What do you guys think about the change in readings?

I just replaced the stock spring and tophat of my wifes Gamo 890S with a crosman spring and custom brass tophat.  I cleaned and deburred everything, polished the piston and replaced the piston seal, polishied the spring ends, and added spring tar to the spring.  Lubrication was applied sparingly.   After putting everything back together I went out and shot it a few times.  Overall the gun felt great, twang was gone, and gun felt alot more solid during the firing cycle.  I was pleased, at least up until I did my chrony readings.  I used RWS Superdomes for testing purposes. Here are the before and after readings:

                                     AVERAGE (15 shot string)

                                   BEFORE................ AFTER
                                   867.58..................766.93

                                                HIGH/LOW
                                  BEFORE..................AFTER
                               880.6/855.1...........780.0/754.2          
                                   (25.5).....................(25.8 )

So why do you think the readings (post tune) are so low??
My only guesses are:
1) the spring isn't as strong as the stock spring?
2) the spring fits very tightly to the sping guide...maybe too tight?
3) the new seal is slowing the piston due to it's tight fit?

Just curious as to what you guys think
Thanks

[DanoInTx]

Is the new spring the same wire OD and the same amount of coils etc?  That would be my first guess, but I'm sure others will chime in too.  Does the new custom top hat have the same amount of brim, maybe you lost a little spring spacing?  And then the obvious other thing, was the air chamber fitting well with the new piston seal?

Dan

[daved]

Hey, Mark,

Is this gun new?  Reason I ask is, your starting numbers seem a little low to me.  Typical speed with Superdomes in the Shadow/Hunter powerplant is usually around 900 fps.  What's really interesting to me is that the spread stayed almost exactly the same.  Based on that, I think I'd suspect the spring first.  But my logic could definitely be suspect :-)!  Seems odd though, I seem to recall Rich saying that the Crosman spring is slightly stronger than the Gamo.  Maybe he'll have a better idea.  Good luck!

Dave

Almost sounds like you nicked the seal installing the piston. The Crossman spring is a little smaller in diameter than the Gamo spring but is a little stronger.

If everything was not over lubed it won't slow down the fps. Like I said sounds like a seal problem.
The seal is a pain to get in with out nicking it, you have to take your time to make sure it don't hang up on any of the slots and edges of the holes in the compression tube.

[vinceb]

There's a similar spring available for another Quest model that has less power - and there's at least one instance I know of where someone ordered and got this weaker spring by mistake. You should have part# C1K77-010.

This spring has a couple of coils more than the Gamo spring but a slightly smaller overall diameter, to give it roughly the same spring rate if not a little more. There's a spreadsheet in the library called "Macarri Spring Chart" that lists the dimensions of several springs, not just Macarri's. When I put that chart together, the Crosman spring I measured seemed to have a slightly thicker wire than the Gamo spring I had measured, however subsequent comparisons showed them to be virtually identical. So maybe I happened to pick up a Gamo on the "low" side and a Crosman on the "high" side.

Regardless, the springs are very comparable. The Crosman spring (because of its greater stacked height - more coils, remember) requires a shorter tophat - if you use a typical Gamo hat the gun probably won't cock! In any case, your preload shouldn't be an issue - the Crosman hat is so thin that the custom hat you use just simply cannot be much thinner.

Also remember that any spring will "set" (get a little shorter) the first time you cock it... and these springs will tend to shorten up on the order of 1/2 to 3/4 inch. If you started out with a little less preload because of a super-thin tophat, all that I think is gonna happen is that the spring will set a little longer (because it isn't being squished as much), and you'd wind up with the same amount of preload.

The spring is a tight fit on the stock guide, that's why it doesn't twang. But remember that the spring grows in diameter when it is compressed - so it probably isn't binding on the guide during the most important part of the compression stroke.


So anyway, I have 2 questions for you:

1) Did the gun diesel noticeably when you first shot it? Even a couple of diesel shots can crunch a spring - had it happen to me a bunch and make it lose 40-50fps. But you seem to have lost much more than that.

2) When you reassembled the gun, did you debur the inside of all the holes and slots that in the compression tube? Did you dress the inside edges with a small file or a dremel tool? If not, you might have cut up the seal when you reinstalled it.

