HW77K in pieces

Tuning a Weihrauch HW77K air rifle at home – Part 3

How to DIY tune a HW77K spring airgun: Part 3

Hi guys, tuning Weihrauchs refers to the HW77, HW77K and HW97K that are the subject of this article, Most of what I discuss here applies to other air rifles too, though I will make continual reference to the HW77K. Apologies for the delay in getting part 3 out but running this site and business on my own just eats up the hours leaving little time for articles. I have simplified this assembly procedure as much as I can to assist those of you who are perhaps tuning their first air rifle even if it is not a Weihrauch HW77K.

Check out part one and part two of this airgun tuning article series if you haven’t seen it already.

I am going to assume that you now have the Weihrauch air rifle in bits, polished and ready for assembly. That said, all the rifle parts you have should be lubed with a thin coat of oil if they are being left to stand for a day or two. Now the assembly and testing I am going to outline here is the way I do it, that is to say, it works for me but this is not the ONLY way to go.

Even though the parts are lubed after polishing to stop corrosion etc., I like to wipe them clean and dry and finally inspect them and re-lube them as I assemble each piece. This ensures that no dirt or foreign matter is stuck to any parts that have been zealously lubricated.

Tuning Weihrauch springers: Assembling your piston seal

Before you start reassembly, clean your bench of tools, parts, beer cans and crap so that there is no chance you can scratch or mark your parts or the bluing of your rifle. With the Weihrauch HW77K, I start assembly by fitting the piston seal. I put the seal face down (recess up) on a clean cloth, (key word here is ‘clean’…) and using a cotton bud I put a small amount of Moly grease on the inside of the seal retaining wall and the rebated edge of the seal. Don’t over-do this guys, as too much grease will only result in it being displaced onto the cylinder wall and then finding its way to the front of the seal, where ‘dieseling’ can occur.

Once I have a thin coating of Moly on the internal edges of the seal, I lightly coat the piston flange outer (front of the piston). Then by placing the piston face down on top of the seal, you can ‘walk’ the piston into the seal recess by pressing down hard and moving the piston side to side slightly from the vertical while rotating it by hand, and so allowing the edge of the piston face to slide into the seal recess.

NOTE: Do NOT use a hammer, screwdriver or the like, as there is a high probability that you will not only bruise the seal and create a weakened area, but you may puncture the seal or mark the leading edge.

Once you have the seal mounted onto the piston, rotate the piston while holding the seal with your fingers to ensure that the seal is correctly fitted, it can rotate freely and that the contact areas have sufficient lube. If you can’t rotate the seal by hand then you need to remove it and check that you have in fact got the correct seal and/or sufficient lube.

Note: On the Weihrauch HW77/97 group, air rifles earlier than 1446048 have a 25mm seal while the later units have a 26mm seal. Don’t get his wrong as it will cost you in time and occasionally damage to the sleeve. The seals are very close in size and that has lead to mistakes being made by some in the past.

Weihrauch seals are pretty soft and flexible and dead easy to mount, however, Vortek Vac-Seals and PTFE seals are another matter. Light lube on the contact areas and vertical positioning of the piston while rotating it is the key. Do not use a press or clamp. If you get stuck and you are living in Australia, give me a call, any time and any day and I think we can sort out any problem on the phone (I will call you back).

OK, so let us assume that you have jumped this hurdle and the seal is fitted and rotates freely. I then fit the spring guide that comes with the tuning kit, ensuring that the open edge is 180 degrees away from the slot in the piston (V-Mach kit). If you are reassembling your air gun without the kit, then skip this last bit as Weihrauch air rifles do not come out with spring guides. If you are fitting a Vortek kit, skip this part as well.

Next, I wipe down the piston and seal outer to ensure that there are no grease or cloth particles sticking to it. I then LIGHTLY (keyword again…) grease the piston and the seal sides ensuring that there is no grease on the face of the seal. Then stand the piston upon the face of the seal somewhere where you won’t knock it over if you are assembling on a clean material.

