Revisit: striker weight only.

[Ribbonstone]

Gave some advice about lightening a Crosman striker. Realized that it had been some time between playing with striker weight and when I did, tended to NOT keep that weight change in isolation.  Just kept fiddling with other things while I had the airgun apart.

Wondered what the results would be if I kept ONLY striker weight as the variable, changing nothing else (something I seem to have failed to do in the past; always fiddling with a couple of other things as well).

Not really wanting to retune the Discovery (as I’m happy with it), looked for something “kind-a-like” a Discovery.  My old beat up HiPac converted 2260 carbine came to mind as its current setting is a little bit of a gas hog.



That would pit a 72cc air tube against a 135cc air tube, so I’d have to expect a shorter shot count, but the basic guts of the system running the carbine is about the same as a Discovery.

Had already lighted the original striker a little bit (to 53 grams).  Got on the bench grinder and took the striker down to 39 grams. BUT, I didn’t change anything else.



In this carbine’s case, lightening the striker by 26% and not changing anything else did the following:

1.   Increased efficiency (in this case, by about 30%)
2.   Decreased energy per shot (about 12%)
3.   Increased shot count  (by about 22%)
4.   Changed the top fill pressure from 2400 to 2200 PSI and the bottom pressure from 1500 to 1400psi.

That’s pretty much what I expect a lighter striker to do.  Lower energy, increase efficiency, increase shot count, and run well over a narrower pressure range (Although in this case, was not as much narrower than I expected).

Normally at this point, would increase the spring tension on the light striker to get it back to running to the same pressure as the heavy striker.

An alternate experiment (as I do have a spare striker) is to reduce the current 39g. striker another 20%.  Pretty easy, not a high skill job, as most of the “nose” of the striker isn’t needed and the “sides” are just along for the ride.

However, I ran out of daylight…so that’s something to decide for tomorrow.

BTW: much more of a “carbine” with the LDC off.  Out in the woods, the noise isn’t objectionable and the shorter length is handy.

[Motorhead]

As the weight comes off, increases in stroke should get the power back.

[SpiralGroove]

Hey Ribbonstone,
Good topic for fine tuning the PCP powerplant.  I recently had a heck of a time (or I made a heck of a time of it) tuning my BSA R10 with the OEM regulator.  I can honestly say that I don't have great instincts to know exactly the right Hammer mods (weight), spring guide mods (weight), pre-load mods (spring tension) or hammer travel that is needed for the job.  I wish there was (and maybe there is) a clear cut hierarchy of what changes to do first, second, third and so on.
On the R10 tune, I was battling to get 900+ fps, 70 shots and didn't really care about the efficiency, even though it was "The Variable" holding me back on the shot count. 
1) I started by reducing hammer weight substantially (aluminum vs. steel) as well as valve spring tension.  Originally, the gun was wasting air, even a 0 pre-load was too much hammer strike.  The change to an aluminum hammer helped reduce the strike, but was still was too much at the same 0 pre-load. 
2) So I made a delrin spring guide instead of steel and that was still too much hammer.  I was getting 900 fps, but it was still too loud, seemingly wasting air. 
3) Next, I reduced preload by changing spring (thickness) and very much got a lower hammer strike - too low to get 900 fps even at max preload.  Max preload was loud, wasting air, but not I wasn't able to get 900 fps anymore.
4) The only thing I didn't try was a longer hammer to reduce hammer travel.

Before this process, I had already increased/reduced all ports to about 79% of bore size (9/64"-.177 cal) so I felt the porting was not a reason for great inefficiency.  Further, I varied the regulator's set-point throughout this process - none yielding exactly what I wanted.
Lastly, the OEM regulator gave me inconsistent velocity readings regularly, which added to my overall frustration.

In summary, for me  PCP tuning is very tricky adventure indeed, and I have yet to come up with a systematic order of mods to be followed every time.  I think I'm at the point, where this area of understanding will be my next breakthrough in PCP tuning.  Keeping it simple, going one mod at a time as Ribbonstone has shown, has proven to be a challenge for me, even though I know it reduces confusion/frustration.

Sorry for the rambling

[Ribbonstone]

Got no problem making power, have had that rifle up to 30 foot pounds with an experimental valve.  Air hog, short shot count, stiff cocking, loud, and due to the 22XX trigger arrangement, stiff trigger.  As I got other PCP’s to fill the high power uses, been bringing this one down in power and liking the results.

Are a multitude of ways to skin a cat.  Transfer port is probably the simplest way to decrease power as it is easily reversed without any mods to the gun itself.  Doesn’t change the cocking effort or fill pressures, so if you aren’t happy about one of those two, transfer port manipulation isn’t really going to cure everything.

Could leave it as it is and live with it (at least until the time comes I want to experiment with something else), but am heading out to the shop to bring that striker down another 20-25% and reshoot.

