Author Topic: SOS ,I don't know what i'm talking about.  (Read 4074 times)

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Offline seagate

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SOS ,I don't know what i'm talking about.
« on: August 09, 2016, 08:23:38 PM »
So brave boys, I'm a bit puzzled.
 

 My boinger hw177 was running sweet as a nut , still is but I notice something that doesn't seem logical ..

 Some numbers to start with.

 7.3  804
 7.9  778
 8.4  748  all these pellets are producing 10.7 near enough. Just lovely.

 Out of curiosity I popped in a spring shim ,2mm maybe.

 7.3  807 *
 7.9  807
 8.4  776   the 2 heavier pellets are 11.5 now, but the falcons haven't gone up at all.

 Also when I take the trigger block out and put my finger over the tp, I can easily push the piston down ,where as on my other hw boinger ,yes the piston can still be pushed down but it's much harder.

 Question being, could this be piston seal related.What do you think?
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Offline landymick

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Re: SOS ,I don't know what i'm talking about.
« Reply #1 on: August 09, 2016, 08:54:41 PM »
Try a new or different seal to check it out.

2 mm preload extra is not a lot but clearly has made a difference,i prefer springs with active coils rather than the washers but not always possible to get the power out put that you desire with out some .

Main thing is that it groups nice & not worry too much on FPS .

Offline Gambo

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Re: SOS ,I don't know what i'm talking about.
« Reply #2 on: August 09, 2016, 09:00:47 PM »
Yes, I'd hazard a guess the piston seal could be the culprit.....but it could also be coz the 7.3 grain pellet is a tighter fit in the bore than the others, and so the extra spring power isn't being transferred to as easily to it, as it is to the looser fitting ammo, which moves earlier.


For what it's worth, I always set a rifle up only ever using FPS as the benchmark, as that measurement is a constant, no matter weight pellets you are using/testing. But if you use a ftlb measurement instead, it's very difficult to directly compare results from different weight pellets, as 1/2ftlb increase from one weight of pellet can be totally different to a 1/2ftlb increase from another.

In short, FPS is FPS....it doesn't change, where as ftlb changes dependent on pellet weight.

It's far less confusing, and is the measurement used by the vast majority of tuners. ;)

Offline seagate

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Re: SOS ,I don't know what i'm talking about.
« Reply #3 on: August 09, 2016, 09:32:30 PM »
The 3 pellets I tested was for a reason, as I know from past form the fps difference between them. If all was well, the 7.3 would be around 840fps.

 These 3 pellets have always sat virtually the same power levels with hw springs.

 So a new seal it is. Quite surprised that this seal hasn't lasted long where as in my hw 22 it's been used  50x or more as much.
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Offline Gambo

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Re: SOS ,I don't know what i'm talking about.
« Reply #4 on: August 09, 2016, 10:12:53 PM »
Is that 7.3grn pellet a looser fit in the breech than the others?

Offline seagate

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Re: SOS ,I don't know what i'm talking about.
« Reply #5 on: August 10, 2016, 07:00:20 PM »
 I think they're all roughly the same, snug but not super snug.  Also tried some hobbies and hnftt green, and none of them are cracking the flags velocity wise.

 

 Done abit of  piston swapping  - the seal is at fault.  Under the magnifying glass it looks like the spru hasn't been removed cleanly compared to other hw seals and protrudes a few thou, that may cause the seal to ''hold off'' the cylinder wall. Also the lip has a few marks which I've dressed out. So when hand testing now the piston offers much more resistence.
 Should have it rebuilt shortly so will know how effective the seal titivation has been.
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Offline Gambo

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Re: SOS ,I don't know what i'm talking about.
« Reply #6 on: August 10, 2016, 07:54:24 PM »
Please keep us informed Mucker. ;)

Offline seagate

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Re: SOS ,I don't know what i'm talking about.
« Reply #7 on: August 10, 2016, 09:39:12 PM »
Alls well.

 The seal has initially responded well to a bit of tlc.

  Soft jsb pellets used as usual.
7.3 808 10.6
7.9 785 10.8 
8.4 758 10.7
 
 This coming weekend i'll give it a whirl with a slightly lighter piston and stronger spring to see if the relationship between pellet weights changes as it theoretically should.   
 
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Offline Gambo

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Re: SOS ,I don't know what i'm talking about.
« Reply #8 on: August 11, 2016, 06:55:43 PM »
Now there speaks a man with enthusiasm. 8)

Offline seagate

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Re: SOS ,I don't know what i'm talking about.
« Reply #9 on: August 12, 2016, 10:09:03 PM »
Just a bit of backgrounf info.

 I 've searched out  a couple of in depth hw177 98  reviews ,and looking at there pellet info  regarding jsb pellets and power. It seems generally that 7.9 make the most power, followed by aa field 8.4 then the falcon 7.3. Which mirrors my findings for my rifle in standard trim. Reading bits and bobs from some leading lights in the fiddling world,it appears the piston and liner weigh 270grammes. Also some have piston weights like my 95 22 did, but some don't like my later 177 didn't. So a bit counter intuitive to my mind, I'd have thought the 177 would have a piston weight to get the power up a little. But it just might be down to the vaguerys of the springs at hand. Anyway enough of all that.

 So what I've got to experiment with is  a piston freshly lightened by 20 grammes ,admitedly a miniscule amount.
 As I understand it, the power will go down a little on all 3 pellets , but in theory the 7.3 should go down a little less quid pro quo than the  8.4.  That's with the proviso that the piston wasn't particularly slammy with certain pellets and bouncey with others. I'l start chronoing after half a tin has been shot to let it all settle down abit after this evening rebuild with light piston. Should have some figures tomorrow evening. I'm not expecting figures to jump out but looking for minor/subtle changes in averages.
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