Air Arms S400/410/500/510 or HFT500 APEX Regulator Installation Guide

Read this in full before installing your APEX Regulator or making any modifications to your air rifle.
- Airguns are not toys and can cause harm if improperly used or modified. We do not endorse HPA conversions from Co2, the working pressures are much higher with HPA, such conversions can be dangerous to you or bystanders when using original parts.
- Installation of any parts supplied is done at your own risk, if you are not confident about fitting the parts by the end of this guide, seek advice and/or the services of a competent installer.
- Always check the maximum working pressure of your PCP, filling above this is done at your own risk.
- Research how the regulator works and how the mechanics of air guns work, so you fully understand the installation and function of a regulator within the assembly.
- The fitting of non-standard parts will invalidate your manufacturer’s warranty.
- The below instructions are a guide, some parts may vary depending on the age of your rifle.
- We recommend looking for the exploded diagrams available on manufacturers websites to help you understand the assembly of your PCP.
- Make sure the rifle is completely empty of air and pellets before starting to disassemble your rifle.
- DO NOT rely on the gauge only, they are only an indicator.
- DO NOT try to unscrew, pierce/drill or force open the pressure tube to degas the PCP if pressure is still present.
- Always check it is empty by depressing the firing valve or slowly unscrewing the gauge unit if fitted so no pressure is held in the PCP reservoir or firing valve before stripping your rifle.
Remove the stock, scope and silencer and any other accessories from your rifle until you have only the metalwork in front of you to begin the work.
Before you start, clear some space to work in, ideally with a protective mat to work on and a container to put the small parts/bolts in as you go through the installation process. Take photos of the disassembly process to help you and keep notes of your settings if you choose to change them later.
Any Anti Tamper Removal is not covered in detail, anyone undertaking this work should be competent enough to remove Anti Tamper measures if your rifle has them fitted.
This guide has been drawn up to install the regulator, with basic tools and understanding. If you have access to advanced machines such as Lathes or Milling machines, use them to the best of our ability to make the process easier for you.
We are here to help you, if you need support, email us with your order number and details of the problem and we will get back to you as soon as possible.
Air Arms S400/410/500/510 or HFT500 APEX Regulator Installation Guide
Are you looking to install a regulator to your Air Arms, with quick and reliable results with minimal modification? If you are curious as to what a regulator does and the benefits a regulator can have, have a look at our other Guides on the website.
The following installation method is for you! If you have access to basic tools and a chronograph have a read through the below guide. As we go through, we will also change the seals on the rifle, giving it a full service at the same time. See our other guides for further modifications.
If you have questions contact us, as we will expand and explain any further points to give you the confidence to undertake these modifications yourself to unlock your rifles performance from home.
This method of installation of the APEX Regulator for the Air Arms models allows the rifle to be returned to standard if required with only a replacement air cylinder needed. As part of fitting the regulator the pressure window in which the rifle will make power is often lower/wider than the original unregulated pressure window, for example, after a regulator you may still be at full power well below the pressure before it was regulated. Giving an overall 30-40% increase in shot count from a full fill to power drop is expected. There are more in-depth guides on how to test your shot count on the website.
Installing a regulator and getting the best performance from it requires the rifle to be completely reset mechanically and work to differently to get optimum performance, again there is a guide on the principles of this which is worth a read for those of you wanting to undertake this work.
Air Arms models have different features, for example the multi shot assembly or bolt action/side lever, and the orientation and placement of the bolts used to hold the action together, but the regulator installation is the same for all. Only the disassembly and reassembly may be slightly different. Further modifications can be made and parts fitted to improve performance further if needed, but the process here gives a great performance increase to get you started.
If you are looking for a faster lock time and reduced recoil due to reduced moving mass, you can fit a lightweight hammer unit which we have available - this is particularly useful for benchrest shooters. Not all the hammer mass is needed when converted to a regulated setup as the hammer only needs to overcome the regulator pressure, not the full fill pressure.
Assembly bolt Anti Tamper on Air Arms rifles has changed over the years, some have caps that need to be drilled through, others have special headed bolts that need a tool to remove. When drilling out the cover caps we recommend a 5mm drill bit is used carefully to expose the allen bolts, or a T15 Torx-H bit is used to tap into the head of the security bolts, allowing them to be unscrewed. With the AT Covers removed it will expose the bolt hex bolt heads required to strip the action.
