Repairing the Damaged Sterling Piston

Slowly bringing the .20 caliber Sterling back to life.   Want to salvage the original damaged piston by reworking it to a different seal configuration.  I don't see much point in using the spare undamaged piston, as the seals aren't just difficult to obtain--they're impossible.  Couldn't warm up to that center-mounted rubber bumper, either.















Started by measuring the ID of the compression tube.  It's essentially a 1" bore.  Next, I went through my various seals.   The criteria:  Obviously fit to the tube was foremost, followed by availability, and to some extent, design.  I wanted the seal to have a relatively thick band near the edge for durability, as I'm concerned about the deep transfer port groove in the front of the Sterling's compression chamber.  Lastly, the seal had to be readily adaptable to the damaged piston.  Picked a spare seal I'd obtained from Crosman to fit my Remington Summit.  It's Crosman's part # B18-04-1A.  I think it was about $4.  (2/3/2012 correction:they cost $1.10) The piston seal requires a 60 degree (included) angle cone for attachment.  I'll get to that a bit later.















The head of the piston is brazed into the front end of the tube.  There's about half an inch of steel inside the tube, leaving lots of metal to work with.   Trued up the piston in a 4-jaw chuck on the Taig.  There's too much overhang here...

Added the steady rest, and started cutting off the damaged grooves.

At the last moment, I elected to leave a shoulder on the front of the piston.  Eventually, I turned this shoulder down to about 0.503" so a steel washer would press fit into place.  Thought this would provide some sort of protection for the actual front of the piston body.

With the shoulder finished, I spotted for a hole with a center drill.

Through-drilled with a #20 bit.

Tapped the hole M5 x 0.8mm.

Broke the edge with a countersink to finish the piston head.

Now, I need a 60 degree cone to mount that seal to the front of the piston.  Mounted the compound crosslide to the Taig set it to 30 degrees.  I cut this taper on a scrap of aluminum rod then realized I was going the wrong direction.  This would be too difficult to do additional machine work to the wide end of the cone...

Flipped the scrap in the jaws and readjusted the compound.  We'll call it -30 degrees.  Cut the cone in reverse.  Remember that the larger end is the front...

Here's where I eyeball everything,  I faced down the end until it just fit into the piston seal.

Through drilled to clear the M5 screw.

Countersunk to match the angle of the M5 flat head.  Cut until the head fit just below flush.

The rear of the seal has a 0.50" hole, so the rear of the cone must match that diameter.

A little layout fluid and some easy caliper work marked 0.500".  Cut just to the line with a parting tool.   

The piston, black steel washer, aluminum cone adapter, seal and bolt.

Couldn't have asked for a better fit.

Here it is above the spare stock piston.  Getting close, but I'm not quite ready to put the gun back together yet.

Check back soon.

Benjamin Sterling HR-81 Fallback Plans A and B

Not to be deterred by the defective compression tube on the Sterling, I went back up to Precision Airgun in Maple Heights, OH and showed the bad tube to Chuck.  He had a couple more Sterlings still in inventory, but not another .20 cal.  Neither of us wanted to cannibalize another gun just for the compression tube, though he seemed less concerned than me.  He dug around for a few minutes and found a cutaway/demo HR-81. ???!!!! Who has this kind of stuff lying around?















The tube looks intact. It was obviously made to show the unique bolt loading design.

Made from early (pre-Benjamin marked) components.  Maybe this was a way to use up some of the leftover British components.

Right side.  This is the bolt loading trough.  Note the line of brass at the brazed joint.

The cutaway on the left side shows a non-lead synthetic skirted pellet just forward of the air transfer port.

The bolt locked in the forward position has an o-ring to seal just behind the port.

Disassembled as in parts 1 and 2.  The trigger housing was cut--probably to show sear engagement and trigger function.  Certainly gave me a better understanding of their relationship.

After looking at it for a few minutes, I decided there just wasn't enough metal left in the housing to support the sear pivot pin.  There were also some small dimensional differences at the front of the tube that would've made retrofitting more difficult.  Reluctantly, I went back to see Chuck with the bad news.

He sent me off with another Sterling--this one in .22 cal--and told me to bring back whatever spare parts were left.  There's probably not a single thing wrong with the Sterling in .22 cal, I just want a .20 for some reason.  Certainly has something to do with losing two .20 cal rifles a few years ago--a Beeman R10 Deluxe and HW-77k.  Replacing my youth.

Took this one apart a bit differently.  This is a better method for the Sterling.  After removing the rear sight, I pulled the cocking lever then unbolted the intact compression tube from the breech.

My spring compressor is cross drilled for 3/4" dowel rods so the bed can be shortened to any length necessary.

The was much easier than working around barreled action with that protruding globe front sight.

OK.... There was almost a second of consideration about using the piston from this gun to simply replace the smashed one from the first.

Except the crash bumper is already deformed.  This is from sitting for years with spring pre-load.  No, I didn't shoot the gun beforehand.

Lemme see if I can get the camera to cooperate.

Remnants of the bumper in the air transfer port groove at the front of the compression chamber.  Which is nice.

Dug it out with a wooden matchstick.

So, the tube itself is a go, but the piston head design is a problem--at least in respect to sourcing replacement ring seals and perhaps also the design itself is flawed.  Then again, the bumper material is twenty years old...  Was the bumper found to be necessary or was it a design error?  Certainly can't presume to know more than the engineers who designed the gun in the first place.

In considering how to remake the piston, I don't know if a parachute seal will fare any better against that groove in the front of the tube.  Seems reasonable to start with the damaged piston and make it work with a readily available commercial seal.  Maybe something with a sturdy looking edge, certainly the groove itself is the issue.   Sorry, I'm just talking to myself here...

As always, more soon.

Benjamin Sterling HR-81 Tear Down Part 2

With the piston looking like this after just a couple shots, I found myself...concerned.  I'm not sure that's the correct word.

There are--or were--two black flat rubber seals with an o-ring underneath each.  The pinkish center piece looks like simply a bumper to protect the face of the piston against contact with the front of the compression tube.

After digging the mess out, it looks like the bumper was glued in place. 

Let's see what's going on.

There's a huge blob of excess brass in the end of the tube. 

Huge amount of overfill from brazing the front of the compression tube cap to the body.  Tube is shot.  I kinda doubt Crosman has any of these compression tubes laying around.  It's been probably  twenty years since the gun was in production.  For most of us, this would be the end of the project.  I think I've got an ace in the hole this time around.  More on that later.

There's absolutely no good reason to remove the compression tube for normal maintenance.  So don't bother with the following step:  The compression tube is affixed to the breech block with two socket head cap screws.  A 3/16" allen wrench didn't have enough swing room.

The bolts were extremely tight.  I didn't think they were coming out without heat, but a longer handle and additional pressure did the trick.

An o-ring seals the compression tube to the breech.

Might as well finish this.  A small set screw retains the bolt handle.

Like everything else on this gun, it's SAE.  I didn't measure it, but I think it's a #8-32.

I expected the bolt handle to pull out after removing the set screw, but it was threaded into the bolt probe and has to be unscrewed.  The probe is pretty long, as is the spring behind it.

Next time around I'll see what I can do to source another compression tube.  I'd rather not return the gun as I haven't seen another in .20 cal.   I don't like the piston seal design so I may retrofit a parachute design seal.

We'll see what happens.