!

For all other above mentioned cases of correctable repair, the proper remedial action is to carefully restore to a smooth surface the sharp edges of any galled, burred, or peened area by stoning with a fine grit polishing stone, without altering the basic underlying contour of the area or shape, such as a bolt lug. The object is to smooth out and remove the deformation(s) present, without changing the underlying contour at all. When using this technique great care must be taken not to alter or remove any of the underlying material, but merely to smooth out and remove the tiny displacements, which if left unattended will continue to focus stress concentrations in that area. (When stoning, never use a coarse grit stone or highly abrasive compound!  When using such materials it is extremely hard to control the amount of material altered and removed. A 400 grit stone is usually the coarsest grade to consider, and one should start with the finest stone available (800 grit is an ideal basic) and move down only if material is not being suitably altered.) A final polishing of the affected area is recommended. Once these initial displacements of wear-in are remedied, and the focus of stress concentrations in that area is removed, it is usually not necessary to continue to need attention. The preceding is however, only true for an individual set of components operating in continuous proximity. If a bolt, operating rod, or barrel is exchanged within a current set, additional instances of parts-mating action may become evident. This may also happen if the firing pin, or operating rod roller bearing, is replaced. In rare instances, the spools of the firing pin may show wear such as galling; this is indicative of excessive wear of the firing pin channel edges. A damaged yoke roller bearing will cause excessive wear patterns on the inside surface of the bolt camming cut in the bolt body. It is therefore prudent to monitor closely these areas for additional signs of galling, burring, or peening if any of the component parts of the operating system are exchanged or replaced in a repair activity.

The operating rod both retains the bolt and helps to cause it to rotate by camming action as it is driven back and forth. The bolt in turn is also supported and guided in the reciprocating movement of the operating cycle by the feed-cam actuating roller, which is supported by the two bolt guide rails and which form an integral part of the receiver. The gas tube, lower receiver operating rod supporting rail, and the bolt guide rails are the primary vertical support members for the operating rod and bolt assembly. Smooth and unbinding free movement along these paths of support is critical for correct function of the bolt and operating rod relationship. As such it is always recommended to check the integrity of these areas first, if undue wear or damages begin to appear on either the operating rod tower, or the bolt lugs. A simple lack of proper lubrication can cause excess drag along these supporting areas, which may lead to sluggish operation of the weapon in general, or failure even to fire a chambered cartridge. Lubrication for the operating group, and its’ supporting members then is critically important!

For virtually the entire military operational use of the M60, there have only been two normally approved MIL-SPEC lubricants, LSA and CLP. LSA (Lubricating oil, Semi-fluid, Automatic weapons, MIL-L-46000B) is a medium weight lubricating fluid, actually an emulsified mixture of multi-grade lubricants designed to provide effective lubrication protection over a wide range of atmospheric and temperature extremes. LSA was approved for field use in virtually all small arms in inventory and is a substantially effective general weapons lubricant when used in areas appropriate for its composition and formulation. Due to its chemical composition, LSA stocks that have been allowed to sit idle for any length of time will drop out of emulsion; to restore the lubricant to its full specification the mixture must be vigorously shaken, by mechanical means preferably. (A quick trick to restore small quantities is too heat individual containers in boiling water for a short period, or a microwave oven set on ‘Low’ for several seconds, then shake by hand.)

CLP (Cleaner, Lubricant, Preservative, MIL-L-CLP) was adopted to address what were considered to be shortcomings when maintaining weapons with the previous standard combination of LSA and RBC (rifle bore cleaner, MIL-C-372B or C). CLP is a complex formulation of cleaners (powder solvents), synthetic and natural lubricants, and anti-rusting and anti-corrosive (anti-acidic) compounds. While touted as a miracle formulation, CLP in general and widespread use has proven to be less than satisfactory for any of its intended uses. It's primary value as a weapons lubricant is dependent upon micro-grannular deposition of DuPont™ Teflon® (a fluoropolymerized lubricant) particles carried in suspension within its formulation. When proper deposition of these minute particles occurs in the grain structure of a treated metal surface, the resulting lubrication properties are phenomenal. The main problem though is that successful, wide-area, Teflon® deposition rarely occurs and the remaining petroleum-based carrier fluid is not an effective lubricant, leaving much of the weapon unprotected from accelerated wear. Primarily, this occurs due to improper usage and application of the CLP as it is mandatory that bulk concentrations of CLP be thoroughly agitated before each use to efficiently distribute the Teflon® particulates in uniform suspension such that they will evenly distribute upon application to a weapon. For maximum Teflon® adhesion and deposition, the area(s) to be treated must be cleaned down to the bare metal surface with no trace of prior lubricants or preservatives remaining. Additionally, CLP does not achieve its maximum potential lubricity without numerous, long-term, continued applications.

