September 13, 2012

Repairing Crosman Air Guns

The quantity of air-gun parts per gun is about the same or less than the number of parts in cartridge firearms. In fact, many air-gun parts look and function like their counterparts in firearms we’re already familiar with. Here's an inside look at the Crosman 760 air rifle.

Gunsmiths who don’t repair air guns should ask themselves, why not? Is it the image of air-powered products that have kept air guns out of the back room? Could it be a lack of knowledge, or even fear of the supposed complexity of the internal parts?

What learning process was used to acquire knowledge about a Ruger 10/22 or any other firearm for that matter? Many learned by taking a firearm apart and seeing how the parts fit and interacted. The same will be true of the air gun. A mechanical device is a mechanical device, and a ‘smith with any experience and savvy at all will soon gain a working knowledge of air-powered guns. Perhaps the revolver most gunsmiths first became familiar with was the Smith & Wesson Model 10 or 19. This S&W revolver has about 80 individual parts. The number of parts in the Crosman 357 is less than 60. The 760 has less than 50. The quantity of air-gun parts per gun is about the same or less than the number of parts in so-called normal firearms. In fact, many air-gun parts look and function like their counterparts in firearms we’re already familiar with.

What may be a problem, however, is the shift in thinking needed from “powder power” to “air power” and how air power is stored and handled. In looking at the 357 and the 760, it can be seen that air is handled differently in each instance. The 357 uses a disposable CO2 container for multiple shots. The 760 is charged by pumping air into a valve, storing it until needed for a single shot.

All air guns, from the cheapest to the most expensive target models, have the common problem of leaking air. The first, and most common factor in air leak-ing, is over-lubrication. Dust and grit col-lect in the oil needed to lubricate sealing and operating parts. This sticky and abrasive mixture slows down moving parts and seems to gravitate to the parts that are most wear-sensitive. These parts in air guns are the seals, valves, and O-rings. Once the valve seals have become scratched, a channel is opened for air to escape. Although there are other causes of leaking, the major contributor is over lubrication. Another cause is the use of regular gun oil. Petroleum products will attack O-rings and seals, softening and expanding them, and destroying their usefulness and this could be another cause. The proper oils are silicone based and a small supply should be ordered from Crosman.

Occasionally, you’ll find that the customer has tried taking the gun apart himself, and will bring you a big basket of trouble.

The second problem is caused by normal wear of parts that slide against each other during operation. Parts affected by wear include the barrel seals on spring-loaded target guns and overtightened end seals of power cylinders. The basic cause of such wear is generally a lack of lubrication and excessive force.

As air-gun work may be new to many, I will begin with an in-depth and detailed look at the disassembly and reassembly process. A person starting in air-gun work can follow these detailed instructions point by point until familiarity with the working parts is developed.

While working on these air guns (and for that matter any project in the shop), the use of eye protection is highly recommended. Parts that are driven by springs and lost are easily replaced, but how about eyes? Replacement eye parts are difficult to find. Don’t risk losing them.

As working on the Crosman 760 is easier than the 375, I’ll discuss it first. The Crosman 760 is one of the industry’s great air-rifle success stories. It is a relatively inexpensive rifle, one that a youngster can afford. It also has enough built-in accuracy for dad to use to discourage amorous night critters. Over the rifle’s 30-year existence, the 760 has gone through several major modifications and design changes. The most notable was the change to a plastic receiver in place of a die-cast metal one. The plastic replacement part, in fact, has served the rifle better than the original material.

These rifles can withstand considerable abuse before problems begin to arise, but when the problems do occur, the rifle will lose its usefulness completely and rapidly.

The 760 is not difficult to disassemble or reassemble and the most needed parts are not expensive. (When was the last time you saw a gun part that sells for 25 cents?)

The primary problem found with the 760 will be excessive lubrication. Many customers believe if a little oil is good, a lot of oil should be better. Too much of a good thing can and will produce leaking problems in the 760. If the rifle lacks power, will not produce pressure when pumped up, or loses pressure in half a day, the problem can most often be attributed to the need for an oil change.

