Idiots guide to classic airgun strips (pt 1)

[ggggr]

I did not remove the piston seal (parachute seal) but it looks like you would have to put the piston rod in some soft jaws of a vice and undo the screw holding the seal on with a large screwdriver. Clean everything up and put it back together. Put piston and mainspring in the cylinder and then the guide and its block (on this one the guide came out of the block but maybe some are in one piece?). Push down on the guide and then locate and screw in the stock screw lug. Refit the arrester and screw on the scope rails. Locate trigger unit and end cap and refit the two pins. Slide breech block/barrel and shims into the jaws and locate the cocking arm at the same time. Replace barrel pivot bolt. Drop action into stock and pull safety arm down and wangle the trigger guard into place and replace the screw. Replace the two front stock screws.
I dont really like gamo,s as I find then pretty souless AND some of the bolts,screws and pins can be really tight. This came with a spring that seemed too small in diameter for the size of the piston so I fitted a larger diameter one of a similar length. As the sights were missing I tried an ASI 4 x 20 with a single post. After 6 or 7 shots that dieseled , the gun settled in and seemed pretty good for plinking. I reckon a better scope would help (even an smk 4 x 20) but this was a period piece. Not my cup of tea but ok for nostalgia for someone. Trigger unit looks like it would be easy enough to strip but I have not checked whether you can get hold of the parts.

[Phil Russell]

CROSMAN T4 CO2 pistol
Here is the strip procedure for our Crosman T4 CO2 pistol. If you doubt your ability or perseverance to do the strip and then the rebuild (part 2) do not attempt it. It is not easy. But it can be done.

Note: This is not a difficult pistol to strip but the rebuild is not easy due to having to locate various components and keep them located while the top slide is pushed into engagement on the lower frame. Some special tools are also needed: if you intend to service the CO2 capsule inlet you will need a broad blade (approx ½”) to unscrew the inlet retaining nut. If you need to service the valve mechanism you will need a pair of circlip pliers with fine ends to remove an internal circlip. The rest of the pistol strips quite easily with a few standard tools.
It helps to download the exploded diagram of the pistol from the Crosman web site. Numbers presented in this guide refer to component numbers on that diagram.
Once the frame has been separated into main body and action, the rest of the pistol is built on modules.
Our pistol, bought at auction, was terrible. To start with the CO2 escaped when we loaded a capsule. I corrected this by adding some Pellgun oil to the seal. It would then not eject a pellet out of the magazine but just went a wimpish ‘phut’. It took a few strips to narrow down the reason ... basically it was either no gas getting into the valve or non getting out. I eliminated the first, which left the latter. It turned out to be the safety link, part 33 that had been fitted incorrectly ... see rebuild.

Strip: Remove the CO2 capsule carrier. Looking at the pistol, note 3 pins through the action. 2 pins through the hand grip (15 + 15) and a slightly thinner pin (18) forward of the trigger guard. There is no need to remove pin 18. Removal of the rear pin 15 allows the grip (14) and piercing lever mechanism to be removed. If you intend to get at the trigger unit and valve system, remove the pin (15) near trigger guard.
There is no need to remove the rear sight.
The top slide is sprung onto the main body and is held by shallow ridges on the slide. There is one ridge each side on the trigger block and a further one each side just forward of the trigger guard as part of the main body. You need to spring the sides of the slide from these ridges. I used a broad blade screwdriver to do this but take care that you do not damage the sides of the slide or the main body. When you have one side free, spring the other side free but keep the slide and main body held together. There are no small components to ping away but be prepared for the next step which is actual separation of the slide from the body.
With the trigger block exposed, hold it in position while you lift the slide and move it forward towards the muzzle. Try to keep the shroud assembly (part 2 which contains the barrel and indexing system) inside the slide at this point. As you lift this off the main body you will release a spring that fits over the guide rod beneath the barrel from its location in the main body. It will not fly away and should simply come away sitting on the rod.
If you manage to leave part 2 inside the action but release it from the slide, do not worry, but in doing so you will release part 4, the shroud release spring on the barrel from its housing and will need to reposition it during the rebuild.
You may now have the top slide plus shroud assembly and the main body with the trigger block assembly inside.
List the trigger assembly away, noting that the shroud release lever ((21) comes with it. This may just fall off its locating peg but do not worry.

