Park RH91 My experience of disassembling fixing and reassembling

[Robsno]

Taking apart , checking, maintaining and re assembling an RH91 (RH93)

This is my own experience, provided as is, in case other owners find it useful.

The primary reason for doing this was that whoever had opened the gun before had not used the correct grease and consequently the gun was dieseling so every time it fired you could smell burnt oil and also I could feel a vibration when firing it. Dieseling is not good for the gun as over time it can cause damage to the internals and seals, as well as potentially altering the power so that it might exceed the limit allowed. I did check it and it was not over the legal limit as it was a mild dieseling but it was hitting 11.6 so quite close to 12 legal limit. The main concern is simply to prevent damage to the gun and hence my need to disassemble, check the components, clean, re-lubricate with the correct grease and reassemble.

Steps to disassemble
When removing parts always place them carefully on a tray, preferably in the order in which they were removed.

1) First remove the 4 screws that secure the gun mechanism to the Stock. These are a screw and bolt that also hold the trigger guard in place and two bolts at either side of the end of the stock. (Advisable if the silencer is fitted to remove it to prevent any damage)

2) Remove the pin securing the cocking arm to the main body by removing the e-clip on one side and pushing the post through. This can be done if you don’t have the correct tool by putting something flat against the open end of the e-clip and pushing open ends. Once partway off, put something plastic in the space the other side of the e-clip from the open end and ease it all the way out, or put a smal screwdriver blade in the gap and twist. Be careful they can ping across the room. Once the cooking arm is loose, you will see the end can pass through a slot and be removed completely.

3) Remove the three e-clips on the posts holding the trigger mechanism in place and gently push them through and then carefully remove the trigger mechanism by sliding out in one piece,and put everything to one side. Do not try and disassemble the trigger mechanism simply put it to one side.

4) Next place the cylinder of the gun in a sash vice or use a gun Spring vice being careful if the gun is a 93 to place a piece of wood on the flat half of the cylinder to protect the safety catch. The winding end of the vice should be at the back of the Spring cylinder, preferably with a piece of wood at either end to prevent marking the metal work.
Tighten the clamp so that it is snug against the back of the cylinder so you can then safely tap out the securing pin on the back plate of the cylinder with a hammer and nail punch or similar, once that pin is removed, the back is then supported by the sash vice. The clamp can slowly be unwound, gradually releasing the pressure from the rear spring allowing the removal of the back, the metal top hat spring guide inside the spring and the rear (longer) spring itself.

5) Next take a piece of wood (in my case a 1cm square piece of hardwood but could easily be dowel et cetera that fitted inside the rear piston and stuck out beyond the edges of the cylinder end. Mark a line on the wood at the point where it meets the end of the cylinder as this will be useful in re assembly. Then put the cylinder in the clamp again with the winding end at the rear of the cylinder and tighten it just enough until some of the tension is released from the motorcycle chain linking the two pistons and it can be moved up and down a little as this shows the vice is taking the tension of the front spring.

6) You can now turn your attention to the chain as you will see at the front on one side there is a clip on the chain that can be removed by applying a bit of pressure on the open end of the clip.
Once the link is removed, you can gently unwind the clamp. This will allow the rear part of the chain to move backwards until the tension in the front spring is released.

7) Once the pressure is released from the second Spring, a second removable link should start to appear in the oval window at the side just before where the trigger mechanism was on the underside of the gun mechanism. It may be that you need to put a screwdriver in the gap just before where the trigger mechanism was to gently push the piston along a little using the metal catch you can see sticking into the gap that catches on the trigger sear, do not try and remove this as the chain will not go through, so first slide it so that the second removable link in the chain is fully in the window Allowing this to be removed using the same technique of pushing a flat edge against the open end of the clip holding the chain-link in.

8) Once the second link is removed, the length of chain between the two removable links can then be taken out. This will allow the removal of the rear piston. it may need a gentle piece of assistance to slide out using a screwdriver to push on the metal protrusion that catches on the trigger Sear.

9) The front piston will then also slide out fairly easily, keeping it upright and level. I used a knitting needle and a few very gentle taps on it with a hammer to ease it out.

10) Lastly remove the chain guide wheel at the front, the post for this simply unscrews

11) having got the gun to bits and all of the parts thoroughly cleaned and decreased, in my case removing a layer of horrible brownish grease that looked like the sort you might see being used in an engine, it is time to check the parts.

Checking the parts
Take the two springs and roll them on a flat surface they should roll evenly, they should not be kinked or bent. Then measure the length of the springs. The short spring from the front should ideally be 148 mm and the long Spring from the rear should be 163 mm.

If the short Spring is too short, you may may want to replace it, in which case the best Spring to use is a standard WEIHRAUCH HW80 spring 23mm diameter 3.8mm wire 270mm long, (NOT SPRINGS FOR THE NEW HW80 as these are 21mm diameter) the length of this will need to be cut down but the rest of the dimensions work well. DO NOT BUY A HIGH POWERED HW80 SPRING LIKE AN OX OR A TITAN AS THESE WILL AFFECT THE WAY THE GUN BEHAVES, PRODUCING SUB OPTIMAL RECOIL SUPPRESSION, MUCH MORE EFFORT TO COCK THE GUN AND MAY TAKE THE POWER BEYOND WHAT IS ALLOWED IN THE UK WITHOUT A FIREARMS LICENCE.

If the long spring is too short, you may also want to replace that, in which case the best Spring to use is a BSA Cadet Major spring 19.7mm diameter 3.05mm wire APPROX 200mm long (not a BSA Cadet, needs to be the Major), again the length of this will need to be cut down but the rest of the dimensions work well.