HW77k Exploded Image for Tuning Weihrauchs article

HW77K ready to assemble (Note Trigger group is in the “fired” position.)

Tuning Weihrauch springers: Compression Tube & Piston Assembly

Next, you need to pick up the compression tube and wipe it clean of any debris, lube and fingerprints etc. It needs to be clean and dry as your hands. Face the chamber downwards and lightly smear a bit of Moly grease into the first 30-40mm of the inside of the chamber wall. Then pick up the piston and carefully marry it up to the cylinder. You may have to ‘walk’ this around by rotating the piston/seal slightly in the open face of the compression tube while rotating it, similar to what you did fitting the piston seal.

Once you have the piston seal entering the sleeve you can push the piston home until it bottoms out, then pull the piston right out of the cylinder and wipe off any grease that has migrated to the face of the seal. Repeat this a number of times until negligible grease is evident on the seal face. By working the piston in and out you should be able to determine if there is any binding or change in the wall diameter as this would be evident in the pressure required to move the piston.

If you use a standard Weihrauch Seal when tuning the air gun, you will notice that the wall tension is much less than that of a PTFE seal, with a Vac Seal rating somewhere between both of these seals in wall tension. Cost-wise the Weihrauch seal is the cheapest followed by the PTFE seal then the Vac Seal with each seal have different unique qualities apart from price.

When you are happy with the piston and cylinder set being assembled and lubricated correctly you need to move onto the chamber. If you have an air rifle that does not use the compression tube like the Weihrauch HW77s and HW97K models do, then treat the rifle chamber as I have detailed above for the compression tube.

I then wash out the chamber with degreasing spray (Cheap Auto-Parts sell cans for around $2.50) and a swab on a wood dowel, usually at a point in time when my wife is out of the house. That enables me to rinse out the chamber and barrel assembly using hot water and detergent in the sink. When clean I wipe it down carefully (the sink that is… and then the Weihrauch action) with clean towelling or one of my tee shirts.  I then blow out the barrel and action points with compressed air. Don’t get caught doing this as women just don’t understand…

Satisfied that the action and barrel are clean and dry, I lubricate the inner chamber wall and then fit the compression tube and piston. With compression tubes that are polished to a high degree along with inner chambers being polished too, you will find that loading the air rifle will be very smooth indeed. When lubricating these 2 parts, there is no real danger of over lubricating them as any excess grease will soon be evident in the loading gate or at the rear where the compression tube is housed. It can be removed then.

The spring is then wiped clean of lube, inspected and re-lubed prior to final fitting. If you are fitting a tuning kit, now is the time to fit the recommended number of washers beneath the spring.

Slide in the compression tube complete assembly and check that the long slot in the tube is facing out (down while holding the rifle horizontally) and that will be evident when the compression tube marries up with the offset barrel port that extends into the loading port by around 3mm. If your air rifle is new then leave the barrel seal in that came with the rifle but if it has done a few miles, change it for an ‘after-market’ seal. These are made in the USA and seal very well with quite a soft material.

HW77K Piston Sleeve. From Tuning Weihrauchs article.

Piston Sleeve and Spring Assembly in the correct position. If you use a V-Mach spring guide then fit it with the join facing upwards, that is towards the top of the air rifle. The above image has a V-Mach kit in it and the lower portion of the spring guide can be seen through the loading arm slot.

Once you have the compression tube pushed home and fully engaged (above) with the barrel port, I fit the loading arm catch into the compression tube and lock it onto the piston. I then tie it up against the air rifle’s action with a cable tie. This is done to stop the piston rotating in the chamber when the breech block is screwed back on. See Below:

Loading Arm – Piston Sleeve from Tuning Weihrauchs article.

The Loading Arm Lever is hooked into the Piston Sleeve

Loading arm in position HW77K from Tuning Weihrauchs article.

This image shows the loading arm is inserted into the Piston Sleeve.