Both LSA and CLP have their places and can be used effectively, though LSA is a far better choice for general lubrication of small parts and assemblies on the M60 if thorough cleaning and complete re-lubrication cannot be accomplished after every shooting session. Using CLP effectively requires more concentration to its proper application, and cleaning thoroughly down to bare metal to allow successful deposition and to efficiently distribute the Teflon® particulates. Without a Teflon® coating of the full surface area of bare metal, CLP offers no significant lubricity as the light carrier oils will generally dissipate outward from the areas of contact!

Even in the best circumstances, neither LSA nor CLP are the best lubricants for several locations on the M60. On small parts, with small surface contact areas and low operating contact pressures, they are sufficient, but for large contact areas under heavy pressures another lubricant type is preferred for Civilian use of the weapon. (Remember- Anything the civilian owner-operator can do to decrease operating stresses, and attendant wear and tear on component parts or assemblies, will serve to prevent unnecessary or premature wear and failure of these parts, adding tremendously to the life expectancy and serviceability of the weapon.)

 Just as was found with long term experience with the M1 Garand, M1 Carbine, and M14 rifles, the use of any other lubricant besides a heavy weight, high viscosity, grease for the bolt and operating rod contact areas led to immediate and disastrous problems. These critical parts often 'froze' under operating loading when used with other common lightweight lubricating fluids, even in ideal atmospheric conditions. The similarity to the operating components of these weapons to the operating system in the M60 warrant close consideration of the same lubricant choice in certain highly stressed areas such as the operating rod yoke, bolt camming cut surfaces, bolt locking lugs, barrel extension cam ways, bolt and operating rod guide rail slots, feed cam actuator roller, and feed cam lever. The supreme benefit of using such a high viscosity, extreme pressure, non-fluidic type of lubricant on these special areas is that even under extreme operational conditions the lubricant will tend to stay where it is needed, continuing to provide vital lubricity as it is much less prone to pressure exclusion or flow like most other fluid-state lubricants. Also, due to its higher viscosity index it will not simply be removed by mechanical action nearly as readily as a thin washing of oil.

The nearly immortal military standard for such applications is the unsurprisingly named MIL-SPEC lubricant, Rifle Grease (Grease, Rifle, MIL-G-46003 (ORD) Amend. 2). GI Rifle Grease is a high grade and purity, high viscosity, temperature stabilized, all-petroleum base lubricant with exceptional qualities for extreme pressure use and is formulated with anti-rust and anti-oxidant properties. While no longer a MIL standard lubricant, it is widely available as surplus stock. If GI Rifle Grease cannot be found in sufficient quantity for cost effective use, it is also acceptable to use a more modern extreme pressure, temperature stabilized, high grade lubricating grease formulated with anti-rust and anti-oxidant properties. The commonly found products should conform to NLGI No. 2 multi-purpose applications, and will usually be found as a lithium-12 hydroxide base, molybdenum disulfide bearing, extreme pressure grease, such as generic automotive “wheel bearing grease”.

