A Service Guide for the BSA Scorpion Air Pistol

A few weeks ago, a fellow airgun enthusiast asked me if I could service his friend’s BSA Scorpion air pistol. I thought sure, providing I can get the parts I will have a go. Of course it was also an opportunity to fully strip one down and write a service guide for anyone else that may need to repair or service their vintage BSA Scorpion.

The initial inspection of the pistol revealed two problems. First, the breech seal had deteriorated and secondly, when cocking the pistol, there were two distinct two stages of movement of the piston. I wouldn’t be sure what the cause of this was until I had stripped it down completely.

safety first!

Before starting work on any airgun, please consider your personal safety. Ensure the airgun is not loaded and not cocked! Your eyes are very precious and are not replaceable. They can be easily protected with inexpensive safety glasses that can be purchased from any hardware store. Airgun mainsprings can be under considerable tension and if not carefully released, can result in parts flying across your workshop leading to injuries or damaging nearby items. When using power tools, do not wear loose clothing or jewellery that might become entangled in the machine. Think are you working safely before turning the tool on.

working environment and tools

It’s important to ensure that you have a good working environment to service your air pistol. A solid workbench with enough surface area to place your pistol without tools getting in the way will be of great benefit. Use an old blanket on the work surface to protect the finish of the pistol as you are working on it. I actually prefer to use a sheet of neoprene or a foam exercise mat rather than a blanket as it won’t slip on the surface of the work area.

In order to remove the mainspring, you will need to manufacture a tool that will pass either side of the mainspring retaining pin. I used a 22mm diameter steel hole-saw to form such a tool. These can be purchased from a hardware store. 

The teeth of the hole-saw can be cut off with a metal handsaw. The slots on either side can be cut out using a Dremel with a cutting disc. Make sure you use a sturdy vice to hold the hole-saw securely whilst you fabricate the tool. Finally, use a metal file to remove any burrs not only to prevent you from cutting yourself but also to prevent damaging the pistol.

Apart from fabricating the tool above, there aren’t many tools required to strip this air pistol. The most specific is a set of parallel pin punches and an o-ring pick. You will also need a sturdy, metal sash-clamp to compress the mainspring in order to remove the spring retaining pin.

For a basic strip and service the tools required are:

1. The spring compression tool to fit either side of the spring retaining pin – see above.
2. A set of parallel pin punches. Don’t be tempted to make do with screwdrivers as you will not only damage the roll-pins but also damage the screwdriver too!
3. A small to medium hammer.
4. An o-ring pick.
5. A set of metric Allen keys.
6. Flat and cross-head screwdriver bits and screwdriver.
7. Two small jewellers screwdrivers – only necessary if the piston buffer washer needs to be replaced.
8. A metal sash clamp.
9. Some rags for general cleaning.
10. A toothbrush or small brass brush for cleaning screw threads of old grease and dirt.
11. Gun grease and oil.
12. A chronograph to test the power of the air pistol.  

If a new mainspring will be fitted, you will also need:

1. A bench grinder.
2. A metal file.
3. Some medium and fine grit wet and dry paper.
4. A blow torch or gas cooker hob.
5. Metal polish such as autosol – optional.

let’s get started!

Remove the Rear Sight

The first step is to remove the rear sight. It is fixed in place with a single slotted screw at the front of the sight. Once the screw has been removed, gently lift the sight off the pistol. The elevation adjuster is not attached to the rear sight or the pistol and might fall off. Don’t worry, it isn’t spring-loaded so it isn’t going to fly off across the room never to be seen again.

The sight blade can be removed from the body by unscrewing the windage adjustment should you wish to fully strip and clean it.

Remove the Stock

Before you can remove the plastic stock, the safety catch must be removed. It is held in place with a single screw. Remove the screw and then pull the safety catch off if it hasn’t come off with the screw.