I hold the loading arm with a Cable Tie on sub 12 Ft Lbs springs but on FAC springs I hold the loading arm by partially inserting the pivot pin and setting the outer arm into the detent as you would do prior to firing the air rifle. See below:

Weihrauch Pivot Pin HW77K from Tuning Weihrauchs article.

The partially fitted pivot pin

At this point screw the breach block on using a sliding clamp as illustrated in part 1 of this article. Before you put it in place, lightly grease the face of the breech block and the loading arm catch that enters it. I also put grease on the rear of the breech block so that I can turn the block when it is under pressure of the sliding clamp. I put a small amount of oil on the breach block threads at this point.

Compressing Loading Spring HW77K

Irwin Clamp – Piston, barrel and spring assembly being compressed into the breech assembly.

I then tighten up the clamp slowly until I see the thread of the breach block mate up with the thread in the action housing. At this point look along the air rifle assembly to ascertain that the breach block and chamber are inline. Then carefully turn the breach block until the action thread is engaged.

Note: Keep an eye on the alignment here as on powerful springers the 2 assemblies can ‘walk out’ of the clamp alignment and possibly exit at great speed. The key here is to do this slowly if you are a first timer.

Then screw the breech block home and tighten until the trigger opening is aligned correctly. You can also use the dovetail on top to ensure that the mated parts are aligned by running your fingernail along the dovetail to check for alignment.

Alignment is done by placing a brass drift that fits snuggly into the trigger recess and tapping it around once it tightens up by hand. I say “tapping” because it only takes a small amount of pressure to tighten and align the two components. If you over-tighten the breach block then you may find that it becomes loose down the track. To overcome that problem you will need to Loctite the two parts with medium grade Loctite or equivalent, but only do this if your components have been over-tightened quite severely or are prone to loosen on older air rifles.

Weihrauch HW77K end cap.

Threaded end cap partially installed while still in the clamp. At this point, you can disengage the clamp and tighten by hand.

Weihrauch Trigger pod from article Tuning Weihrachs

Tightened End Cap Assembly ready for Trigger Group.

Weihrauch Rekord trigger from Tuning Weihrauchs

Trigger Group in the ‘Open’ or ‘Fired’ position

Press down the Top Lever and catch it with the trigger sear floating arm and it will then be in the ‘Loaded’ position and look like the image below here:

Weihrauch Rekord trigger 2 from Tuning Weihrauchs

Trigger Group in the ‘Loaded’ position and ready to install.


Once you have the breech block tightened up and aligned you will need to fit the trigger assembly. Firstly you need to ensure that the trigger group is sufficiently lubricated and then you need to “load” the trigger as per the images above. Once the latch is engaged you can drop the trigger group into the recess and work it down keeping it parallel with the housing. The safety catch is then lubricated and inserted with the spring fitted on the catch end. Hold the safety in against the spring pressure and gently manoeuvre the trigger group into position on top of it.

At this point, I feed 2 tapered pins through the locating holes so that they engage the trigger group and align it ready for the long and short retaining pins. I put them in from the side as shown below and just far enough to locate the trigger group and hold it in place. Then you should be able to slightly rock the trigger group while still holding the safety catch to ensure no binding of the safety catch or trigger assembly occurs. I now slide in one pin at the opposite side and lightly tap it through so that the tapered pin is dislodged, and the retaining pin replaces it through the trigger group locating the hole. The safety catch should still have a small amount of movement in it without binding.

Assembling Weihrauch Trigger from Tuning Weihrauchs

Locating the Trigger Group and Safety Catch to ensure that there is no binding prior to installing location pins. Apologies for the crappy image.

Then repeat this with the second retaining pin while still gently rocking the trigger group and safety catch until the tapered pin is replaced. I then drift the pins into place using a small ball pein hammer and pin punch until the pins are equidistant through the housing. You can then press the trigger and the loading catch will be released with an audible ‘click’.