Liberal, but careful, application of this type of high viscosity lubricant in the above mentioned areas will provide long term wear prevention benefits significantly greater than if the weapon is maintained solely with CLP, LSA or other light weight, fluidic base, firearms care lubricants. Synthetic weapons lubricants such as MilCom TW-25B, and other such purpose-designed lubricants, are excellent for general use on the components of the M60, for those weapons in military use. However, the key here is that civilians are not bound to consider all the same operational considerations as is a military user, and use of the high viscosity greases are more beneficial on certain mechanical components in the long run for the private owner. While it is true that such a grease application may cause an increased attraction and collection of dust and other foreign substances, the Civilian-use protocols are considerably more conducive to allowing such use. As with all firearms care protocols, significant application of common sense is required for maximum benefit.

The last area of the operating system we need to devote attention to, to prevent unnecessary problems, is that part of the operating rod group that interacts with the fire control mechanism and counter recoil components.

The firing mechanism of the M60 is a relatively basic and straight forward design employing a simple spring loaded, single form sear which acts in combination with another simple machined cut on the underside of the operating rod. The sear is pulled into or out of engagement with the sear-cut on the operating rod by mechanical action with the movement of the trigger and its spring, and the sear spring, effectively starting or stopping the firing cycle. Or at least, that was the intention of the design. In use though, the firing mechanism has two unpleasantly distasteful habits that must be understood and examined. The following applies to any variant of the basic design, and although with the M60D aircraft weapon variant the trigger is replaced with a linkage and trigger bars, the actions involved are exactly the same as in the ground guns.

The sear engagement surface cut on the operating rod underside is neither overly large, nor particularly aggressive, in its engagement with the sear. This has lead to a reasonably accurate branding of the weapon as being prone to experiencing “run-away” operation, that is, un-commanded firing after trigger release. While the inherent design does indeed play into it, there are a few things the operator can do, or not do, to help prevent the situation from happening. As a relationship of the way the sear is forced to engage and hold the operating rod to cease firing, the operating rod sear engagement cut is placed under considerable impact loading which can over time cause the face angle to distort, lessening the available engagement contact area. This may continue to the point that the mere impact of the operating rod hitting the sear nose will cause the face on the rod to simply “jump over” the sear nose allowing the gun to fire another round, etc., ad infinitum. The successful engagement of the sear with the rod face is dependant upon the nearly perpendicular mating of the two surfaces. Any proclivity for either face to lessen that angle of contact through damaged contours, may be enough to prevent the trigger mechanism from holding fast the operating rod. While a certain amount of this distortion is unavoidable, the greater part of this wear is an unnecessary event. The most important thing any operator can do to prevent wear of these surfaces is to control the trigger mechanism in such a way as to prevent the two surfaces from experiencing “casual contact” as the gun operates. “Casual contact” occurs when the operating rod sear notch comes into partial contact at any point with the sear nose. This will inevitably cause frictional wear and low-level impact loading, forming galling or peening of the top edges of these surfaces, greatly increasing the tendency for a future uncommanded firing event. For the sear nose and operating rod to function with the least amount of contact wear, they must never be allowed to “rub” or otherwise touch, except when placed into sudden, full and complete engagement, as when intending to stop the firing cycle. The problem is that too often operators will not fully activate the trigger to its complete extension of rearward travel at the stop, allowing a small portion of the sear nose to continue projecting into the path of operating rod as it cycles. The gun will fire in this condition, but accelerated wear of the firing control parts is guaranteed also. The telltale give away sign of this circumstance is to view the underside of the operating rod and see if there is a polished “track” along its bottom edge. There should only be evidence of contact at the sear engagement notch, and nowhere else! If a polished “track” is present, the sear nose, via the trigger action, was not being pulled down out of the way! Again as with the other areas of the operating system that are subjected to galling or peening, the correct remedial action is to restore these surface faces to remove the damages, without altering the underlying angular relationships of the base contours. Past a certain wear point, this may not be possible and only replacement of the offending component part will cure the problem.