The stock is attached to the underside of the pistol with two screws. One of the screws, actually an Allen bolt, is hidden in the heel of the grip. There is a small hole in the base of the grip in which you can insert a tool to lever the plastic cap away. You do not need to insert the tool much, say half a centimetre or so, then lever the cap off.

Remove the Allen bolt that was hidden by the plastic cap. The second screw is cross-headed and can be found at the rear underside of the pistol. Remove this screw and the plastic stock can then be slid away from the action.

Make sure you place the stock, the safety catch and the screws together to one side. I find it helpful to place parts of each sub-assembly in zip lock bags which not only helps to prevent lost parts but also prevents parts from being mixed up when it is time to reassemble the airgun.

Remove the Trigger Assembly

The trigger assembly is attached to the compression tube with four cross-head screws. There are two on each side at the front and rear of the trigger assembly. This time I decided not to strip the trigger assembly down but just clean it with a degreaser and then re-lubricate it.

Be warned that the piston sear lever is spring-loaded. Hold the trigger assembly firmly in place until the last screw is removed. Then ease it away to prevent the sear flying up and scratching the bluing of the compression chamber.

Detach the Barrel Assembly

The barrel assembly is connected to the compression chamber with a roll-pin. BSA fitted plastic caps to either side of the visible roll-pins to improve the aesthetics of the pistol. If you try to lever these off, they will snap. Luckily, you can buy new caps from www.airgunspares.com. Notice that the barrel pivot roll-pin end caps are larger than those of the cocking link roll-pin end caps. So be sure to select the correct size of cap when ordering.

Rather than attempt to prize off the caps, it is easier to leave the caps on and knock out the pin using a pin punch and hammer. At least one of the caps will be saved and you might find the other survives the procedure too. If not, buy some new ones. Make sure you use a parallel punch that is of the same size or just slightly smaller than the caps of the roll-pin otherwise you might just drive the punch through the cap and into the pin rather than push out the pin.

Once the pin is out, the barrel assembly and cocking links can be removed by sliding it out of the jaws of the compression chamber.

Remove the Barrel Latch – Optional

The spring-loaded barrel latch is retained with a roll pin. This pin does not have any plastic caps as it is hidden when the barrel is closed. Drive the roll-pin out with a pin punch. Then hold the latch in place as you remove the pin punch. Now gently release the latch and remove it along with the spring.

Remove the Mainspring

The mainspring is held in place and under some preload by a large retaining pin that is normally hidden under the rear sight. It cannot be knockout like other pins. To remove it, the mainspring must be slightly compressed to release the tension on the pin. Once this is done, the pin can be removed simply by pushing it out with a finger if it doesn’t fall out by itself.

However, to compress the mainspring, you will need to make a tool that passes either side of the retaining pin and can then be pushed in with the aid of a sturdy metal sash clamp. I mentioned earlier in this article how you can manufacture such a tool for the job out of a hole-saw.

Insert the tool into the end of the compression tube such that it makes contact with the spring-guide washer. Now place the compression tube and tool into the jaws of the sash-clamp. I must point out here that there is a risk that the sash-clamp could slip whilst the spring is under pressure. Providing you take your time and make certain that the jaws of the sash-clamp do not slip you should be ok. You may wish to strap the compression tube and clamp it down to prevent them from moving although I found this was not necessary.

Gently compress the spring using the sash-clamp until the pressure on the retaining pin is relieved so that it can be dropped out of the compression chamber. Frequently check the jaws of the clamp are not at risk of sliding off the compression chamber or the compression tool. Once the pin is removed, gently release the pressure on the mainspring. The mainspring and guide can now be removed.

Remove the Piston

The piston will probably need a fair bit of persuasion to come out if the o-ring seal is still strong as it is quite a tight fit. You could refit the barrel using a pin punch as the pivot pin and refit the trigger assembly which will allow you to use the cocking action of the barrel to push the piston back. However, this will only move the piston so far. You will then need to remove the trigger assembly again and the barrel to complete the removal process.

Alternatively, use a piece of wood and hammer to drive the piston out of the compression chamber. You will need to do this for the final stage of removal anyway.

