Where angels fear to go…or How I fixed B.B.’s Sterling air rifle
by B.B. Pelletier
Some time ago, I sent my Sterling air rifle to blog reader and all-round nice guy Vince. He knows his way around old airguns and has worked his magic on this old one.
If you’d like to write a guest post for this blog, please email us.
Now, here’s Vince!
I just sort of blunder in because I generally don’t look where I’m going. Like when B.B. attempted to test his unusual Benjamin Sterling underlever rifle and found he was rather perplexed about the ridiculous velocity variation he was getting — on the order of 50-150fps. I let it slip that I have a smidgen of experience with these somewhat oddball rifles, and next thing I know it’s on its way to me.
So, I wandered into another tear-up, beat-on and generally-trying-not-to-screw-it-up airgun repair. But even when I blunder thus, I do it slowly, so B.B.’s unfortunate rarity sat in my basement for oh, about 9 months or so before I finally got around to it. Heck, he could of had a baby in that time! Well, sorta, coulda — if you know what I mean.
By the time I finally got to look at this gun, all my hard-earned experience and knowledge from working on exactly two examples a few years ago had leaked out of my head. “No problem,” I thought. “If I figured it out once, I can figure it out again!” A little before Christmas, I finally bit into it.
Right! OK. Here we go. Let’s see. Lefty loosey. The gun starts to come apart in pretty much the usual fashion by removing the rear triggerguard and front stock screws located in the usual positions:
I’m pleasantly surprised to see those small, thoughtful gestures you should expect on such a high-class instrument of plinking. Like the metal bushings in the front stock holes that allow you to tighten those screws without squishing the wood:
After the screws are removed, the action simply lifts out of the stock. To get the guts out of the action, however, is a bit more involved.
The first thing I need to do is remove the mainspring, which on most rifles means compressing the rear spring stop and removing some pins. But this gun follows an old practice that you generally doesn’t see much of anymore — a threaded end plug.
There’s nothing inherently wrong with a threaded end plug, although it IS expensive to manufacture and can lead to some interesting histrionics during disassembly. Problem is that you can’t just push on the end cap, remove the anchor(s) and release the pressure. You have to merrily unscrew it (which takes many revolutions) while applying enough pressure to keep the spring compressed and eventually prevent it from sproing-ing all over the place. This can be a bit tricky with a standard spring compressor (which I never use anyway), but no matter — I’VE GOT A PLAN!
First step is to loosen the spring plug with a screwdriver, like so:
We really don’t need to worry about the plug trying to escape until it’s at the last few threads. I continued to unscrew it until I can get a thin screwdriver all the way through it and put it on the floor, holding it this way:
This lets me put much of my weight (Anybody need any extra? I’ve got plenty!) downward while I rotate the rest of the action counterclockwise. My foot holds the screwdriver that keeps the plug from turning; and when the threads finally disengage, it’s easy to let the spring uncompress in a controlled fashion.
Especially when the preload is, oh — about 10 lbs, which it is on this gun — very anti-climactic. But no matter. The plug unscrews and the spring guts slide out the back:
Obviously, though, we haven’t entirely dismembered the rear of the rifle. There’s another threaded plug in the tube-thingee above the cylinder, which we need to remove if we’re going to take out the bolt and feed mechanism. There’s no spring behind it, so it just unscrew it without any drama:
Now, I remember a major boo-boo I made when I first worked on one of these. I have to pull off the bolt handle, but that won’t happen until the setscrew in the end of the bolt is loosened:
Finding that setscrew, by the way — AFTER trying to unscrew the bolt handle — means you’ve made a bit of a mess. But since we’re not repeating that mistake, the bolt handle unscrews and everything just slides out the back.
Note the itsy-bitsy o-ring circled in yellow.
The piston’s going to come out next, but there’s some stuff in the way — most obviously the trigger. It’s not modular like, well, most guns anymore, so it actually has to come apart.
First thing out is the sear pivot “A,” otherwise the safety lever “C” will fight until the cows come home. But once that’s removed, the circlip “B” and safety “C” pop right out:
Unfortunately, there’s also “D” in this picture, and I’m pointing it out because the OTHER side of this pin (which we can’t see because I forgot to snap it) contained a spring-loaded detent ball for the safety.
Notice that I said “contained.” So, I find myself singing (yet again) that ancient tune of the absent-minded tinkerer: “Where, oh where, did my detent ball go?”
Not a clue, but more on that later.
Pin “D” slides out, and now the trigger blade can be removed:
Best to grab those trigger springs before I lose them, too:
The cocking lever pivot comes out easily enough — first by removing the lock screw on the port side of the gun, then the pivot from the other side. After that, the entire cocking lever/cocking link assembly can be removed:
FINALLY, the piston slides back and out, and (disturbingly) the piston o-ring seals don’t look that bad. I say “disturbingly” because I was hoping to find an obvious reason why the gun was running so badly:
The Sterling is unique in several respects, but perhaps the most evident is the way the spring cylinder sits below the barrel and bolt tube. This does necessitate something of a convoluted air path (ensuring it will never become a “magnum” rifle), and it also means that there’s a seal between the tubes. Since B.B. was having so much trouble with this particular gun, it seemed prudent to replace it, which means they have to be separated.
