How to DIY tune a HW77K spring airgun: Part 2
If you have followed Part 1 of Tuning a Weihrauch HW77K air rifle at home, you should by now have your Weihrauch HW77K or another air rifle completely stripped down with the exception of the trigger group that remains assembled.
At this point, we need to de-grease the entire collection of air rifle parts, inspect them for wear and damage and replace anything as a result. You will need to carefully prise the piston seal off if you are going to keep it, however, if you are going to replace it then you can cut it off.
Caution: The Weihrauch HW77K /HW77/HW97K have 2 piston sizes, one is 25mm up to serial #1446048 with the 26mm piston seal fitting all rifles with the serial #1446049 and above.
Identifying the Air Rifle Components for Polishing
The HW77K Air Rifle trigger group sits in what’s commonly called the Back Block which forms part of the barrel and action and is not sold separately. You will see that the end facing forward that faces the spring end is rather crudely machined, see below:
This face is the first part that I polish, by using a Fine Buff and the Autosol Metal Polish. Be careful to keep the Buff on the face and do not let it stray onto the thread leading off from the face as this will wear rapidly under the Buff friction and polish. Basically, we are improving the Weihrauch HW77K’s parts by reducing friction and make it air rifle load and fire more smoothly. It works, period.
If your back block face has heavy machine marks, then I would advocate doing the initial work on some oiled Wet & Dry 400 Grit Carborundum paper on a sheet of glass to keep the face edge flat. Once you have taken off the machine marks for the most part, then continue with the Buff.
Finish the Buffing with the Ultra Fine Buff with NO metal polish, just aim for a high lustre.
The idea is to polish the face and not grind the crap out of it. You will likely still get cutter marks (aka Tram Lines) on the face as seen on the image below but as long as the face is polished these marks are of no concern. See below:
While you can’t see down the bore of this back block, I did run a light Carborundum paper in a slotted brass post down it and took out the worst of the drilling marks.
Next, I focus on the air rifle cylinder and start by de-burring the loading arm trap at the end of the cylinder. Failure to de-burr can result in a small nick to the piston seal when you are assembling the unit. I use a Dremel with a small stone (2mm) and come in from above onto the rectangular slot as seen in the image below.
If you mark the bore at the end of the cylinder where the loading arm slot is, that should not pose any problems as the piston seal does NOT come back that far that it would ride over any grind marks in this area. All the same, care should be taken.
If you shake like a Granny at a Christening then it would pay for you to put some masking tape from the opening in as far as the loading arm slot. This way as you shake your way through de-burring, you are less likely to penetrate the tape and damage the cylinder bore.
You can see the de-burred loading arm slot in this image below:
From here I move to polish the air rifle cylinder outer using a Fine Buff and Autosol Polish. I rotate the cylinder slowly while keeping the Fine Buff working a small area of metal polish. The idea is to either run up and down polishing it and slowly rotating as you go, or polish around the cylinder and work your way from one end to the other. Your choice.
Once you can see the machine marks are on their way out, swap over to the Ultra Fine Buff with no polish and buff to a lustre. Keep an eye on how much metal you are removing, because in fact that is what is happening, be it slowly. I would suggest that you do the buffing very slowly and take care not to mark the cylinder with some careless buffing or by dropping a hot cylinder. That is correct, you can expect the cylinder to get pretty hot so I also suggest gloves and glasses to protect yourself further.
Your cylinder should end up looking like this on the left of the piston:
Note: Do NOT polish the inside of the cylinder as it has hone marks in it to trap lubricant so just leave it be after cleaning.
I then polish the piston (without the seal fitted…) and the release catch the same way as we have polished the cylinder. Below I have 3 images showing the Cylinder, Piston and Back Block polished:
When polishing the catch on the end of the piston, do NOT overdo this and end up changing the lug’s leading edge profile, you only have to polish it using metal polish and buff it with the Ultra Fine Buff with no polish.
Once you have got this far, you might be wise you lightly oil these components as humidity or damp fingers can leave unsightly marks and even slight rust stains.
Next, we need to focus on the spring and you need to consider swapping it out if the spring has a lateral bend in it, is worn (inconsistent gaps between coils) or has done a lot of work. Of course, if you are fitting a Vortek kit then you would be replacing the spring, but it still needs work as follows:
The spring on the left has been removed from a new HW77K and has been de-greased.
You will notice the grind marks on the spring end and the sharp edges of the spring.
Firstly I would polish the spring ends on a Carborundum paper on a sheet of thick glass with oil to take out the worst grind marks. See following image below.
Once you have done this then the sharp edges need de-burring with a Dremel or similar: don’t get carried away, just take off the sharp edges.
Then it is time to buff the spring but take care that the buff does not catch in the spring ends as it can tear out of your hands easily.
Once the grind marks have been polished on the Carborundum paper and the edges de-burred, polish the spring ends to a lustre as below:
Repeat this with both ends of the spring and use your fingers to check for sharp edges by rubbing them into the ends of the spring. Any sharp edge, stone it and polish it out.
Next comes the Spring Cushion Guide which has a finish as below left before polishing on the right:
You will note that even after polishing the spring cushion still has machine marks that are OK as long as they are polished.
So if you have come this far, you should have a collection of parts as in the above image, cleaned, highly polished and lightly lubricated until you are ready to reassemble the air rifle.
Tuning the HW77K: Summary of Polishing the Air Rifle Components
While this article is based on the tuning the HW77K, the components for the HW77 and the HW97K are exactly the same. However, if you have another model of Weihrauch Air Rifle or a different make of spring powered air rifle, the process is still the same. You may or may not have a separate cylinder like the HW77K as the piston may run inside the air rifle outer action, or you may have a side loading action, break barrel or whatever, it is still the same process.
You need to focus on cleanliness when stripping an air rifle and then going to these extremes to polish up the components. Your working environment needs to be clean and have a soft cotton cloth (preferably) base to put the parts on to minimise scratching or marking.
The next article is Part 3 and I will cover assembling the air rifle, lubricating it and testing it through a Chronograph. The last article on tuning the Weihrauch HW77K, Part 4, will cover pellet selection and tuning using pellet types in your air rifle.
Author: Ian McIntosh, Gunroom