Replace the Piston Seal and Buffer Washer

The piston comprises of the main metal body, a white plastic piston head in which sits the o-ring piston seal, a buffer washer, a retaining clip that holds the piston head assembly onto the piston and a washer. The washer sits between the mainspring and the inside of the piston.

The buffer washer (the thick black washer shown in the photograph), on this particular Scorpion had deteriorated so much that it disintegrated when touched. The purpose of the buffer washer is to cushion the blow of the piston as it reaches the end of the compression cylinder. You can tell if this will need to be replaced without opening the pistol during the cocking cycle. As you cock the pistol, if the buffer washer has deteriorated, the piston will move back noticeably easily a short distance. Then once the slack has been taken up, the compression cycle is much harder as the friction of the piston seal takes hold as it too is pushed back. A good buffer washer will provide no slack and the whole piston assembly will travel together in one movement.

If, as with this Scorpion, your pistol has the original piston head retaining clip, you can use a flat-bladed jewellers screwdriver to prize open the clip, twist and then lever the clip out of the groove that it sits in. This might take a few goes to work out how to do it.

Replacing the o-ring is a matter of removing it with an o-ring pick, cleaning the piston head and fitting a new o-ring.

Piston head kits can be purchased from www.airgunspares.com and are shipped with an o-ring, a buffer washer and a new style retaining clip. I reused the old retaining clip to keep the pistol as original as possible. It was also easier to fit than the new style clip. You may need to close up the gap between the ends of the original retaining ring with a pair of pliers so that it fits tightly in the piston head groove. Do this before fitting it to the piston head.

Replace the Breech Seal

This one is very simple. Pull out the old breech seal and fit a new one. You can use a jewellers screwdriver although I used an o-ring pick. Scrape out the remnants of the old seal then fit a new one. It’s just a press fit. If your buffer washer has deteriorated, there is a very good chance that the breech seal is in a similar condition and also needs to be replaced.

Fitting a New Mainspring

Inspect the old mainspring. If the coils are not evenly spaced and/or it isn’t straight then it will do you well to replace it with a new mainspring. You can see just how warped the old mainspring is when compared to a new one in the photograph below. Notice that the new mainspring is supplied much longer than necessary. So much so that you will not be able to cock the pistol as the coils will touch each other and physically prevent the spring compressing during the cocking procedure.

You will need to cut the new mainspring to size. You can use a Dremel or grinding wheel of a bench grinder to do this. I found it best to cut the spring so that it is about two coils longer than the compression chamber when inserted into the pistol with the piston in place. Cut the mainspring, then collapse the end to flatten it as you can see in the photograph above. To collapse the end, heat just the last coil with a blow torch or gas hob until it is red-hot and then push down onto a block of wood or something suitable until it is flat. Try to keep the spring vertical during this process. You could use a bolt that fits the inside of the spring snugly as a former to help keep the spring straight. Do not cool the spring in water or oil but allow it to cool naturally. This will soften the end of the spring allowing the metal to be easily ground and polished. Once the coil has been collapsed, grind the end of the spring so that it is flat. Use the end of the new spring or the old one as an example to follow. You should be able to stand the spring vertically without it toppling over. The idea is to make the ends perpendicular to the spring so that the ends will have maximum contact with the spring guide and end washers.

Once you are satisfied that the end is square use a file to remove any burrs from the spring both on the inside and outside edges where it has been cut and ground. Next, we work to remove the grinding marks and polish the end of the spring to a mirror finish. Place some medium grit, say 800, wet and dry paper onto a flat block of wood or suitable surface. Wet the paper and then sand the end of the spring by moving the spring across the paper. Keep the paper wet during this process. Once the grinding marks have been removed and only sanding marks are left, switch to a fine wet and dry paper, say 1000 grit. Repeat with the fine paper until the marks from the medium grit paper are gone. Remember to keep the paper wet. It’s best to move the spring in one direction only at all times, e.g. forwards and backwards. Finally, use some metal polish like Autosol to give the ends of the spring a mirror finish. This all helps to allow the spring to twist during the cocking and firing cycle without binding on the washers which in turn will produce a smoother firing cycle with less recoil.