Servicing the valve: Look at the trigger housing (11) and note a screw (10) holding the lh plate to the rh plate. Remove this screw and gently lift the lh plate away. It helps to keep a finger on the trigger to keep the trigger against the rh plate while you do this. With the lh plate removed you can see all the components ... trigger (35), trigger spring (34), hammer with spring (32), the valve assembly (30) and a component called the safety link (33) which is a piece of bent rod approx 2mm diameter that locates through holes attached to a bar on the trigger and sits between the valve pin and the hammer.
The valve unit body just lifts away from its location on the rh plate. To access the capsule seal, undo the nut using the broad blade and remove, in order, the seal (30B), the piercing pin (30C) and the filter/screen (30D). Note that if all you need to do is replace any of these components, this can be done without the full pistol strip as they are accessible through the pistol grip once the capsule carrier is removed (rear pin 15).
To access the exhaust valve, remove the circlip (30J) and let the valve seal and valve stem come away. Releasing the circlip allows these parts to come out under slight spring pressure. Remove spring, a steel washer and a seal (30F). An O ring (30K) will most likely stay inside the valve body. Replace parts as necessary.
At the front of the valve body is the part that transfers gas to the pellet ... a black push on cap with a sprung transfer port. If needed, the cap just pulls off to release a small spring and the transfer port tube.

This is really as far as you need to go with a normal service strip. Other components, shroud assembly and CO2 capsule carrier can be stripped if needed but I had no need to do this.

Rebuild follows
Cheers, Phil

[Phil Russell]

Part 2: Now the fun starts.
The rebuild:
Valve assembly: Clean the valve body and replace components in order, making sure the seals etc lie flat and locate properly. To refit the circlip, assemble the circlip on the circlip pliers and compress it; holding the clip in the pliers push down on part 30I (check this is going to seal against the O ring, it goes with the wide seal side, narrow brass side, into the valve) to compress the spring and release the circlip when it is in position. Check it has located in the groove.
If you removed it, replace the black cap, transfer port and spring. Note the cap has 2 small lugs that need to be in the correct position on the lower side of the valve body in order to locate in the trigger housing. Refit the capsule components and tighten the locking nut.
Check the trigger safety link part 33: This is a bent rod in a sort of horseshoe shape with one arm longer than the other. One arm locates through holes in the flat bar that connects the trigger to the hammer. When the trigger is pulled, this rod moves up to cover the firing pin of the valve. The hammer then strikes the rod which pushes the valve pin in to release gas via the transfer port. Our rod was fitted with the longer arm located through the trigger bar holes, indeed the exploded diagram seems to show it this way. When I failed to get our pistol to fire after checking everything else, I reversed the position of this rod, placing the short arm through the holes in the trigger bar. The result was remarkable ... instant power. I wonder if the pistol had been assembled incorrectly from new.
When happy that all is well with the valve and safety link, make sure the trigger spring is located in its recess and replace the lh plate. You can now check trigger action. It should be smooth and if you look inside the hammer as you pull the trigger you should see the safety link rise to be struck by the hammer.

The time has now come to rebuild the pistol into the main body.

