Idiots guide to classic airgun strips (pt 1)

[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.