There are plenty of guides on YouTube that show you how to cut, grind and dress an airgun mainspring. Perhaps I will write an article specifically on this topic at some time in the future.

Reassembly

By now your BSA Scorpion should look like a bag of parts and you are probably wondering why you started. Fear not though as they often say, reassembly is the reverse of disassembly. However, here are some tips that will help along the way.

Place some molybdenum based gun grease on the piston seal o-ring. Do not use automotive grease as that does not have a high enough molybdenum content. You may also wish to put a small amount of grease along the outside of the body of the piston. Insert the piston into the compression tube and use the sash-clamp to push the piston in. When the clamp reaches the end of the compression tube, open the clamp and place a length of wood into the piston. Now use the sash-clamp again to push the piston all the way to the end of the compression chamber with the wood as an aid. Remember to keep the slot in the piston in line with the slot in the compression tube.

Fit the washer into the end of the piston. Apply some gun grease to the end of the mainspring and insert it into the piston. Apply some grease to the other end of the mainspring and along the spring guide and insert the spring guide into the piston. Did you remember to fit the spring-guide washer?

Some people grease the entire spring in an attempt to reduce twang. I prefer grease just the ends of the spring to help it twist more easily.

Using the compression tool and the sash-clamp, compress the mainspring into the compression tube taking care to ensure the sash-clamp is not at risk of slipping off! Once compressed past the retaining pin hole, insert the retaining pin with the recess for the rear sight at the top of the compression chamber. The sash-clamp can now be released safely.

Refit the barrel to the compression tube. I use a pin punch that snugly fits the pivot pin hole and use it to hold the barrel in place. If you don’t have a pin punch that is a good fit, you can use the shank of a drill bit instead. This ensures that the barrel pivot hole is correctly lined up with compression chamber pivot holes. Then drive the pivot roll-pin into the hole pushing the pin punch out as the roll-pin goes in. This ensures the barrel and compression chamber remain perfectly aligned which ensures the roll-pin passes through with ease and without damaging the pistol. Do not fit the roll-pin end caps yet! You may need to disassemble the pistol once again if the mainspring needs to be shortened to reduce the power of the pistol.

Fit the trigger assembly, stock and safety latch.

Now test the power of the pistol using your chronograph. For UK residents, the power must not be greater than 6 ft/lbs. I try to remain at 5.5 ft/lbs or lower. Make sure you measure the power over at least ten pellets and not just one. If the power is ok, happy days! Fit grip cap, the roll-pin caps, the rear sight, crack open a cold beer and congratulate yourself on a job well done!

If the power is too high, tough luck. You will need to strip the air pistol down and remove the mainspring to shorten it. You could just collapse a coil and refit and test. However, I prefer to cut off a quarter of a coil or a half, collapse and refinish the ends as described earlier (yes, all over again), refit and test. I wouldn’t worry too much about achieving 5.5 ft/lbs. If your pistol is running anything over 3.5 ft/lbs it will do just fine but you should be able to achieve much better with this air pistol. Remember, it’s easier to remove small amounts of the spring but impossible to put it back on.

Maintenance

Some words of advice with regards to the maintenance of the BSA Scorpion. By now you will have realised that the piston head assembly is plastic and rubber. Therefore you should not apply oil to the chamber as this might react with the plastic and rubber causing them to perish. This is probably the reason why the buffer washer and breech seal of this air pistol were in such poor condition. This pistol requires a drop of oil to the pivot pins now and then and a wipe over with a lightly oiled rag to protect the blued finish from rusting. That’s all. In fact, that’s exactly the recommendation in the BSA user’s manual.

I hope you have found this service guide useful and have the courage to tackle the job yourself. As always, if you have any questions, add a comment below and I will be only too happy to help.