Trigger housing and shroud release catch: Take the trigger housing and place the shroud release lever on its locating peg on the lh side, the long arm to the left. Now look inside the main body and note that when you push down on the components at the muzzle end, they move up and down under spring pressure. Note there is a small lever on the rh side of these bits that comes up as the lh side bits go down. When reassembling the trigger block into the main body, you need to locate the shroud release arm under this small lever such that when the shroud release ‘thumb release’ is pushed down, the small lever moves up, pushing the shroud release mechanism down ... this releases the shroud and it springs forward under the action of the spring(s) over the barrel and guide rod. This can easily be checked when you think you have the trigger block in place .. just push the release lever down and see if the release mechanism moves down. While doing all of this, note that at the forward end of the centre of the trigger housing (just below the black cap) there is a groove (approx 1.5mm wide) facing inwards on each side. The shroud assembly will locate into this groove soon.... With the trigger housing assembled into the main body and the shroud release catch working, turn your attention to the slide and shroud assembly.

Shroud assembly: If the shroud assembly came free from the slide, you need to refit it. The barrel goes through the top hole and a guide rod goes through the lower hole. The barrel has a spring and retaining C clip. The spring needs to fit into a recess in the slide with the C clip next to a ‘guide plate’ in the inside of the slide such that the spring is pushing the barrel into the shroud. At the same time, the spring on the locating rod needs to be in place over the rod. It will be located properly later ... While this is going on, the barrel and locating rod both need to go through their respective holes in the end of the slide.
I found the critical part was getting the barrel spring and circlip to locate properly. I solved it by: With the barrel/shroud off the slide, slide the barrel spring over the barrel and add the C clip (if you have not removed the C clip, it will be in this state). Now take about 15cm of thin garden wire ... mine was green covered and maybe less than 1mm total diameter. Thread one end through the barrel spring. Compress the spring completely and use the wire to hold the spring compressed. Now, with the other spring over the guide rod, pass the barrel and the locating rod through the holes in the slide, making sure the C clip is on the inside of the locating plate in the slide housing with the end of the spring away from the C clip against the shroud body ( such that when you release the spring it will push the shroud back away from the muzzle). Remove the thin wire. You should now have a slide with the shroud assembly attached by the barrel spring and with a locating/guide rod with a spring spinning freely on it.
Look at the shroud assembly and note a guide bar on the lh side at about guide rod height. It is about12mm long. On the rh side is a much smaller guide, about 3mm long. These will locate in the grooves in the trigger housing.
During the many strips I did on my pistol I managed to damage the small guide (soft plastic!) but replaced it with a small steel peg (actually part of a steel rod from a needle roller bearing) superglued into the shroud assembly.

Now for some fun: you now need to mate the shroud assembly / slide unit into the main frame while: 1. Keeping the shroud release lever operational 2. Locating the guides on the shroud assembly into the grooves on the trigger housing and 3. (most difficult) locating the spring on the guide rod into its correct position in the main frame as the slide and main frame come together. This is how I (eventually) did it. There may be other ways but this worked for me even though it took many attempts to get it right:
Hold the main frame in left hand with trigger housing upwards. Hold it so that the trigger housing is slightly angled up at the rh end; it will not move much anyway but all movement helps. Keep the lh end in the mainframe or the shroud release lever can become detached.
Offer up the slide assembly. You need to keep this almost parallel to the main body and in the correct position such that the guides on the sides will slip into the grooves on the trigger housing. At the same time, the lh end of the spring on the guide rod needs to be caught against the rh face of the plastic plate that sits a couple of cm away from the rh end of the main frame. As you push the slide and shroud to the left, this spring compresses. If you get this wrong and the spring is not compressed against the plate in the body (by the plate being positioned somewhere along the spring) then the shroud release will not work. As you succeed in locating the spring and the guides / grooves, the slide will begin to engage again onto first the trigger housing and then the body at the muzzle end. The parts should snap together.
Check the action: Does the slide release and shroud work? Does the safety work? If not, then you must remove the slide / shroud assembly again and start again.

If all is well, replace the lh pin 15, replace the CO2 lever unit with the rh pin 15 and check pistol function.