As a first step, I'd strongly recommend pulling the mainspring out and measuring its length (now that it's been shot). It should be around 10 3/4" long. If it's significantly shorter than that, you've either got a wrong spring or a bad one.

Let's see,
I debured the chamber and slots with a small file then finished the beveled edges with sandpaper so that they weren't as sharp.  I used a dremel on the cocking slot of the piston itself and finished with sandpaper also.  I was very careful installing the piston into the chamber but noted that it was extremely tight in the chamber all the way in.  On this point, I've only installed one other piston seal ( JM seal) in my life, and that was in my Shadow.  It was tight, but not nearly as tight as this one (crosman seal) was.  This gun is about 2 1/2 years old so thats why the numbers were a little low at the outset.  As for the spring itself all I know is that it is a crosman spring and was marked with yellow paint (not sure what power designation that would correspond to).  When I first shot it, I used a couple of heavy (Beeman Kodiak) pellets and there wasn't any dieseling that I could tell.

[DanoInTx]

could the seal be so tight that it's riding inside the air tube more like a taco than a dinner plate?  Just a thought.  The few, and I mean very few springer seals that I've done all seemed to slide smooth, but still hold air.  Of course they were a bit hard to inititially get into the tube, but once in there was a good tight, but smooth friction when moving the piston in and out of the tube.  I even machined up some seals from Delrin for my Fast Deer and they rode pretty smooth inside the air chamber bore.  Sounds like maybe it's a little tight?

Dan

The seal is just a little tighter than a JM seal but I never have had any problem with them, they are in all but two of my gamo's. If the seal was lubed properly it will get better as you shoot it.
You did put a film of moly behind the seal before you installed the piston didn't you?

I just did a Quest tune for a customer and I installed a new crossman seal and a JM spring and the first shots were in the 820's after half a tin of playing with it I just cronyed it and the averege was 924 fps. It's getting packed up tomorrow and shippd back to it's owner.

[vinceb]

I've bought about 7 or 8 Crosman springs over the past several months, and NONE of them were marked with any kind of paint or dye. I strongly suggest yanking it out and measuring it, counting the coils, and checking the wire diameter.

How's the before/after cocking effort? Noticeably different?

As for the seal, the Apex seal from JM is softer and will therefore install easier. Another poster asked if you lubed the seal before installation, may we assume that you did? Also, frequently one needs to use a small, blunt instrument to push in the seal lip to get it past some of the wider opennings, even if they were properly deburred.

[vinceb]

...so far? Or is it too early to tell?

I mark the springs with yellow paint so I can find them in the box of springs !!!!!!!!

I was very careful installing the piston and new seal in the chamber....I'd be down right amazed it it had gotten cut...I was that careful. But I have to say again, that puppy was TIGHT going in.   I did apply a small amount of moly behind the seal as well as a very  very thin coat of silicone grease to the piston itself.   When I first removed it there was a little discoloration in a couple of small areas on the interior of the piston that looked like corrosion, I wanted to make sure that didn't happen again.  I'm going to shoot it again for about 50 - 100 shots and try and chrony it again.  I'm sort of hoping that the seal needs "working in" a little.  If the readings are the same after shooting it some more, I'll open it back up and do some measurements on the spring.

It's a Quest spring and it has 34 coils and the wire size is.119

Thanks Rich.
Do you think that the seal could still be cut, or is it possible that it fits so tightly that it is slowing the spring down thereby losing velocity??

Took it apart again to check everything (I used Rich's spring compressor BTW..worked great!).  Everything, including the seal looked fine.  I did notice one thing though, when re-inserting the seal into the chamber this time, it was no where near as tight as the first time.  After assembling it again I took it out and shot a few pellets to make sure it was working.  I then got out my chrony and noticed that the readings have gone up a bit:
LO  813.7
High 824.6
Ave 820.3
max spread 10.9
These readings were obtained using RWS Superdomes also.
Even if the readings don't get as high as pre-tune, I'm really liking that the max spread has cut to more than half of what it was....before around 25 fps, now about 11 fps!!
I'll check it again after another 200 pellets or so.
Thanks again everyone for your help.