FWB 124 low power, and seal questionsOctober 11 2017 at 8:41 AM
|Steve D (Login sd5782)|
I have a pristine San Rafael vintage 124 serial 38XXX that I bought from the original owner in 2010. The factory seal went right away, so I put in a JM Old School kit. This was in my earlier learning stages. The piston seal was very snug. I fitted a new breech seal at the time also.
I never got much velocity: 740 with Exacts and 770 with Express. I have maybe 1000-2000 shots through it now. Currently 730 with AA Fields. I checked the breech seal with tissue, and put the factory one back in with the same results.
After seeing all the posts of folks getting 800+, I opened it back up today. Actually, the tight piston seal wore in nicely, and is now just "snug", but no more so than most of my other guns.
OK, here is the question: When looking into the receiver tube, the face does not appear to be flat. There appears to be a raised center "plateau" that is perhaps dime size. It seems that the piston seal bottoms out on this center part and the outer lip of the seal never hits bottom. Indeed, my seal only shows signs of wear in the center part, and seems as if the lip never bottoms out.
I did get a long metal piece to feel along the face of the receiver, and it does seem that the center part is raised perhaps .010"-.015". Is this how this gun is made? No 'F" marks on gun, just standard Beeman marks. It would seem like to me that I am having lost air volume at this spot, and perhaps machining the center face of the piston seal down a bit would be the cure. I know that currently "cup" style seals do exactly that.
Low power and seal question
|October 11 2017, 9:29 AM |
Piece of old seal stuck to front of receiver maybe ? ?
No seal remnants
|October 11 2017, 4:00 PM |
Nope, no seal debris in there
Three Things (Edited)
|October 11 2017, 3:49 PM |
1. Your current velocity isn't really that much off the mark for other 124s out there, and it's good to keep in mind that there is some variance from one rifle to the next of the same make and model number. I'm trying to say that you could spend a lot of time chasing that last 30 fps and never get it. So, what's it worth to you? Is the lack of that 30 fps. keeping you from doing something you need to do? (Edit: Oops. It looks like it could actually be more like 60+ fps. lower than some others, so I can see your point. I misread it the first time--sorry).
2. What Mark said about some remaining old piston seal debris is often right on the money when it comes to the 124. Of all the air guns on the market, the 124 is notorious for having a piston seal that disintegrated and cluttered-up the compression chamber when it went bad. Seeing it, then removing it can be a daunting task. When I tuned a 124, almost all of them had a ring of old seal debris at the end of the compression chamber closest to the breech. The beige color of the factory seal material tended to blend in with the color of the chamber. Even using a Mini-Mag flashlight (which will nicely fit inside the tube), you might not see the debris that needs removed to get complete piston travel and thus full velocity. Sometimes I had to use one of those little fiber optic attachments on the Mini-Mag light (see link below) to focus all around the perimeter of the chamber to see what was there (or not there). Then it took using the right tools to get into the edge of the chamber and remove the debris, even when I saw it. As long as you have the rifle taken apart again anyway, I suggest you look for any such debris and remove it if it's present.
3. Your question about removing that little hump on the face of the seal has some merit as it pertains to velocity, but probably not by a lot. You can sand it to a flat surface and probably pick up a few fps. Just keep in mind that the original design of the seal (with the hump) might have added a little cushion to the shot cycle as the piston landed on the chamber wall at the end of its travel. How much? I don't know. I can tell you that the originally-designed piston seal for the R1 had the same hump on it, and when it was removed the velocity of the rifle increased but the shot cycle didn't become anymore harsh to a noticeable degree. Now, as far as to going further and cupping or concave-ing the face of the seal as you mentioned, you may be in some uncharted waters on the 124 with that. The piston seal on the 124 has a slotted-parachute design versus a non-slotted-parachute design and that could make a difference. I don't want to type all of the negative possibilities, but although it works well on some other air rifles, there could be some negative effects and you might actually lose some velocity and need to order another seal and start over.
I hope that helps. Best wishes on your project.
Safe and Happy Shooting!
Ed Krzynowek, The Airgun Tune-Meister
"We can rebuild the squirrel. Make him stronger, faster...We have the technology"---Skyler M.
|This message has been edited by ekmeister1 on Oct 12, 2017 11:17 AM|
|October 11 2017, 4:02 PM |
Thanks Ed, I will keep in mind
Twenty years ago Tom Gaylord removed old crumbled 124 seals...
|October 11 2017, 8:46 PM |
Scratches on breech face
|October 11 2017, 4:20 PM |
On closer examination, I notice some scratches on the breech face. I will have to check to see if that is causing the breech seal to not do it’s job
|October 11 2017, 7:42 PM |
730 with AAs is ~10 fpe, and yes, that's pitiful. No other way to put it.
The last JM spring I measured was 37 coils, 0.128 wire diameter. That boiled down to a 4.74" compressed length; not much, if any room left to space it up. That spring produced a titch over 12 fpe.
I have also run 33 coil springs (also .128 wire, a common diameter)that perform in the 12- 13 fpe range. I did not note whether I had spaced up the spring, but expect I would have.
I want to say the "Old School" kit was billed as "kitchen table installation", that is, much shorter spring than the stock unit. If you have room to do so, try spacing up. That will usually coax out more power. You've got at least 4.74" to fill.
By what you've written I expect you knew not to over goober the innards with tar -- that'll suck the life right out of 'er!
|October 11 2017, 10:03 PM |
This kit came spaced up to nearly max. It is one of his "older" kits and has 29 coils of .128 wire and a delrin spacer of 1.16". Maxed out at 4.87. The easier installation is with the spacer rather than compressing more coils.