If it all works, congratulate yourself on a job well done. While correcting our pistol I must have removed the top slide about 8 times while trying various methods to get it all back together. The method described here worked, many others, including assembling the trigger housing and shroud into the slide first (quite easy to do) failed because I could not then locate the shroud release lever into its correct position while fitting the slide unit back into the main body. Maybe perseverance would have seen me find a way.

Cheers, Phil

[ggggr]

Here we have the RO72. Undo the screw on the foresight and the two on the rearsight (you will have to remove elevation wheel to get at the rear one and dont lose the little post it pushes against) and remove sights and put somewhere safe. To remove the grips there are two screws on either side parallel to the cylinder and a larger and smaller one in the pistol grips (one locates in the other).Lift the action out and remove the endcap. You can remove the cocking arm pivot pins at either end of the cocking arm and remove the cocking arm.( the 2nd pin goes through a slot in the trigger housing and connects to a slider which has a ball bearing on it).Undo the nut on the RHS of the breech jaws with a forked screwdriver or similar and undo the barrel pivot and remove. If you want to change the breech seal do it now. If you want to grease the barrel plunger and spring, you will have to remove the small pin in the block that holds them. I would do it now and replace in the block. There are 2 pins near the middle of the trigger housing----if you knock these out you can remove the trigger. Slide out the cocking slider.The remaining pin at the rear of the housing holds the sear. Watch out for the hairpin spring that goes on the sear and locates on the out side of the housing, when you remove the sear. Go to pt2

[ggggr]

Use a bit of forked tube smaller than normal (see first few posts in thread) to compress mainspring and remove cylinder pin. Ease pressure and remove guide,mainspring and piston. The piston washer on this was rubber (see end of this post). Clean and lude and replace piston,mainspring and guide. Compress mainspring and refit cylinder pin. Replace barrel and locate the pivot pin and lockscrew and tighten. Replace the cocking slider in the trigger housing and then replace the cocking arm and the 2 pins (one at the breech block and the one that goes through the slot in the trigger housing and joins it to slider). Locate the trigger and replace its 2 pins. Locate the sear and make sure it,s spring is located on it, with the other end hooked on the outside of the trigger housing. Fit the endcap with flange pointing into grips and repalce the grips and the 4 small screws and the larger and smaller one in the pistol grip. Replace sights. This is not a bad plinking pistol although crappily made. You could replace the piston washer with leather or get someone to turn a bit of ptfe or alloy and fit an O ring. The trigger is adjustable via the large screw visable through the underside of the grips but does not do much. It is possible to move it too much so the gun will not cock. At least you can strip this pistol because it has pins instead of Rivetts but I would guess spares are hard to get. Not a bad "Tin can gun" though.

[Gareth W-B]

Sterling work Guy, sterling.

[Phil Russell]

gggr (Guy) has already posted about the RO72. Today I had reason to strip a RO71 ... basically the same pistol ... but did not need to completely dismantle it to correct the fact that although it would cock it would not fire. Here are a few notes:

Supplement for RO71: if all you want to do is service the piston, it is possible to do so without disturbing the main components of the trigger:
Remove grips as above. Undo and remove the barrel pivot pin as above but keep barrel and cocking arm roughly in position. Place pistol on a flat sheet, barrel to the left. Remove the 3 screws that secure the lh side plate of the pistol. Carefully lift the plate away ... the trigger assembly will stay assembled on the rh plate.
Lift the rear sight brass studs away from the top of the action. Lift barrel and cocking arm away to the left. Piston assembly will now lift out.
Take assembly apart as above ... there is little preload but take care.
On our pistol the trigger would cock but the piston was sticking and pistol would not fire. Examination of the piston assembly soon showed why: I guess someone had had the pistol apart before and had caught the leading edge of the plastic piston seal on the cocking slot during assembly. This had shaved a fine sliver of seal away from the leading edge of the seal which was jamming the piston. The unit was also lacking any lubrication. I cleaned up the piston seal and applied a smear of SM50 to the seal, a smear of moly grease to the spring and piston outer surface and re-assembled. All works perfectly now.