- Check if the rifle is completely empty of air and pellets before starting to disassemble your rifle.
- DO NOT rely on the gauge only, they are only an indicator.
- DO NOT try to unscrew, pierce/drill or force open the pressure tube to degas the PCP if pressure is still present.
- Always check it is empty by depressing the firing valve or slowly unscrewing the gauge/manometer so no pressure is held in the PCP reservoir or firing valve before stripping your rifle.
Removal of the AT on the transfer port/Venturi screw needs to be done to accurately set the power after regulating. There are different methods to do this.
- Drill out the middle of the cover to allow a torx bit to be tapped in, use a 2.5mm drill and a T15 bit for this.
- Using a small (10-15mm) disk on a Dremel cut a slot in the cap to allow you to turn it and tap it around.
- Using a pin punch and hammer, tap the cover out, slowly rotating it round tapping gently.
Any of these methods can be used and if careful won’t damage the main body.
Remove the screw on QF cap, on the bottom of the spacer ring there is a 2mm grubscrew, undo this and the spacer ring (or barrel band) can be removed. This reveals the 2 holes used to remove the Quickfill body from the cylinder end. On models with barrel bands, remove the 2 grubscrews at 5 and 7 o’clock and the band will slide forward off the cylinder.
Unscrew the gauge slowly using a 22mm spanner, if any pressure remains it will leak away slowly.
Drill out the cap on the gauge banjo carefully to expose the allen bolt head, undo this bolt slowly, any pressure left in the rifle will leak out after a sharp crack.
Using a 10mm spanner on the 2 flats at the side the the T-bar nipple, remove the inner part of QF unit from the end of the cylinder.
Remove this part, with the spring and plunger as a complete set to service unit later on. We recommend removing the QF unit entirely to make cleaning the tube later on easier.
On Multi Shot models, (S410/S510) remove the 3 bolts securing the magazine side plate to the breech, the whole assembly will come off the rifle. Now remove the x6 securing bolts fixing the breech to the main body. (You may have to drill out any caps/use a special tool as mentioned at the start of this guide).
With all the bolts removed, the 2 halves will separate, you should have a breech seal between the two parts, this will be replaced on reassembly.
Put the breech and barrel assembly to one side.
On some sidelever models there is a bolt in the hammer that connects to the banjo pin, to cock the rifle. Remove this bolt with a 3mm allen key before this next step.
Undo the 5mm allen bolt at the back, the hammer and trigger unit including hammer tube will come off the assembly. Remove the hammer spring top hat, hammer spring, with any spacer washers and the hammer. Put these to one side.
Remove the air tube from the firing valve body, use a vice with some protective material in the jaws to hold the cylinder. If it is tight, thread a long M6 bolt into the stock bolt hole and use that to gain extra leverage to undo the assembly.
With the firing valve assembly removed from the cylinder, check the distance of the lip of the firing valve pot with and make a note, you will be resetting this part to this measurement later. This will be different depending on your rifles model and power. Don’t measure the overall length as the ‘pot’ is being removed.
Undo the locking grubscrew and unscrew the pot off, watch out for the little nylon ball under the grubscrew if there is one fitted. Remove the valve spring, spring top hat and valve also.
Using a vice or pliers and a heat source, remove the firing valve pot from the threaded part of this assembly, also clean any Loctite from the threaded part. When clean of any remaining Loctite put these to one side.
With the AT Cover removed from the transfer port adjustment, using one of the steps outlined above, use a 1.5mm allen key to wind out the grubscrew until the port isn’t restricted.
Put this assembly to one side.
Some outer QF units can be removed without the use of the QF removal tool, if you need to use the QF removal tool, clamp the cylinder in a vice with some protective material in the jaws, fit the pins in the correct spaces and undo the QF unit out of the end of the cylinder. Some heat to the outer of the cylinder may be needed to free off the end.
Replace all orings on this assembly, including the cylinder seal, to fully service the QF end. Put this assembly to one side.
If there is contamination present on the phosphor bronze air filter on the QF unit, clean it off before reassembly.
Screw the firing valve block back on to the cylinder hand tight. Using some masking tape and a rule, mark the middle of the underside of the cylinder. Place the edge of the ruler on the bench and lay the tube against it to mark the middle of the 2 holes onto the masking tape.
Stack the regulator body, plenum and firing valve on top of each other to see where the 2 seals on the regulator body fall. Mark both with lines on the masking tape. (You don’t need to refit the threaded valve pot so do this, it was only for demonstration.)