Disassembly

The disassembly of the 760 requires no special tools, and those needed should already be on hand—safety glasses, screwdrivers, vise, heat source, pliers that will grip brass valves without marring, cleaning tank, wood dowel, and a clean work area.

The pivot pin between the pump tube and lever/link holes must be checked. These holes can become enlarged.

As with any firearm, make sure the chamber is empty. This can be done by pulling the trigger to release any pressure in the valve and to expel any BB or pellet in the barrel. A thick telephone book makes a good BB or pellet trap. Run a rod through the barrel to make sure the bore is clear of any stuck BBs or pellets.

Study the EPV (exploded parts view) to become familiar with the parts names and general locations in the rifle.

Start the disassembly by unscrewing the stock bolt, located through the bottom of the stock grip. These seem to be put on at the factory by a person with bulging biceps—they are tight. A proper-fitting screwdriver and a deep breath will help in loosening the stock bolt. Then remove the three screws on the right side of the receiver and then the right receiver plate can be carefully lifted off.

The inside parts should not spring apart, and their locations are easily visible. How they interact with each other can also be easily determined by moving the bolt, trigger, and safety. The BB retainer and stock retaining plate are the only “loose” parts in the receiver and should be removed before removing any of the other parts.

Next, release the trigger spring from the safety. The safety will drop out of the left receiver. Remove the trigger spring. (This spring really performs the function of a safety/sear spring. The real trigger-return spring lies under the trigger and interlock.) Remove the sear and sear pin.

Lift up the front leg of the trigger-return spring, and pivot it into the trigger-guard opening to release pressure on the interlock and trigger. Pulling the trigger pin out of the left receiver makes removal of the trigger, interlock, and trigger-return spring easier.

Turn the rifle over and, while holding the barrel and receiver together, remove the single screw in the left receiver. As the barrel assembly and pump tube are lifted out of the receiver, the tube plug and hammer spring are removed. The bolt and hammer assembly are easily removed from the barrel and pump tube. Note that the hammer pin is a small, loose-fitting pin at the rear of the hammer. Use care to keep from losing it.

Unscrew the barrel-band screw and drift the pivot pin through the pump tube, barrel band, and lever/link assembly. Opening the pump about a quarter or a third of the way will take pressure off the pivot pin and make driving it out easier. These pivot pins are generally very tightly fitted. Using a roll-pin punch or other close tolerance punch will make the job of removal easier. If the pin is already loose, check to see if the holes in the pump tube and lever/link have been enlarged.

Holding with non-marring pliers, heating the center joint can help disassemble the valve assembly if it has been “Loc-Tited” into place.

After the barrel band has been slipped out of the pump tube, the barrel can be lifted off the pump tube. The fore-end and pump assembly are with-drawn from the pump tube.

The pin that holds the pump rod to the lever/link assembly is loose and can fall out as the pump assembly is removed. It is a good idea to remove it as soon as the pump rod clears the tube to prevent accidental loss.

If the rifle has been in service for any length of time, the pump assembly will be a black, gooey, oily mess. Removing the pump cup from the pump-rod assembly with the fingers is messy—but nevertheless recommended. A screwdriver, while more sanitary, could damage the pump rod.

To remove the valve assembly from the pump tube, insert a closely fitting wood dowel through the rear of the pump tube to push the valve assembly out the front. Gentle persuasion may be needed. This valve will also be a black and gooey mess if the rifle has seen use.

Getting the valve assembly apart will require heat, as the two valve halves are “Loc-Tited” together. Remove the O-ring from the front of the valve and the gasket from the vent hole. Place the rear or stem end of the valve in a vise with non-marring jaws. With care to prevent damage to the O-ring, apply pressure to the front of the valve with non-marring pliers, and apply gentle heat to the center joint area. Be careful not to force the parts or grip them hard enough to gouge the soft brass parts.

When the locking material has softened, the two valve parts will unscrew easily. Disassemble the internal parts from the body halves. A fine wire or punch may be needed to poke the check valve out; it may be stuck in the front valve section with thickened oil.

