FWB 124
The FWB 124 I am overhauling.

Part 1
Part 2

History of airguns

  • This report covers:
  • The piston seal
  • Assembly and lubrication
  • Slide the piston into the spring tube
  • Lube the shoe rails
  • Mainspring in
  • How did I do?
  • Summary

Oh boy! These blogs just keep getting more and more fun and today I finish assembling and lubricating the resealed FWB 124 that I did for Glenn who I met at the 2022 Texas airgun show. Then I test its velocity to see how successful things were. Put on the coffeepot and grab a couple donuts — this one will be long and fun.

The piston seal

We ended Part 2 with me telling you that the new piston seal I got from Pyramyd Air was almost the perfect size for the compression chamber. Almost, but not quite. That’s the way you want them, so you can take them down to the perfect size. I sized (ensmalleated/reducified) the seal by rubbing its edge against 400-grit sandpaper. I turned the seal slowly as I went to keep the edge round.

124 piston seal sizing
I sanded the edge of the piston seal against 400-grit sandpaper.

I worked on the seal for about an hour. After 45 minutes I brought out some 240-grit paper because 400-grit was going too slow. The coarser paper sped things up. I kept fitting the seal to the spring tube, knowing I wanted a snug fit that was short of a force fit. You want the piston to stay in place no matter how the spring tube is turned. It should take about 15-25 pounds of force to move it.

Fitting a piston seal is an art, and for bumblers like me there is a lot of luck involved. Fortunately for Glenn it was good luck this time.

124 piston seal fit
When the seal could be pushed into the spring tube with light pressure, the job was done.

Assembly and lubrication

Once the piston seal was the right size, I pushed its bottom end into the piston head. Because the Pyramyd Air seal material is so flexible, it went in in less than a minute. I then lubricated the edge of the piston seal with moly grease and set the piston aside.

124 lubed seal
The sized seal took the moly grease well.

I put a section of paper towel on a 14-inch long half-inch dowel rod and used a rubber band to hold it tight. I then coated the paper towel with moly grease and used the dowel as a long applicator to coat the inside of the compression chamber and spring tube with moly. 

124 moly applicator
Paper towel on the end of a longer (14-inches?) dowel rod makes the perfect applicator for the inside of the compression chamber and spring tube.

Remember — the compression chamber is not a separate part. It’s just the forward end of the spring tube. That’s where you want most of the moly to go. And know this — moly is not a grease. Molybdenum disulfide is a solid particle. It is suspended in grease to get it to the place you want it, but when it gets there the solid particles bond with the metal and don’t rub off. So you aren’t greasing the compression chamber; you are coating it with a dry particle that is about 500 times slipperier than grease.

Don’t use a lot of moly grease in the compression chamber. Try to get a thin, even coat. The moly that’s on the piston seal will help with this. The goal is to have a solid wall of moly for the piston seal to slide on. The piston itself never touches the inside of the compression chamber, just the seal.

You also grease the rest of interior of the spring tube with moly, but the application there is much lighter. The only thing that ever should touch the wall of the spring tube is the slightly swollen end of the piston body.

Slide the piston into the spring tube

Now insert the piston seal into the spring tube and slide the piston inside. It should slide and yet still be tight, as I described above.

124 piston in
Slide the piston into the spring cylinder. Moly the slightly swollen back end of the piston (arrow).

Lube the shoe rails

Okay, those who are careful observers — this is in the last picture, but I didn’t call your attention to it yet. The sides of the cocking slot in the piston are flat “rails” for the sliding “shoe” that connects the cocking link to the piston.  I lubed these rails with moly grease so the shoe will slide along them with very little friction. Let’s look.

124 piston rails
I put moly on the rails (arrows) in the piston body where the cocking shoe rides.

124 cocking shoe
Then I put moly on the sides of the cocking shoe and inserted it into the piston body through the enlarged hole in the rear of the spring tube.

Now we can put the barrel and spring tube back together. Insert the end of the cocking link into the sliding cocking shoe in the piston. Then lubricate the washers on both sides of the base block (the flat block the barrel is pressed into). Remember that the flat washer goes on the left side of the base block and the formed washer goes on the right side. Also put moly into the bolt hole.

124 washers
Put the flat washer in the moly on the left side of the base block and the formed washer in moly on the right side. The moly helps hold the washers in place when the base block is slid into the action forks.