Double check the lines you draw are in the correct place. Now mark the middle of these 2 lines. This is where you will need to drill the regulator vent hole in the cylinder. This measurement should be around 57-58mm from the end of the cylinder body on a sub12 plenum spacer.
With the hole carefully drilled using the provided 1.5mm drill bit, remove any burr on the inside of the cylinder with a small, rounded file.
Some cylinders have internal scoring from the manufacture process which need to be removed for the regulator to seal. Some tubes are better than others, shining a bright light down the tube helps detect any scoring/marks left in there. Polish the inside of the cylinder with sandpaper or a 'brake cylinder hone' to remove these and make sure there is a clean sealing surface for the orings to seal. The sealing surface should not be a polished bright finish, a dull, burnished finish is better for orings to seal onto - a 400-600 grit polished finish is perfectly fine.
This process can take some time, and different grades of sandpaper or emery cloth wrapped around a mandrel, hold the cylinder securely in a vice and use a battery drill to polish the inside of the tube.
On some rifles this process will take 5 minutes, on others it can be half an hour of polishing. Taking your time over this process will pay off later, as you won't have to strip the assembly down to re-polish and clean everything if you do have a slow leak because of a remaining score in the cylinder surface.
When you are happy there is no scoring left, thoroughly clean the cylinder (and threads) internally, washing it out with brake cleaner, using a toothbrush in the threads to clean out any abrasive. When it is clean, repeat the clean again and inspect the polished area. If there are any lines on the inside of the tube running along the length of the polished area, repeat the internal sanding/honing until there are no marks. These lines prevent the regulator from sealing correctly.
When the marks are gone and the cylinder is clean, lightly lubricate the internal surface of the cylinder with silicone oil to help the regulator slide into place.
Check the outer body seals of the regulator for anything that will stop them sealing correctly before fitting. - This is important!! Taking a few seconds to check this may save you time later on - even a tiny spec of dirt or a hair will cause a leak.
Place the regulator body on the bench and hold the tube upright and straight , push down gently, rock the tube slightly to make sure the regulator does in correctly.
With the regulator seated into the tube, put this assembly to one side.
Pick up the firing valve body and remove the large cylinder oring. Inspect the firing valve body, we recommend cleaning the hammer rail at the back of the valve body with brake cleaner to make sure it is free of oil or contamination.
See photo of hammer rail contamination, this will cause inconsistency and power problems. Make sure the rail is clean, polish the rail if the contamination can’t be removed with brake cleaner.
Clean the inside of the hammer unit, check for any rust inside the hammer also.
Place the hammer on the rail and shake the assembly lightly, making sure the hammer is free to move on the rail and does not bind at any point. Once clean and free, put the hammer unit to one side.
Slide the valve into the hole in valve body and hold your thumb over the threaded hole, shake the assembly to check the valve is free to move. You should feel it and hear it moving. If it is not, clean it again and polish the valve stem with some plain paper, repeat until it is free to move, if these parts are not free to move it can cause inconsistency and power issues.
Refit the valve spring, spring guide and the threaded part of the pot, screw it into the original setting you had before. Make sure you gently nip up the grubscrew to lock the pot into place with the nylon ball at the bottom of the hole first.
Replace the large oring on the firing valve body with the new one. If you haven't already, use some silicone oil to lubricate it slightly. Make sure the oring cord is not twisted, if it is, use a pick to get under the seal and run the pick around the oring a few times to untwist the seal. If any oring seal is fitted twisted it can cause leaks and will reduce the lifespan of the seal.
Now place the plenum spacer on the bench, add a small amount of silicone oil to the contact face that touches the regulator body and push this into the cylinder behind the regulator, sitting the plenum on the step on the regulator body.
Push in until flush with the end of the cylinder.
Now put the firing valve assembly into place, the brass pot in the slot in the plenum spacer.
Push the 2 parts together and then thread the firing valve in slowly, if you feel it go tight, stop, undo a slightly and screw in again, repeat until it is screwed in fully.
With this part complete, put it to one side.
Remove the gauge from the Gauge banjo assembly. Use a 12mm spanner (or something with a hole larger than the cap) as a spacer and close the assembly in a vice to push out the Anti-Tamper cap.
Cap removed easily with no damage.
Replace the seal underneath the head of the bolt, the seal on the gauge, and the seal that sits in the recess on the gauge banjo where it screws into the firing valve body.