All the parts of the rifle will need to be cleaned with a good-quality water-soluble cleaner. Harsh cleaners can attack plastic parts and damage them badly enough to require replacement.

After cleaning and drying, the parts can be inspected for wear and repairs can start. Be sure the reassembly area is clean. Remember, these parts are more sensitive to dirt than a powder-powered gun.

Disassembling the internal valve parts may take a fine wire or punch if the front valve section is too oily.

Reassembly

Several parts will be needed after cleaning, as old O-rings, gaskets, valves, and valves stems will be ruined by the abrasiveness of dirty oil. The hammer spring may have lost some pressure and it may be good insurance to replace it as well.

Make sure the parts ordered are for the correct version of the gun. Consult the parts list for parts numbers for specific versions. The gun serial number, parts number, and name should be included with the order. The parts numbers listed in this article are for the latest version, those guns with a serial number beginning 688.

What costs are involved in vital valve-parts replacements? A grand total of $3.25 to $3.50! These prices are current retail and are offered (plus shipping) to anyone who contacts Crosman. Dealer prices apply only to warranty stations.

It is important to remember that Crosman air guns require special oil, and that a little goes a long way! Order a small can or several consumer tubes when placing your initial parts order. All rubber and sealing parts will need to be treated with this. Do not use gun oil.

When reassembling the valve, install the smaller O-ring against the shoulder of the threads on the threaded body section. The larger O-ring is slipped into a groove at the pointed end of the valve body. Both O-rings should be treated with Crosman oil.

Insert a new check valve into the pointed end of the valve body (bevel forward), followed by several drops of Crosman Pellgun oil. The valve stem (well oiled) is then inserted into the rear body. The valve spring is placed between the stem and check valve. A drop of Loc-Tite on the threads before screw-ing the halves together completes the valve assembly. There is no need to screw the parts together in a vise or with pliers; finger tight is sufficient. Insert the valve gasket into the vent hole so it is at, or below, the surface of the valve assembly.

Barrel and Pump Tube

The newly assembled valve is pushed into the front of a clean and lightly oiled pump tube. It will seat in position against stops at about the middle of the pump tube. Line up the air-vent hole on the valve with the corresponding hole on the top of the tube before sliding the valve into place. This will make the final hole alignment adjustments easier.

Reassembling the receiver group shouldn’t take long at all.

Assemble an oiled pump cup to the pump rod and attach the rod to the link/lever assembly with the pump-rod pin. Insert this unit into the pump tube, install the barrel band into the tube, and fit the lever/link arm into the tube between the slots in the barrel band. Align the holes in these parts and drive the pivot pin through the pump tube, barrel band, and lever/link. Use care to make sure the holes are lined up when starting the pin to prevent breaking the band. The use of a slave pin and beveling one end of the pivot pin before reassembly will make insertion of the pin easier and help align the parts as the pin slides through.

Insert the hammer pin into the hammer, and slide the assembly into the rear of the pump tube. Install the bolt in the barrel with the hammer pin fitted into the slot on the bottom of the bolt. Insert the hammer spring and tube plug in the rear of the pump tube and position the barrel and tube in the left receiver. Seat the retaining screw stud on the barrel in place in the receiver. The rear of the tube is positioned over the locator in the rear of the receiver. Pull the bolt handle back slightly to make sure you have spring pressure and that the hammer pin is correctly positioned in the bolt slot.

While holding these parts together, turn the receiver over and install one of the two short receiver screws to hold the unit in place. The mid-length screw can now be screwed into the barrel band.

Receiver

Place the trigger-return spring over the trigger-spring boss with the long end of the spring inside the guard opening. Insert the interlock under the pump tube with the top arm resting behind the bolt, and pin it in place with the trigger pin. Press the interlock down on the pin and lift the long end of the trigger-return spring to rest above the trigger guard inside the receiver. Pull the interlock up against the spring and place the trigger in position on the pin. Release the lower leg of the interlock so that it rests on top of the trigger.

The trigger spring is installed with the hooked end of the spring extending through the upper hole in the sear. The rounded portion of the sear is slipped into the pump tube above the trigger, and the spring is seated over the trigger pin. The sear is pinned in place with the sear pin (the trigger pin and sear pin are interchangeable). Insert the safety into the receiver with the small end up and, while holding it in place, seat the “V” end of the trigger spring in the thinnest section of the safety.