The moly helps hold the washers in place when the base block is slid into the action forks. But even with that you must be prepared to shove the washers back into place, especially the flat one, as the base block enters the action fork. I use a flat-bladed screwdriver for this. When you get the alignment close a screwdriver or pin punch through the bolt hole helps line things up perfectly.

Now coat the outside of the pivot bolt with moly grease and slip it through the hole. Thread it in by hand and when it gets tight use a screwdriver. Then the jam screw goes in the right side. Keep the moly off this one. Snug it down but it’s tiny so don’t overdo it.

Mainspring in

With the barrel attached and the piston connected to the cocking link, it’s time to put the new mainspring in the gun. Now we start lubing the spring with Tune in a Tube, or some other sticky red grease. And it’s also time to lube the piston rod (the central rod inside the piston that grabs the sear when the rifle is cocked). You could have lubed the rod before you put the piston into the spring tube, but I have that neat little grease gun I recently wrote about that allows me to reach in and apply TIAT grease to the rod while the powerplant is holding the piston. It’s cleaner for me and for the rifle that way.

124 lube rod
I’ve reached through the cocking slot in the spring tube and in the piston to put TIAT on the piston rod. The action of the piston will spread the grease.

Now spread red grease on half the mainspring and then push the that half of spring into the piston inside the spring tube. Look at the picture to see how much grease to use.

124 spring half
I used this much TIAT on the first half of the spring. After taking this picture I rubbed the grease evenly all around the coils.

I also put a large glob of TIAT on the open end of the spring before putting it inside the piston. That helped distribute grease to the piston rod.

With the spring halfway into the powerplant where it was held I could now grease the other half.

124 spring complete
While the powerplant holds the front half of the mainspring you can grease the back half. Spread this grease evenly, as well

Now we finish the powerplant assembly. The end cap holds the spring guide that’s coated with TIAT. A big glob of TIAT is put in the hole through the spring guide to get onto the piston rod as it passes through. You HAVE to use a mainspring compressor to get the end cap in the gun. And, as it enters the spring tube, the safety bar and spring are placed in the end cap and the rear of the safety is held down as the mainspring compressor pushes the end cap into the spring tube.

124 safety spring
The safety spring is actually the trigger return spring.

124 safety bar
The safety bar goes on top of the safety spring and then the mainspring compressor pushed the whole assembly into the spring tube. The mainspring isn’t in the gun in this picture or the end cap would be several inches outside the spring tube.

When the end cap threaded hole that’s in front of the trigger blade lines up with the hole in the spring tube, the threaded bushing is screwed in and the powerplant is together. The barreled action is dropped into the stock and the three stock screws are tightened. Then the new breech seal is pressed into place and we are done.

Build a Custom Airgun

How did I do?

I cocked the rifle and fired it into my pellet trap and it fired dead calm. That’s the first success!

Then I tested it with JSB Exact RS pellets, RWS Superdomes and RWS Hobbys. The fastest 124 I ever tuned before this pushed Hobbys out at 838 f.p.s. This one shoots the same pellet at an average 895 f.p.s., making this the fastest 124 I have ever tuned or tested. The low was 885 and the high was 901. This rifle actually broke 900 f.p.s. — something I have never even heard of with a stock 124 mainspring — with aftermarket custom springs, perhaps, but not with the factory spring. The spread was 16 f.p.s. and, at the average velocity, the 7-grain Hobby develops 12.45 foot-pounds. Glenn, you are one lucky boy!

JSB Exact RS pellets averaged 885 f.p.s. with a 7 f.p.s spread from 882 to 889 f.p.s. At the average velocity this 7.33-grain pellet developed 12.75 foot-pounds of energy.

RWS Superdome pellets averaged 810 f.p.s. with a 12 f.p.s. spread from 805 to 817 f.p.s. At the average velocity this 8.3-grain pellets develops 12.09 foot-pounds of energy.


We have just seen an FWB 124 pellet rifle go from not shooting a pellet out the barrel to one that shoots over 900 f.p.s. All it took was a standard mainspring, a new breech seal and a Pyramyd Air piston seal — and many, many hours of careful cleaning.  This is phenomenal! I give most of the credit for this result to the Pyramyd Air piston seal that I was fortunate enough to size so well.

Accuracy comes next. I can’t wait!