Before installing the assembly, lubricate the surface where the oring goes for the banjo bolt, place the seal into the recess on the Gauge Banjo, and screw the bolt into place, slowly screw it down into place. Be careful not to damage the oring. It should come to a solid stop. If it does not, the oring may be damaged and will leak.
Tighten the gauge up onto the Gauge banjo using a 22mm spanner or socket.
Refit the QF end and slowly fill the tube assembly, we recommend filling slowly to prevent any temperature shift making a slow leak more difficult to find due to temperature shift causing movement of air inside the chamber between the 2 orings on the regulator body in the tube.
Fill to slightly above your regulator pressure (which is engraved onto the side of the APEX regulator body). You may get a small leak from the firing valve (leaking from the transfer port) until you reach 50bar or so, this is because the valve is being forced shut and creating a seal. if this leak does not stop there may be some contamination keeping the valve open, or the valve face may be damaged. Unfortunately the valves are soft injection moulded plastic so can damage easily and take a 'set' in use, and once removed they may not seal again so need to be replaced or re-faced.
To check the regulator vent hole for leaks, put a dot of silicone oil over the vent hole. If it does leak, the silicone oil will be pushed out of the hole by the escaping air. Watch for 30 seconds and see what happens. If you are unsure, leave the assembly while you do the next part and come back to it to allow the air/tube to cool before testing.
If you do have air escaping from the vent hole, the assembly will need to be stripped, the regulator orings removed from the body and inspected for any contamination, such as hair stuck to the seal. If you are sure the orings are clean with no contamination found, the cylinder will need to be internally polished again.
When it is polished and cleaned again, reassemble and test. If the leak improves repeat until it does not leak. If the leak does not improve, check all the orings on the regulator assembly and re-test. When finished, refill to 150bar on your bottle. Remember, the standard gauge will no longer read fill pressure, it is regulator pressure only.
At this point you can rebuild the rifle with the standard hammer and spring, however we would recommend at least shortening the standard spring. Doing this allows the valve to close faster, reducing air usage and boosts the shot count considerably. You want to aim for a built rifle making slightly above your goal power, which you can finely tune on the transfer port restrictor. For example, aim for 12ftlbs if you want 11.5ftlbs in the end. The closer you are to your goal power, the more shots you will have from a fill as less air is wasted.
We recommend shortening your spring by 1.5 coils to begin with, more shortening is usually needed but this is a good place to start. You will want some preload on your hammer, to stop the hammer and spring moving when not cocked, but not more than you need because this means the valve will take longer to close on firing, dropping shot count. For more info on how to set the hammer/valve and regulator relationship up and the benefits of doing it, look at our Guides section where we cover the different principles.
See how much ‘uncocked’ preload you have by measuring the amount of tension set on the hammer spring by checking the distance between the two faces. This distance is with the standard hammer spring.
With the hammer spring shortened
The shorter your spring the less preload you will have, so your valve will shut faster, but you may not have enough spring to make the power you want, setting this balance with the hammer spring/valve spring can take some time and effort.
Cut the spring using a Dremel and /quench it in water when it is pressed flat on a metal surface. We also supply shortened springs for this if you can’t do it yourself.
With the spring shortened or replaced, fit the hammer, spring and spring guide, leave any factory fitted power shims off for now.
Refit the trigger unit and the rear bolt. make sure you pull the trigger so the rifle is not cocked, otherwise when you come to cock the rifle later the hammer will jam the sear and the rifle will need to be stripped again to sort it. Only screw the rear screw in until half a turn from nipped, it will need a final tighten when the 6 breech bolts are refitted. Align the trigger roughly with the gauge so everything is nearly in the correct place. Put this to one side.
Before reassembly, we recommend replacing the barrel to probe seal, this is the most common cause of inconsistency, due to damage or contamination from lead, hair from cleaning patches, or other debris, this process is different on different rifles.
HFT500
Remove this 1.5mm gubscrew near the transfer port, first if your rifle has one fitted.
To remove the barrel, there are 3 different types of fixing, for the HFT500 undo the 3 2mm head grubscrews, (check you know the orientation, they are not all the same!)
You can see the breech seal sat in the groove here, remove the seal with a sharp pick or pin,
Clean the groove carefully, remove anything that may be in the bottom of the groove, kitchen towel soaked in brake cleaner and pushed into the groove and moved round works well to clean the groove out very well. Any dirt/contamination in here may cause a breech seal leak!