The trigger spring is installed with the hooked end extending through the upper hole in the sear.

Place the stock-retaining plate in the receiver, concave area facing to the rear, and install the BB retainer in its slot at the top of the receiver.

The right receiver is now placed in position and pressed gently down. A slight back-and-forth motion will help position the pins in their respective holes. Positioning the safety to line up in the safety hole in the right receiver may be necessary as well. When the receivers are together and mated, pull the bolt back to cock the hammer. Test the safety and let the hammer fall. There must be spring pressure against the bolt handle as it is pulled back, the safety must work, and the hammer must fall when the trigger is pulled with the safety off. There will be no air released as none has been pumped into the valve. Check the BB retainer to make sure it moves freely and clicks open and shut.

If the gun passes these four tests, use the short screw to screw the barrel to the right receiver and insert the two long screws in the remaining locations. Snug them down but do not overtighten them. Plastic receivers can strip.

Install the stock bolt tightly and pump five to eight pumps of air into the valve. Set the safety to On to test it under load. Set it to Off and pull the trigger. A pop of released air should result. Pump it again five to eight times and let the charged rifle sit undisturbed for about 12 hours. If it has retained air pressure for that length of time, the valve is functioning within factory specifications and the rifle can be released to the customer.

The factory recommends that no more than 10 pumps be used, as excessive pressure could damage the rifle and may leave residual air in the valve.

Common Problems and Solutions

l No pressure or rapid air loss. This is probably caused by oil and dirt buildup in the valve parts. To cure the problem, disassemble, clean and replace the valve and pump parts.

l Pumps up and maintains pressure but has little power. This is caused by oversized pivot-pin holes in the pump tube and lever/link which keep the pump cup from fully compressing air into the valve. Enlarged holes could be caused by excessive pumping. To fix it, you’ll need to replace the pump tube and/or the lever link.

The interlock should rest on top of the trigger.

l The valve is pressurized, but no BB or pellet comes out. The bore is obstructed with stuck or jammed projectiles. This is a common problem caused by insufficient initial pressure to shoot the projectile out of the barrel. Subsequent shots merely add to the problem. To cure it, first try pushing the stuck BB or pellet out with a closely fitted rod. BBs generally won’t obstruct as easily as pellets. Measure the length of the blockage and if it is more than several pellets long, it may be difficult to remove. To compound the problem, the customer may have tried to remove the obstruction himself, staging the pellets against the sides of the bore and wedging them in place. If the obstruction can’t be removed by pushing it out, a new barrel assembly can be ordered.

Another option may be to carefully heat the barrel at the obstruction to melt stuck lead pellets. As soon as the lead melts, the obstruction is easily pushed out. (The factory reaction to this radical cure may be one of complete dismay and horror. It must be recognized this is not a recommended factory procedure.) If this method is going to be tried to save the customer the cost of a new barrel assembly, the plastic elevator, breech, and barrel band must be protected from heat and kept cool. Judicious use of heat will not discolor the blue. If this method fails, a new barrel will have to be ordered anyway and nothing has been lost in the attempt to save the customer a charge for a new barrel.

l Bent or broken lever-link assembly or forearm. These problems are most often caused by abuse and require replacement of the older parts with new assemblies.

Conclusion

Labor charges to clean, oil, and replace the necessary valve and related parts in the Crosman 760 should be in the $12.50 to $15 range, since the time to disassemble, clean, and replace the parts should not run over 20 minutes. Parts charges will be extra and, given the cost of standard valve and pump replacement parts, a charge of $5 would not be excessive. The total should run no more than $20, as a new 760 from discount stores like Wal-Mart is only $29.95. Charging much more than two-thirds the cost of a new rifle may be enough to cause the customer to purchase a new one rather than repair the old one.

What’s the return on Crosman 760 gun repairs? Twenty minutes work per gun equals $45 per hour in labor charged! Where else can such easy and profitable work be found?