Looking at the spring cylinder (from the muzzle), we can see two Allen-head screws. Back those out, and the spring cylinder just drops away:
The seal I was talking about is circled in yellow.
Unfortunately, that seal also looks fine. So far I’ve got this thing completely torn down, and I see nothing wrong.
Next, I inspect the spring cylinder, and I’ve got mixed feelings about peering inside. On the one hand, I hope I find something horribly wrong that explains the poor performance; on the other hand, I know that a damaged cylinder will be almost impossible to replace. Well, to my suprise and delight both hopes were fulfilled!
And what did I find loose inside that cylinder? Say “rubber baby buggy bumper” three times fast:
Since the Sterling uses o-rings as seals, there’s nothing but metal on the front face of the piston. That’s not good. When the piston slams home, a metal-on-metal interface doesn’t make for the gentlest of landings. This rubber bumper was somehow fitted to the front of the piston to cushion that shock.
And I really mean “somehow.” The face of the piston has no provision for a pin or screw or anything else that will actually hold this bumper in place, so I have no idea how it was originally secured. Or theoretically secured. Regardless, whatever Benjamin did obviously didn’t cut it.
Setting that aside for a time, I turned my attention to cleaning out the cylinder. I’ve come around to the general opinion that if the inside surface finish of a cylinder is good, there’s really no reason to do anything more to it.
First thing I did is scrunch up half a paper towel, jam it into the cylinder and push it all the way to the bottom:
I then squirt a generous amount of some sort of solvent down the cylinder and give it some time to soak into the paper-towel plug…
In this case, I’m using simple mineral spirits.
…and use “claw” to retrieve it while rotating it and moving it back and forth:
Repeat, repeat and repeat until it starts coming out clean. I follow that with a couple of dry ones to mop up the remaining solvent. Another peek inside the cylinder shows a nice smooth surface, so it’s ready to go.
Now, I’m back to that stupid rubber baby buggy bumper. I’m determined not to repeat Benji’s mistake, so I check with B.B. and get his okey-dokey to drill and tap the end of the piston:
Now all I need is a bumper. Using the original was out of the question; and since I harbor an odd desire to stick pieces of dead cow into airguns, I made one out of leather. I used a flathead machine screw to attach it to the end of the piston AND KEEP IT THERE:
The next issue is, of course, to find replacements for the seals in this gun. JGairguns.biz doesn’t show them; and if they exist anywhere else in the US, they managed to remain hidden from me. Besides, these are simple o-rings that I have to replace. How hard can THAT be?
Starting with the piston:
We see that it dispenses with a standard piston seal. Unfortunately, what it uses are not exactly traditional o-rings, either. First, they’re metric. And 2mm width has no decimal substitute. Second, they’re square in cross section. Two strikes and you’re out. I couldn’t find exact equivalents. So, round ones will have to do, and I ended up using size 2×19.5mm.
Finding a replacement for the itsy-bitsy bolt o-ring was also a bit tricky, but it turns out that a 1x3mm seems to be perfect:
Fortunately, the spring cylinder transfer seal is virtually identical to a common #106 o-ring. A little grease holds it in place for reassembly:
Now, the entire gun can start going back together. A process which, I’m sorry to report, I sort of forgot to photograph. But, no matter, really — it’s essentially just all the above in reverse order.
There are two home-made moly lubes I generally use for springers. The first is a mix of “tacky” grease and 25% moly powder, the second is a sticky chainsaw bar oil (30w) with about the same proportion of powder. I use sticky base lubes for two reasons: lube clinging to surfaces lubricates better; and if it’s clinging to surfaces, it isn’t atomizing and exploding.
The seal and the inside of the compression tube gets a light coating of the oil mix, while the piston and the cylinder behind it (after the piston is reinstalled) get a light coating of the grease. I use Maccarri’s tar on the spring — nothing heavier than that on a medium-powered gun.
And, so, the pieces start coming together…tubes, piston, cocking linkage, bolt — uh, oh. Right. I still have one little problem. That stupid safety detent ball the aliens beamed up. I need to replace it. As far as I can tell, it’s a 2mm to 2.5mm ball, and loose ball bearings aren’t exactly in stock at any of the local hardware stores.
This turned into one of those problems that I just NEW I could address with SOMETHING I had in my house SOMEWHERE. It took a couple of days, but I finally remembered some sliding door pullies I had laying around:
So I cut one apart…and am I gonna get lucky?
BINGO! Although, in all honesty, these balls were a smidgen too big — and I mean a SMIDGEN. I opened up the detent well by a couple thousandths, literally, and it fits like a glove.
It took a few dozen rounds for the excess lube to work its way out of the gun’s innards, but eventually it put down this string of velocity numbers with Crosman Premiers:
That’s a 12-shot average of 690 with a spread of 16 f.p.s (pretty close to B.B.’s subsequent 705 with a 14 f.p.s. variance), so we can safely say that the old Sterling is back up to snuff.
And that about wraps it up. I think the overhaul went fairly well; the only thing I’m tempted to be concerned about is the piston o-ring substitution. But considering how consistent it seems to be shooting now, I think my concerns are unwarranted. It appears that after an anxious 9 months, B.B.’s baby is finally ready to wail!