Refitting the new seal, poke the edge of the seal into the groove first, then push the seal into place, it is easier to push the seal into place, rather than pull it back out of the barrel into the groove.
When complete, you may want to replace the 2 barrel to block seals which are machined into grooves inside the block, this can be difficult to remove and refit the seals as they are so deep in the block, and unless you had a previous consistency issue, they are very rarely damaged.
Refit the barrel into the block, nip up the 2 pointed rear screws first, making sure the locator holes are aligned correctly, if they are not, your transfer port will be blocked off, and you may get low power. When tight, screw in the final top screw, and finally the smaller grubscrew from the underside.
Other Models, S400, S410, S510 etc
For the S400/S410/S510/S500 models, it is possible for the transfer port/barrel sleeve to remain stuck in the block on disassembly, requiring more work to remove, so we suggest changing the breech seal with the barrel in situ. The process is the same but the access is not so easy as the HFT500 with the barrel out. If you’d like to attempt this, on the S400 both screws will be obvious, on the Multishot models undo the magazine retaining plate with the 2 panhead bolts on, this will reveal the rear barrel fixing grubscrew, undo this and the one at the front, make sure the 1.5mm one next to the transfer port is removed if you have one fitted. With all 3 bolts removed you can now pull the barrel out of the block on both multi and single shot models. On these models, the orings around the brass sleeve that fit either side of the transfer port often get damaged on assembly, replace these with the ones provided in the seal kit to prevent leaks on firing leading to consistency issues. When refitting the barrel to the block, check the hole is clear/free of any old orings or contamination. lubricate the brass sleeve/orings well with silicone oil before reassembly to reduce the chance of any damage.
Refit all bolts and check the orientation of the barrel/Transfer port when fitting the barrel back in, if you don’t align the barrel/transfer port you will have no air delivered to your pellet!
Now it’s time to rebuild.
Fill the rifle above your regulator pressure, we suggest around 30bar above, keep a check on the pressure. If your pressure drops below the regulator output pressure your power readings will be incorrect. Before assembling the breech, check the transfer port restrictor in the valve block is not obstructing the port. Refit the breech seal between the 2 parts of the assembly, align the 2 halves with the sidelever closed to make sure it still operates correctly.
Refit 3 of the 6 bolts, this will be fine for a quick power test. Fit either side of the transfer port, and one of the screws at the back of the block to orientate the trigger assembly. Nip up the rear bolt so it is tight before power testing as it can alter the output.
Cock and fire the rifle in a safe direction to check it all functions correctly. Check the power on a chronograph over 5-10 shots, if you need less power, shorten the hammer spring by 1/4 coil and repeat. When you get close to your power setting you can change how you alter the power.
If you shorten the spring too much you can use a spacer to shim the spring back up, adding preload, or change the firing valve pot preload if you prefer, half a turn is a good amount to use. Less preload will give more power, more pre-load will reduce the power also. This can be done in different ways depending on your needs. Make sure when you are stripping/rebuilding that you nip up the 5mm bolt at the back, as this can change your hammer spring preload!
With the power set to above your goal setting, fit all 6 of the breech/hammer tube bolts, and check the sidelever still works smoothly, alter the tensions on the bolts to get the fitting correct if the sidelever action is not smooth. The bolts can cause the block to twist, so always check the sidelever works fully without binding.
Screw in the transfer port adjuster to set the power to do a final set on the required level you will find the first 2 turns or so will have little impact on the velocity. You will reach a point where it will begin to have an effect, from here 1/4 turn adjustments and trial and error will set your velocity.
When the installation is complete, fill to slightly above your regulator pressure (engraved on your regulator body) and shoot a shot string until you fall out of your power window/FPS drops. You are looking for a steady drop off in FPS, if your FPS rises from your average when you get below your regulator pressure, the regulator pressure will need to be reduced as it is too high and stopping the mechanics of the rifle from working efficiently.
Drop the regulator pressure by 5 bar and repeat the test, resetting the power each time. If needed, repeat until your velocity does not increase after the fill pressure drops below regulator pressure. You can also add some valve preload added to reduce the valve dwell, this will reduce any spike, and will reduce power. Balancing the hammer, regulator pressure and valve spring together will give you the greatest benefit of fitting a regulator, in terms of shot count, ballistic gains and the power difference between pellets too.