On another gun, I once thought the JM spring had weakened, when in fact the leather to synthetic piston head had loosened and unscrewed and caused a shorter stroke. I don't doubt the JM spring in this one is more than fine also.
I just took .020" off the seal face where the center was protruding a bit, it is now flat or perhaps .005" concave. The seal still had the annular mold lines around the outer ring, so it was obvious it never bottomed out except in the center.
As you said 10fpe is not great in this gun. I like the 8-12 fpe guns, but unless I make an effort to detune something, I like to know that my guns are performing properly, and THEN I can do what I wish. And yes, no loads of tar. In fact, I almost never use it in anything now.
As far as "tar" is concerned, I stopped using the stuff years ago.
|October 14 2017, 8:56 AM |
I found that if the spring guide is "properly tight" there is no need for tar, but if I used tar to "de-twang" a "twanger" the excess tar required to do so also made the poi very "temperature sensitive". I found that using molly paste for all internals was just fine if the spring guide was a "tight fitter" and the poi didn't shift as badly with changing temperature as when the spring was slathered with "tar". Now I only used Dupont Krytox GPL205 for internal lubing but prior to that I found molly paste to work OK with careful application.
Short tube Dianas
|October 11 2017, 10:31 PM |
Just an aside to anyone interested and to hijack my own thread: the first year "short tube" variants of the Diana 34 series guns had the same 28X77 B&S measurements as the FWB 124. Perhaps they thought that was a good starting point. The early Dianas however, didn't make the same power. I think maybe it had to do with their transfer port which was long, bent, and quite large in diameter. That being said, one of these detuned to 10.5fpe is VERY pleasant.
|October 12 2017, 8:42 AM |
Lessen learned. I guess the 124 needs the domed seal. Cutting .020" off the center to make the face flat LOST 30 fps. Now down to 700 with AA fields. Very smooth though. Nothing else was altered. It looks like time for a new seal, although first I will machine off .030 from the outer sealing lip to get back where I started from and retry. As an aside, I guess I have a hard time learning that the original designers MUST have learned the same lessons too.
ARH generally has 2 seals ...
|October 12 2017, 11:46 PM |
A standard sized one and oversize ....
No clue what was installed originally during rebuild, but if TIGHT, be sure to order the standard as your replacement.
I Had a Bear..
|October 13 2017, 7:31 PM |
of a time with my 124D getting the breech seal worked out.The JM seal was a pain to stuff in the hole,but my breecn face had deformed surface.I used emery on a popcicle stick to get the machine marks out,then touched up the blue.
Likely you`ve checked this,but just to mention.
Mine does 830 with JSB lights,800 CPL.Don`t remember which kit I used,but it was the one for dummies that you really had to compress to get in there.
|October 13 2017, 9:34 PM |
I had a very hard time with the new JM breech seal years ago chasing around a vertical zero. My breech face had some tooling/scratches that I have now sanded also. I think it is sealing well, but I will keep a focus there as the breech face isn't perfect. I probably ruined my piston seal and will get another. In the meantime, I took .025" off the outer lip to match what I took off the face. May as well try to get it back where it was. I also have been trying to size the snug diameter down too. I have never had much luck with that. One thing about a semi-ruined seal is that more experimenting won't hurt it any more. I would like to see 11-11.5fpe at least with this gun.Thanks
I Can`t tell you..
|October 14 2017, 7:19 PM |
how many times a new idea lost another 30fps on an already slow gun.
I also had an older,short tube Diana 36,that was shooting cpl 780fps when I bought it used,I`ll fix this I thought.I took out the original almost chrome looking spring,installed a piston seal&spring,still 780fps.Then I noticed the shorter tube.
Bought a new M34 Compact,that was hard to find.it would only do 730fps.
Problem was a slightly small tube,fitted a seal for it.So many times a factory Diana seal is so snug,but down sizes it`s self in a short time,not with this gun,or I`d had it fixed years earlier,now 830 fps,most 177 cal Diana`s do 860fps cpl here@ 2500' elevation.
Let me know the end of your story!!
|October 14 2017, 8:05 PM |
Thanks Dizz. I have done that many times myself. An HW35 being a very confounding gun too. On the 124, it isn't that I really have to have the extra fps, but it just bugs me knowing that it isn't what it should be. I have many nice springers, and I truly enjoy tinkering with them nearly as much as shooting them. Many I detune, so I guess I am nuts asking for more on the FWB.
I found your observations on the breech face to be interesting, as mine looks like someone went to it with a dental pick. I think I will try the powder trick to see how it seals. I used a bit of sandpaper on it. I am trying to size my piston seal for a less snug fit first. On my short tube 36 I really like the shot cycle with the JM HW50 spring. 750fps, but Oh so nice feeling.
It`s Been 10 years..
|October 15 2017, 8:26 AM |
or more ago I had that 36.I was not able to find a pellet it liked,so I sold it.To date the only Diana(except the 177 cal 12 land Chinese barrel Diana`s)that gave me serious accuracy problems,I sold it.
I`ve not in the past tuned guns for less power,and sold my R-7,I like them hot,but no overly harsh.
I think Dizz is right
|October 15 2017, 3:58 PM |
Dizz, I think you are correct. I have been playing around with shimming the stock seal, and also the JM replacement seal. It fails the tissue test with whatever I do. The breech face does indeed have scratches and machine marks. They really don't look like much, but it must be enough to blow past when shooting. With spring out, and pushing on piston with finger to a tool and other finger over end of barrel, it seals. I guess that is lower pressure than when shooting however. I will try your idea of sanding breech face with emery paper to get it smooth.