On the opposite end of the firing control spectrum is another basic problem that may exist and give rise to a similar tendency for an uncommanded firing situation. The sear must rise forcibly into the path of the operating rod in order to engage and hold the rod to stop firing. The sear is under spring pressure biased to provide upward vertical travel of the sear nose. But, in order for the trigger pressure to be within acceptable limits (trigger pull should be tested to be within a 6.0 lbs minimum, 11.5 lbs maximum pull rate range), this spring cannot be overly powerful. When a “run-away gun” condition is encountered, the first item to check is the sear spring. Examine the sear spring to be sure it is not broken, or damaged. Additionally, serious problems can arise if the sear spring has taken a “set” over time. It can progress to the point where the operating rod is able to “over-ride” the sear nose if it is not being pushed far enough vertically upward to induce the mechanical locking action required to engage the operating rod sear notch fully. Sear springs are an often-overlooked maintenance item, but should be routinely replaced with a progressive maintenance policy every 3000 rounds at least, as a precaution. It is also acceptable to replace the stock spring with a custom spring with a higher spring rating. The compression length must remain the same though to prevent jamming of the mechanism. Proper trigger manipulation combined with a higher rate custom sear spring will greatly increase the useful life span of sears and operating rods as it will encourage proper, full face surface, engagement of the two parts.

Finally, the actual recoil spring (driving spring) assembly is due consideration, as problems that may eventuate themselves here can be easily avoided with a little care and attention. There are two variants of main recoil springs currently available, and each has its own advantages and disadvantages. The gun was originally fitted with a multi-stranded wire-rope type spring to combat the well-known tendency for single wire coil springs to potentially lose some of their strength rating due to "spring wrap". The use of a wire-rope spring is exceedingly effective in the M60. However, with use it can become damaged as the spring itself is in nearly full contact with the inside surface of the operating rod tube and over time this will cause wear to occur in the form of flat spotting of the spring coils. There is nothing that can be done to totally prevent this from happening; it is inherent in the design of the recoil system. In order to allow the spring to deliver its greatest possible useful life though, the coils should be checked often for flat spots, and the spring wire itself should be rotated to a different indexing each time the gun is disassembled so that wear is spread around the full exterior of the wire rope coils as much as possible. If a flat spot has completely worn through one strand, or is close to doing so, the spring should be replaced. The newest single strand coil spring introduced with the E3/E4 variants is intended to help eliminate the flat spotting associated with the wire-rope type spring. While the new spring design is also quite efficient, the single strand spring also requires the use of its own dedicated guide rod as the coil wire diameter is different than the older version spring. The new style spring can also be prone to wear, and is more susceptible to spring-wrap damages if strained. If this occurs, it can jam inside the operating rod spring tube, and become difficult to disassemble.

Though either version of main recoil spring is perfectly serviceable in the civilian realm, the surplus older style wire-rope springs are comparatively much less expensive and no new guide rod is needed. For use of either type recoil assembly, the spring guide rod should be smooth and free of any nicks or burrs that may catch a wire strand. Polishing of the spring guide rod to eliminate as much friction as possible is beneficial. Also, liberal use of lubricant inside the operating rod tube, the spring, and along the spring guide rod is highly beneficial to longevity of the spring, as long as this excess lubricant is thoroughly cleaned after each shooting session to preclude destructive accumulations of abrasive grit and dust!!

The last element of the recoil assembly to be aware of is the recoil buffer unit  and it’s retention yoke. The buffers in use since the mid-1960’s and currently still issued are sealed hydraulic spring type, and essentially are maintenance free except for periodic inspection for damages, including leaks. If the buffer starts to fail, often the first sign will be marked increase in pounding felt at the shoulder stock by the Operator, The buffer retainer yoke will also begin to deform in a backward direction. If left unattended, the yoke may cause damages to the receiver channel and guide rails, along with the butt stock eventually. If the rear face of the butt stock appears dented or deformed outward, immediately inspect the buffer assembly and buffer retainer yoke. If any obvious signs of fluid leakage from the buffer body are present, replace the entire buffer unit!

The butt stock itself is supplied as a manufactured assembly, and cannot for all practical purposes, ever be disassembled or repaired beyond minor touchup of the finish. Individual spare parts are simply unavailable for this assembly, unless from a salvaged unit that has been broken down.

Part 5 brings us to the last and final area of consideration on the M60, and the one that brings the most common problems and costly repairs for the civilian owner-operator, and yet virtually all these problems are avoidable!

Part 5 - Feed  System Group


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