Cheers, Phil

[robs5230]

Just a bit of basic info for anyone who hasn't stripped one of these gas ram riles. Probably one of the easiest break barrel rifles to strip and work on as once the gas ram has been fully depressurised, there is nothing under tension.
Mine is a 1997 model serial number TB11176. The rear face of the cylinder is flat ( no external O ring showing. It has a slotted plug covering the schrader and a grubscrew directly below ( unlike a HW90).
Ensure you have the means to recharge the ram and have a chrono to check after reassembly.

This is meant for info only and any work carried out is at the risk of the person carrying out the work. Other variants of this rifle may have different components inside

1 ensure rifle is uncocked and unloaded.
2 remove the 2 forend bolts and the forward bolt from the trigger guard and remove the stock.
3 remove the slotted plug that covers the schrader valve ( this has an o ring on the threads to seal it)
4 using a 3mm punch or similar and something not too heavy to tap it with, gently tap the schrader valve a number of times until all the air is expelled. When you think all the air is out , depress by hand with the punch to make sure.
5 carefully break the barrel just to take the tension off the cocking linkage. The cocking link pin will be lightly narrower on one side of the breech. Drift the pin out with a suitable punch tapping the narrower end of the pin. when removed from the breech, there is a ptfe or similar washer between the arms of the cocking linkage. catch this and dont lose it. swing the cocking link away from the breech rotate the link 90 degrees and lift the cocking shoe from the slot o=in the cylinder.
6 remove the grub screw from the rear face of the cylinder with an appropriate hex driver ( this retains the cross pin that goes through the rear of the dummy piston.
7 gently tap the large pin through the rear of the clinder and keep safe.
8 using a punch or similar push the piston down the cocking slot. it will meet solid resistance and at this point, the sear at the rear of thetrigger needs to be lifted to allow the piston to pass it and be removed. ( same applies on reassembly)
9 remove dummy piston rod
10 remove wire retaining clip
withdraw ( by tapping the piston on some wood or similar) the seal unit. This may catch a little at the groove that the wire clip sits in but you should be able to get it out far enough to gently grip it with some pliers or similar.
11 remove any other parts that may still be in the piston. in my case there was a black oil scraper/seal and the inertia weight. The inertia weight can be difficult to remove. a magnet may be handy.
12 inspect the piston crown. any deformation or brown colouring may indicate this has been over pressured and is in need of replacement.
13 clean, relube and renew all seals. be very careful as the seal unit also has some oil scraper rings fitted inside it.
14 reassembly is the exact reverse of the strip.

Just a few futher notes. If you find / suspect a new schrader valve itself is needed please be aware that depending on the age of your rifle, the dummy piston may need retapping to suit the threads of the new schrader valve. Luckily mine was ok.
After conversations with the lads at impact it appears that these rifles were subject to constant research and development and many things were tried and the innards may differ between rifles. Seal units may have been of a different type of metal (mine was alloy) and the number and types of seals may be different. mine has square section seals inside the seal unit. fortunately these can be replaced by standard o rings.
As parts varied / changed during the production of this range of rifles, please be aware you need your parts at hand to make sure you are ordering the correct spares.

hope this helps

I have been asked to include something about refilling the ram which I will below but firstly with a couple of caveats. The Theoben gas ram is a self contained unit that "should" need little if any maintenence, but the sealing inside the ram is via O rings and O rings do perish and fail. The schrader valves can fail too.
If pressure loss / drop in power/velocity is detected the trick is to find why, not just to pump up the ram with more air. Any excess pressure added to the ram causes premature failure of the piston crown resulting in less power output.. add more air.. further destroy the piston crown etc etc. The sears can also give way under higher than normal pressure. Dont ask me how I know. It was 20 years ago and probably resulted in the first know gas ram to spring conversion ( landymick on here can vouch for this)

