Saturday, February 11, 2012

Quick Update on the Sterling

Dissatisfied with the 570 fps the Sterling was making, I tore the gun down yet again.

At least the compression tube can be disassmbled from the gun after removing only six bolts.

No damage to the face of the piston after over one hundred shots, so that's the good news.

No pics, but while I had the gun apart, I checked the bolt o-ring, but it was fine.  The next largest o-ring I had on hand didn't fit into the breech anyway.

Went through the box of mainsprings and found a similar spring that was about 0.750" longer.  Same OD, same ID.  I'll probably end up ordering a new higher quality spring from Air Rifle Headquarters.  The gun went back together and velocity increased to 600 fps for about twenty shots then dropped off to 580.   A drop of oil on the sides of the transfer port o-ring showed a leak like a breaching whale.  I'm getting bored with all the gun's idiosyncrasies, I stripped the compression tube again (six bolts!) and degreased the seats for the o-ring seal with acetone.  Glued the o-ring into place with a cyanoacrylate.  With the port sealed, velocity climbed to a high of 620 with the .20 cal H&N wadcutter. Another hundred shots and the velocity has stabilized at 615.

The washer in the compression tube is certainly disrupting the airflow, not to mention the square-shaped channel of the air transfer slot transitioning to round then making that ninety degree turn inside the bolt nose.  Way too much turbulence in the original design.  So, a replacement spring seems like potentially the most bang for the buck in terms of effort.  When (if?) I strip the gun again after sourcing a different spring, I'll also try a different piston seal.  I've got four more on order.

Probably something different next time around.  Need a break from this one for a while.


Ryan said...

A little too late to mention now, but I wonder if a much larger hole in that washer at the end of the compression tube would have had less impact. Wouldn't you need only a very narrow "surround" to protect the piston head anyway? Seems like the original damage was due to the odd design where the center "bumper" of the piston stood proud of the metal rim.

Now I wonder how much surface contact (between piston seal and the end of the comp. chamber) is needed to absorb the impact with a modern seal. The air cushion in the chamber probably does enough alone. That the washer might not have been needed at all. Just by looking at the pics, the washer cuts the airflow through the transfer port by more than half.

Just a thought.

Ryan said...

Ping! Forgot to check "Email follow-up comments" option.

derrick38 said...

Yes, I think you're right. The larger hole should help. As far as the air cushion being enough, it probably is at the moment of firing, but there's still enough preload at rest that the seal lip will be damaged. All three of the Sterlings I opened had bumper damage from the slot.

paddyfritz said...

"square-shaped channel of the air transfer slot transitioning to round then making that ninety degree turn inside the bolt nose"


Ryan said...

I don't think there's any way to get good performance from this design and protect the seal. Right?

In my mind's eye I see a very very thin steel shim instead of a washer. I would guess the inside diameter should be just smaller than the inner diameter of the grove in the flange of the piston. If the shim is very thin, the face of the piston would only deform very slightly from preload - only the thickness of the shim. The inner face of the seal, where the shim wasn't, would bottom out again on the compression tube/transfer port don't you think?

Lol, will you really abandon this and move on? Or is that low FPS eating at you?

derrick38 said...

The fps is about the same as an R7 in .20 cal--so I can live with it if this is about as good as it gets. There's always a point of diminishing return on tuning any particular gun and I usually work on it until I feel like I'm hitting that line. The gun has so many transfer port airflow issues, the washer seems relatively minor. I feel like I'm fighting for 10 fps at a time at this point--and, let's face it, no matter what I do, this gun is never going to hit 800 fps. Does that bother me? Not a bit. It's now sound mechanically, it's very accurate and it should stay that way for a long time. The walnut stock is nicely figured, the metal work is excellent. This is a very fine rifle.

Jon B. said...

I'm wondering if swept volume might also be a influence on low velocity. Do you think you may have lost some in the conversion process?

derrick38 said...

Good question. I actually took that into consideration before doing the work. The swept volume is virtually identical before and after. The converted piston's OAL is a couple hundredths shorter and the washer is a couple hundreds thick.