Remember when you cut springs down, cut it half a coil longer than it needs to be as you need to make sure that the ends form a flat circle and are smooth so you will need to use a blow torch and heat the final coil to squeeze it closed with some flat pliers. you can then use and angle grinder or bench grinder to flatten the end of the coil to that when against a surface most of the spring is in contact. Then file the end and inside and outside of the ground area so that it is smooth and there are no rough or sharp bits otherwise you may introduce a judder. I like to get some fine emory paper with a drop of oil on and give the end a final polish. It is worth the effort as whe the spring is being compressed to cock and uncompressed while firing the spring will rotate and if this is not smooth you may feel a judder.

Check the front piston seal condition and the two “O” ring seals on the pistons. Mine were in good condition but should you see damage then the O rings are the same as the diameter of the piston cylinder they are 20.22x3.53mm. I understand Airsporter O rings will fit. For the piston seal, it has been said on the Internet that Feinwerkbau seals do fit but require a small adjustment as they are a tight fit.

NOTE
Having checked my own springs it turned out the front spring had been replaced, but whoever did it had simply chopped both the ends of the spring without closing or smoothing them so less than half of the base of the springs were in contact. Because when compressed and uncompressed the springs will rotate inside the cylinder, if the ends are not smooth and flat they will not rotate smoothly giving a slight juddering sensation. As the ends had not been closed were I to close them and make the ends smooth by filing them it would make the Spring noticeably shorter, I also noticed the gauge of the wire was thicker so this was clearly an Ox or Titan type spring (I had wondered why it was so stiff to cock) so I decided to order a replacement Spring as per the description above. The smaller Spring it turns out was actually longer than it should be so did not need replacing , just shortening.

Prior to reassembly you will need to apply a fine layer of black molybdenum sulphide air gun grease to the chain, Spring guides and the parts of the Pistons behind the seals before it is reassembled using a stiff artists brush as for most areas you do not need too much, the Pistons behind the seals and the inside and outside of the bases of the Spring guides can have a bit more but be careful not to overdo it. Everything should be double checked in case there’s any damage. You are then ready to reassemble.

Re assembly

1) Having greased the front Spring, spring guide and piston, making sure the flat ends of the spring and the base of the cylindrical spring guide, place the spring guide over the end of the spring , hold vertically with the spring guide upwards and with the front piston in the other hand chain downwards feed the chain through the spring. Then angle the cylinder so the open end is raised a little and feed the arrangement them into the cylinder with the end of then chain resting on the bottom of the cylinder so it can eventually come out of the hole at the end while maintaining the gentle angle as you slide it in to stop the chain buckling or getting stuck. once the front cylinder is pushed all the way in the chain will appear out of the front of the cylinder. you can now re attache the guide wheel/pulley to the front of the cylinder

2) Next, maintaining the slight angle to keep the chain on the front cylinder remaining flat, put the rear piston into the cylinder and slide it fully in, it can only go one way due to the catch for the trigger. Then maintaining the slight angle, get the piece of wood we used used to release tension in the front spring when disassembling (and marked the depth when under tension) and place it inside the rear piston and push the piston gently until part of the piston the trigger catches on and the length of chain removed attaches to, appears in the oval window in front of where the trigger mechanism fits. you can now feen one end of the chain you removed through so the end appears in the oval window and re connect the chain to the front piston by re fitting the removable link.

3) Now the cylinder need to be put back in the sash vice/spring vice with the front end of the cylinder at one end and the piece of woof inside the rear cylinder at the other end. Start to tighten the vice whic will push the wood in and compress the front spring. As this happens more chain will come through the front of the cylinder and the section if chain attached to the rear piston will move closer to the front. The chain may need to be helped along at both ends but will eventually meet and allow the other link to be inserted making the chain complete. You can now release the tension on the vice, and remove it allowing the chain to get taught.

4) You can now insert the top hat spring guide you greased earlier into the spring, remove the wood and insert the spring into the rear piston then put the end cap on, ensuring the threaded hole is facing downwards as this is where the long bolt from the trigger guard screw in eventually and the two holes align at the sides align with the holes in the sides of the cylinder. Put back in the the vice and tighten until the end cap is flush with the cylinder, the holes align. You can then put the large pin that secures the back cap in place into the aligned holes and tap home so it is flush either side. you may need to increase or decrease the vice pressure slightly to allow it to go in.

5) Next you need to put the trigger mechanism back and re insert the securing pins and secure with their E clips.

6) Insert the end of the cocking arm into the the slot made for it under the gun a bit in front of the trigger mechanism slide in , and put large pin that holds the front two thirds in line with the holes under the gun the at at the front and replace the fixing pin with the e clip.

7) put the mechanism back in the stock , replacing the 2 securing bolts at the front and then fit the trigger guard with the long bolt at the back and the screw at the front.

All should now work well, certainly in my case the gun is now fully recoilless with no judder from the springs so when you pull the trigger the only indication is the sound and the pellet hitting the target. It is also is not a challenge to cock it now. Have just tested the power and it is on 10 so a good working power without risk of exceeding the legal limit of 12. If I have missed anything let me know and I will edit the post. Thanks Rob

[micky2]

Hi, thank you for your detailed explanation of the stripping down of the RH91 rifle, l am sure some members on here will find it very help full.

[Garvin]

That's brilliant Rob, thank you. If you have no objection, I'll stick it in my airguns gallery where it may help others too?

[Robsno]

That's brilliant Rob, thank you. If you have no objection, I'll stick it in my airguns gallery where it may help others too?

No problem , it may help somebody else. Rob