as for filling there are 3 options
1 the slim jim type pump. it works and may need 80 or so pumps give or take plus a few to charge the hose itself for an empty ram after rebuilding. the downside is you cannot measure the pressure in the ram and this pressure can be an indicator of how good your piston crown is. in any event check by chrono
2 my personal preference - the stirrup pump. comparatively few strokes and for me, charge to about 600psi, then check with a chrono .because of the efficiency of a stirrup pump, when the rifle is in a near charged state, even half a stroke can have a dramatic effect on pressure. as said above once you know what your ram takes its a good indicator of the state of the piston crown. more pressure needed..... shafted or wearing piston crown.
3 not a way I'd go but a bottle. for me not enough control over the amount of air going in and the volume inside the ram is tiny compared to a pcp bottle / tube.

Please note. the adapter that fits the theoben ram is not the same as that for the HW90. An adapter can be purchased and some do have both the HW fitting and the Theo fitting and you unscrew one which exposes the other. A simple male/male 1/8 bsp adaptor can be had for a couple of quid that will connect the fill adapter to your pump whip/ fill hose.

There are some good companies out there for anyone not competent or confident in taking this stripdown on. My vote would be Impact Airguns.

[Phil Russell]

Hi ... excellent thread but could you add a guide to the pressure to pump the ram to, please? I realise you cannot be spot on, but it would help to get a guide pressure so that someone could then fine tune for a given output.
A guide to equipt needed to pump up the ram may be useful as well.

Cheers, Phil

[robs5230]

Hi Phil.
Charging info now included.
Rob

[Phil Russell]

Thanks for this very detailed explanation Rob. It goes further than I expected but is all the better for it. I would be very surprised if any problems were caused by publishing a charge pressure ... I am sure it is available elsewhere on BBS threads ..... but I thought it would complete the picture of an excellent thread. I always find it interesting that the HW90 uses a lower pressure but have not really investigated why ... are cylinder sizes different?
Overpressurising can be a real issue as I know from experience. Many years ago I bought a cosmetically perfect .177 HE at auction for a very good price. Full of anticipation when I got home, I chono'd it ... a measly 5ft lbs with a very hard cocking action. I had no experience of the Theoben gas ram so did not know the correct cocking effort but was surprised it was so hard. Similarly I had never serviced a ram so took it to Theoben. The diagnosis was over pressurising and a burnt out piston head. In this case I think it may have not been possible to pump it up over 12. My auction buy was still good value but it was a salutory experience.
So ... thank you,
Cheers, Phil

[robs5230]

Thanks for this very detailed explanation Rob. It goes further than I expected but is all the better for it. I would be very surprised if any problems were caused by publishing a charge pressure ... I am sure it is available elsewhere on BBS threads ..... but I thought it would complete the picture of an excellent thread. I always find it interesting that the HW90 uses a lower pressure but have not really investigated why ... are cylinder sizes different?
Overpressurising can be a real issue as I know from experience. Many years ago I bought a cosmetically perfect .177 HE at auction for a very good price. Full of anticipation when I got home, I chono'd it ... a measly 5ft lbs with a very hard cocking action. I had no experience of the Theoben gas ram so did not know the correct cocking effort but was surprised it was so hard. Similarly I had never serviced a ram so took it to Theoben. The diagnosis was over pressurising and a burnt out piston head. In this case I think it may have not been possible to pump it up over 12. My auction buy was still good value but it was a salutory experience.
So ... thank you,
Cheers, Phil

I can only comment for the size of my piston crown which is approx 27mm dia. Other theobens may have had a 31mm dia one. The 90 i understand has a 30mm one.
The 90 I have shot is a lot harsher than my fenman but as I stated above, a wide range of internal parts have been used over the years in theoben gas ram rifles which may have an effect on how they perform. Also drilled and undrilled inertia weights have been used. Mine is undrilled.