The loading arm can now be cut free of the cable tie and removed, cleaned and assembled with the ‘bear trap’. The loading arm pivot pins are quite often a loose fit and if your air rifle’s pin falls into this category, do the following:

The bear trap and loading arm are refitted so that the loading latch is located correctly and the pivot aligned with the hole. Prior to alignment, lightly oil the pivot pin arms and locating hole.  With the pivot pin clean and dry, insert it through one side of the chamber boss and into the 2 pivot flanges until about 3mm is still protruding from the boss. Put a drop of Loctite on this dry pin and then press it home. Note that the final 3mm of travel must be free of oil or lube so that the Loctite can securely hold the pin against any movement.

Before you engage the loading arm into the end of the barrel detent, make sure that the bear trap is located at the end of the latch, otherwise, there is a better than even chance the loading arm catch may bend the bear trap. At this point, I push the bear trap platform towards the trigger so that the anchor stud can be threaded into the action. While tightening the anchor stud you need to rock the loading arm up and down to ensure that the stud does not crimp the bear trap. The anchor stud has a small spacer under it to give clearance to the bear trap. Once you have tightened the anchor stud, make sure that the bear trap platform moves under the trigger pawl the moment the loading arm is released from the barrel detent. Lightly oil the bear trap once you are satisfied that it is functional.

Note: Never remove and discard the bear trap as the rifle may then be fired (inadvertently) while your fingers are putting a pellet into the barrel. It WILL hurt if it lets go…

Tuning Weihrauch springers: The final assembly

So now we should have the HW77K air rifle assembled and ready to mate with the stock. Wipe down the action with a clean rag will a small amount of oil on it. Make sure that the small nut is still in place at the rear of the trigger group as this has a habit of dropping out of the trigger housing when inverted. Then I suggest that you wipe down the internals of your gun stock to ensure that there is no debris, oil or grease that can migrate onto your clean action.

Assemble the two components together and fit all 4 screws (we are talking about the Weihrauch HW77 and HW97k here) but do NOT tighten them at this stage. Now I hold the rifle vertically with the butt pad on the bench and while pressing down on the barrel, effectively pushing the action rearwards into the stock, I tighten the anchor stud first. Follow this with tightening the two forward stock screws and lastly the small rear trigger group screw. Always undo the screws in the reverse order.

Doing it this way ensures that the rifle action is hard back against the timber/synthetic stock and ultimately puts less strain on the anchor stud through recoil. Do not tighten up the rear screw ahead of the anchor stud as this screw is small and does not bottom out when tightened. The anchor stud is capable of bending the trigger group when tightened if the small crew is, in fact, pulled down first.

If you have experienced your rifle coming lose in the stock due to recoil, what I do is the following:

With all the screws loosened I stand the Weihrauch airgun vertically and measure the distance between the rear of the breech block and the stock with feeler gauges or similar. I then cut a half moon shape out of black plastic (you can use whatever it takes) from a piece of scrap laying around and fit it between the breech block and the stock rear (make it slightly thicker than the opening when under pressure). If you push down on the barrel while being held vertically, you should not be able to pull the plastic spacer free. Then tighten the screws in the order above while downward vertical pressure is applied.

By fitting this small half-moon spacer we are effectively transferring the recoil through the spacer into the stock rather than through the anchor stud that is prone to coming lose with some of the heavy recoiling Weihrauch air rifles.

The Weihrauch air rifle can now be test fired once you are satisfied that the loading arm effectively activates the bear trap and that the safety works correctly. After tuning you should notice a definite reduction in recoil and spring noise that will help with accuracy while putting less pressure on the mechanicals of your scope. Part #4 will cover pellet selection and tuning the Weihrauch HW77 air rifle. Remember, if you have a problem with a Weihrauch Australia wide, not just when tuning Weihrauchs, then please contact me, any time, any day and I will endeavour to assist you.

Author: Ian McIntosh, Gunroom