Stripping a spring-piston rifle
09:39 31 October 2011
Having spent a decade and more taking guns apart, I know that each of them has its own identity and design quirks. Yet very few guns are entirely revolutionary in their design and almost all tend to follow a number of basic principles. So, whilst its never a case of done one, done em all, you do seem to get a sense of deja-vu after youve stripped a dozen or so. If I had to distil my experience into a single one size fits most strip, then here are my suggestions which should provide some useful guidelines.
First and foremost, always check that the gun is unloaded. It should go without saying, but you must check that its neither cocked nor loaded with a pellet. I know of shooters who have stripped guns down with a pellet in the breech, only to have it fire when the gun was assembled and tested, one such breaking a window in the process. It could have been far worse
When working on the action its always advisable to remove the scope and any open sights as much as is practicable. Sights get in the way and are very easily damaged if the gun slips on the bench or the action rests upon them.
Always keep a set of clean containers into which parts and assemblies can be placed. You also need to have spare containers into which components can be transferred after being cleaned and degreased. Clear plastic bags also work well to hold small components such as trigger mechanism parts.
Actions are invariably held in with three or four screws. I cant emphasise enough how important close-fitting screwdrivers, or turnscrews to give them their proper name, are to prevent damage.
A proper service will start with a quick check over the chronograph, to see how the power is doing before doing any work. The chrono will show up problems such as loss of power and inconsistency, and it may also give an indication as to the condition of the mainspring. If the power is acceptable and the mainspring is still looking straight when inspected, then you neednt replace the item just for the sake of it.
Getting at the internals usually involves removing the back end, generally referred to as the cylinder block. Most airguns have a pin running transverse through the rear of the cylinder to hold this block in place though others, such as the older BSAs and many Weihrauchs, have the trigger block screwing directly on the rear of the cylinder. Almost all airguns have some form of pre-load on the spring - so it will come out to bite you if youre not careful.
A spring compressor is the safest tool to use but failing that, replace the main axis pin with a drift a tad smaller which will allow you to gauge the pre-load within by the movement of the smaller drift.
If unsure, or if the back end unscrews and therefore gives you no warning of pressure, put the action on a padded workbench, wrap the back end in a towel and wear thick gardening gloves. Unscrew the back end and let the innards meet the padding of the towel rather than the bare skin of your hand.
With the back end away, the spring and guide usually come out cleanly well, not always what youd call clean! with no problem. Check the springs condition by looking for bends, kinks or, worst of all, breaks. Replace if necessary.
You can check the piston seal to a lesser extent by tipping the gun up and seeing if the piston falls out. If it should slide out slowly, this is acceptable, but if it still falls out when you have a finger blocking the transfer port, its very likely that a replacement is needed.
Getting pistons out is usually involves removing the cocking link and sliding it out of the cocking slot. If you have a break-barrel gun, I would recommend removing the barrel rather than drifting out any pins in linkages. These pins have a habit of sliding out once disturbed and need peening over again which can mark the surface, so use some judgement.
Pistons usually meet up with the trigger mechanism as they are extracted from the cylinder. If the mechanism is a stand-alone assembly, such as you find on the Gamo series of rifles, then this is no problem, just remove it. If the mechanism is built into a welded trigger cage, look into the cylinder to find the sear jutting up slightly. Find this component in the trigger cage and try to remove this single item rather than the whole assembly, invariably thats all thats required.
Piston seals are most commonly push-fit but there are numerous and obvious variations. Piston washers that are difficult to fit usually can be softened in hot water if needed, though this rare these days. Sometimes you will find that seal is an O-ring, such as on BSA Mercurys and later Airsporters.
Breech seals are often overlooked but need replacing if worn since they allow air to escape and thus reduce power. At least its a simple job, just pick out the old one and push-fit the new one. Fairly obvious stuff really.
Lubrication is essential for smooth and consistent running and the basic rules are straightforward enough.
1. Nothing down the transfer port - even if the old instruction booklet might suggest it! Dieseling, the explosion of minute particles of lubricant under pressure, will occur which is most undesirable
2. A smear of grease wiped onto the side of the piston washer and then wiped off will help ease it in to the cylinder.
3. A wipe of moly-grease around the piston sides is sufficient, as is a very small teaspoon of grease distributed around the mainspring and spring guide.
4. Pins and links like a drop of light oil to keep them moving. A general rule is oil for small parts and grease for the larger load-bearing parts.
Reassembly is almost always the same procedure in reverse so just remember the sequence and then do it backwards.
The final bit of advice is to check your rifle over the chrono again after carrying out any servicing work. That new spring or a packing washer here and there might have sent the power wayward, in either direction, so do consult the chrono to be sure your rifle is safe and legal.