Will likely add back in a cocking indicator mod that I’ve removed.  It adds a bit of weight, so would gain back some of the striker weight loss, but I kind of like the indicator.

[SpiralGroove]

Hey Ribbonstone,
Good Luck with the hammer weight reduction and cocking indicator

Sorry for my indefinite ramble on tuning my R10.  Sometimes you have to remember the Rolling Stones song "You Can't Always get What You Want" especially on 200cc's of Air and a marginal OEM regulator.
Or maybe it's "I can't get no Satisfaction"..............

[Ribbonstone]

Got on the grinder and files again to produce an even lighter striker.

But first: a few words about stroke.

All the striker weights (the previous 53g. and 39g. plus todays 31g.) already had been shortened to give a little more stroke.  It’s probably not worth the doing.
Issue striker is about 1.305-1.31” long.  Can shorten the NOSE by about .1”, which gives about .1” more travel.

HAVE TO DAY "ABOUT"!  Quite a bit of manufacture variation in striker length, valve stem length, cocking slot length, etc.

The trick is the cocking pin slot. With the striker in the rifle resting on the valve pin, measure the distance the cocking pin still has to go in the slot.  The one illustrated has about .12” of travel to go, so it can only depress the valve stem that amount (a very little more as the cocking pin tends to have some “wiggle”).   That seems enough travel (which would be equal to valve lift), but if that space gets any smaller, may find some trouble ahead.



Have not had to do it, but could likely extend the cocking slot a little bit to increase the forward motion a little without getting into trouble.

Not completely finished either.  The longer stroke/deeper travel of the striker PROBABLY will require the rear end of the bolt to be filed/ground down 1/10th or so.  The rear end closed locked bolt can act as a “stop” for the cocking pin, which limits the valve stem compression.

OK…back to the shortened striker. Ended up at a little under 31 g. I’ll admit, it’s a rough job.  Will go back and clean it up/polished it up just for visuals next time in.





Basically just removed most of the sides (grinder and file) and 3/4ths of the “nose”.  Leave the top and bottom alone. Bottom runs the sear, so don’t want to change that, and the top runs the cocking pin, so don’t want to change that either.

Tested:




Results:

53g. ran right at 2400psi – 1500psi  and produced 16-17 shots @ 17.1 foot pounds.  That would be  291 foot pounds per fill.
39g. ran right from 2200psi – 1400psi and produced 21 shots @ 15.1 foot pounds. That would be 317 foot pounds per fill.
31g. ran right from 1900psi – 1000psi and produced 24 shots @ 14.2 foot pounds. That would be  341 foot pounds per fill.

So can guess the following:

Lighter striker weight decreases velocity/energy.
Shot count and total energy produced increases with lighter striker weight.
Some indication that lighter striker weight narrows the sweet spot pressure.
 
Can’t count this one as I changed more than one thing:

Added some of the weight back by installing a cocking indicator (bring it up to about 31g.).  Basically it’s a reverse spring guide long enough to poke through a hole in the rear end cap.  Can adjust spring tension by the length of the “head” (which fits down inside the striker).

So the cocking indicator changed the spring tension a bit.  TRIED to get the thickness of the guide’s “head” the same as the previous tests, so the spring tension would be the same, but am not sure it was exactly the same, so excluded it from the tests (like first stated, wanted to test only ONE change, not the possibility of two changes).







Looks like it splits the difference between 31 and 39g.  Running right from 2100- 1300psi, giving 22 shots @ 14.8 foot pounds (325 foot pounds total).

BTW:Cocking indicator makes it easy to measure the stroke (from full cock to just touching the valve stem).  Starts (resting on the valve stem) .329" deep and pokes out .25" when cocked, so looks like .579" of travel.

[Ribbonstone]

Upping the spring tension didn’t work out so well. Am going to halt and give this a good thinking.

Though out the striker weight tests, the fell of cocking the gun was the same. Same effort, which in this case is pretty light and smooth. 
Did up the spring tension on the 31gram striker a little (basically 1/10th inch) and brought it back to operating at 2100 psi. Energy back up to 15.5 foot pounds. Shot count back down to 20 shots (total of 310 foot pounds per fill). Could make out the increased effort in cocking, but it was minor.

Cranked it up a little higher (another 1/10th) and got back to running at 2400psi. Energy back up to 16.6 foot pounds, but back down to 17 shots (total of 282 foot pounds per fill). Increase in cocking effort more noticeable, and the trigger is getting rather heavily loaded, so the trigger pull noticeably worse.

So basically, cranking the spring tension back up did bring the power back up,  but once the power was basically equal, the shot count was basically equal.

So for now…until I give it a good thinking...back to that last posted shot count (24 shots @ 1900-1000psi/ 31g. striker).