Feinwerkbau Sport air rifle: Part 7
by Tom Gaylord
Writing as B.B. Pelletier
This report covers:
• Disassembling the Sport
• Spring guide was loose
• Remove the piston
• Piston comes out
• Mainspring tube/compression chamber finish
Okay, today the Feinwerkbau Sport air rifle comes apart, and we’ll start looking inside. This report is huge, so it will take today and tomorrow to complete.
Some of you might like to compare what you see in the Sport to the FWB 124. That can be seen in the 15-part report I did on the FWB 124.
Okay, enough explanation. This is what you’ve been waiting for, so let’s get to it!
Disassembling the Sport
First, the barreled action came out of the stock. FWB now uses Torx fasteners all around; and although I’m one of those dinosaurs who doesn’t like when technology advances and forces me to buy new tools, in this case it was the right move. I’ve stripped both Allen screw heads and Allen wrenches in the past when working on airguns, and the Torx seems to put an end to that. Three screws out, and the stock came off.
Now, I could see the gear oil that was still oozing from the action. If nothing else, today was going to get rid of all of that so I didn’t have to put a pan under the rifle like it was a 1949 Harley panhead (an old American motorcycle that leaked oil all the time)!
Fortunately, Feinwerkbau finished the inside of the stock wood as well as the outside, so no oil got into the wood. That’s something you never see on a breakbarrel!
Next, I placed the action into my mainspring compressor and put some tension on the trigger block. The rear of the trigger assembly block has a flat that is perfect for pressing against with a compressor, so I used a wood block to press on it. That relaxed the tension on the crosspin that anchors the trigger unit (keeping it from rotating in the spring tube) and also the bolt that holds the trigger unit in the spring tube.
With the bolt out, you can relax tension on the trigger assembly block. The mainspring will push it back out of the spring tube. The FWB 124 has a huge amount of spring tension on the trigger block and comes out several inches, but the Sport only comes out about 1.50 inches.
The trigger assembly (which is contained inside the trigger block), the mainspring and the spring guide can all be taken out of the spring tube. I note that the gear oil I put in the gun has diluted some moly grease that was inside the gun to the extent that it’s impossible to tell exactly how Feinwerkbau had lubricated their gun. There was moly-filled oil everywhere! But moly seemed to be all they used; and with the tolerances I found, it wasn’t enough.
Spring guide was loose
The spring guide was very loose on the mainspring, and I tried both ends of the spring. Neither one made it better. The mainspring is a thick wire that’s not very long. This is a complete reversal of the 124’s spring, which is made from thin wire and is very long. So, the Sport’s cocking stroke is short, and the effort to cock is higher than the 124’s. And the rifle is also more powerful.
I don’t have a lathe, so I’m not going to machine a new spring guide, but that would be part of a real tune. I saw that the mainspring is perfectly straight, which means it’s doing as well as can be expected.
Remove the piston
It’s time to take out the piston from the spring tube. To do that, the barrel has to be separated from the spring tube. This is when I really came to like those Torx fasteners. The pivot bolt on the Sport is hollow with petals at the threaded end. A screw with a tapered head locks the pivot bolt in place by spreading the petals of the bolt as it is tightened. First you remove the locking screw, then the pivot bolt is removed.
After removing the locking screw, the pivot bolt can be removed. The pivot bolt has a Torx head, making is easy to control and difficult to strip. This will come in handy during assembly when it’s time to tension the pivot joint again.
The pivot bolt petals are spread by the beveled head of the locking screw. This gives you superior control over the tightness of the pivot joint. This level of engineering is not found on most spring guns today.
Once the barrel is separated from the spring tube, you can see the baseblock washers, which act as bearings. These were correctly lubed with moly grease, just as I would do when tuning the rifle. The washer/bearings are shaped differently, so there’s no danger of swapping them to the opposite side during assembly.
Now that the barrel is out of the action, the piston disconnects from the cocking link at the cocking shoe and slides out. As you slide out the piston, the cocking shoe has to be lifted out of an enlarged opening in the cocking slot of the spring tube.
The cocking shoe on this Sport is very well machined and smooth on all surfaces. Where a 124 benefits from stoning the cocking slot and the piston slot that the cocking shoe rides in, the Sport needs nothing — it’s already as smooth as can be.
Piston comes out
Once the cocking shoe is out of the piston, the piston slides out of the spring tube. The 124 piston was machined, but it was left a little rough. This piston is a jewel! The discoloration is from heat treatment.
I noted that the piston does fit the spring tube/compression chamber very well, though not overly tight. A top tune would probably button the piston (put bearings around the piston body, front and rear, to keep it away from the spring tube walls), but it wouldn’t be absolutely necessary for most shooters. The piston seal is the parachute style, and its free to rotate freely on the piston. It may not be as tight as it could be in the compression chamber, but only in a top tune would exchange it.
Inside the piston is a central rod that connects the piston to the sear when the gun is cocked. It also serves as a spring guide. At its front, there’s a short, bevelled swelling that serves as a forward spring guide. The mainspring does fit this swelling rather well, though not absolutely tight. Anything short of a master tune can leave this as is.
The rear of the piston rod is hook-shaped to grab the sear when the rifle is cocked. You will want to lube this hook with moly during assembly, but I will lube the entire piston rod, as well, since the inside of the mainspring will contact it at times.
Mainspring tube/compression chamber finish
The gear oil was coating the walls of the spring tube, so the first job was to dry the inside of the gun. I did that by stuffing a shop rag into the tube as far as it would go. It absorbed all the excess oil With that done, I could inspect the finish of the tube walls.
The inside of the mainspring tube/compression chamber is beautiful! I’ve never seen a spring rifle that was finished this well, though a vintage Walther LGV target rifle from the 1970s does come close. The FWB 124 does have smooth walls, as well, but there are often machining marks on them. This tube was completely smooth!
Leave your brake hones on the bench, boys — you cannot improve on this! It’s not a mirror finish, either, which would be detrimental. There are enough micro-hone marks for lubricant to grab, yet the finish is ice-rink smooth. I would have taken a photo, but have you ever seen a picture of the surface of a mirror? There’s no definition — nothing to be seen.
I’m going to end this report here, because it has already become too long. I’ll return to finish the job tomorrow, and that will include testing the rifle with the same pellets that were used to test it in factory trim back in Part 2.