Wednesday, February 1, 2012

"Fixing" the Sterling's Compression Tube

Been ruminating about the transfer port in the Sterling for a few days now. 

Those sharp-edges worried me.  It looked like it could/would eat the new piston seal within a few shots.  Thought about making an insert to fill in near the edge, but it seemed like difficult work considering it's so deep in the tube.

This 1" stainless steel washer should do the trick, but it's just a couple thousandths too large in diameter to fit into the compression tube. 

Mounted it on a mandrel and touched the edge with a mill file.  In about two seconds, it was sized.

The washer barely drops to the bottom of the tube.  (OK, it's technically the front of the tube, but you get the idea.) 

Washer is so thin so there won't be any appreciable change in compression stroke.

Used a good degreaser to thoroughly clean the inside of the tube. 

Cut a piece of aluminum tube to fit inside the length of the tube when the end cap is threaded down. 

The aluminum tube will push and hold the washer against the front of the tube as the J-B Weld epoxy sets up.  There's really not another way to secure the washer in place--at least, none that I could envision.

Not using the quick-set version.   Want the longer set-up time so I don't have to rush--as well as the higher strength. 

A plastic spacer wrapped near the end of a 3/16" steel rod allowed me to get down to the bottom of the tube without the epoxy covered end touching the inside wall.

Washer dropped into place and the rod and end cap installed.  I cleaned the excess epoxy from the transfer port and set it aside.  I'll look at this again in about twenty-four hours.  In the mean time, I'll clean the barrel and degrease the mainspring.  The stock could probably stand another coat of oil, too.

Reassembly should happen in another day or two.  Please check back.  I hope it all works.


James said...

Good luck. Cant wait to see how it turns out.

Anonymous said...

I was sure you would decrease performance with your washer mod.. It would appear you have fitted a restriction in series with a void (air channel) in series with a restriction (the port), this arrangement called a labyrinth seal is used to great effect on turbine engine shafts to reduce gas leakage! Maybe if your washer was aluminium with a 3/4"cut through the middle and you fixed it with the cut aligned with the channel and then after the stuff had set you could have formed the edges with a bit of doweling, this would have left you with your channel having rounded edges for good flow characteristics in restricted volume. You could of course make a tool to do the same thing to your existing mod., Maybe a length of 1" diam., ally bar with a bit of 3/8" tool steel sharpened like a chisel and set in the end and aligned radially, a few swift smacks with a hammer might cut and form the edges or, if you are lucky it might just dislodge the offending item. I look forward as always to following your antics and learning much on Nick's site. Monty

derrick38 said...

Yep, I knew it would screw up the flow characteristics, too, but I don't know if it lost 30 fps or 50 fps. Saving the piston seal from certain destruction was the utmost consideration. In a perfect world, this asinine design would never have made it out the door. No good reason Benjamin couldn't have just drilled a round holed port. The o-ring sealing the upper and lower halves is also problematic.
Having some real world numbers off a couple .20 cal--or even .22 cal--Sterlings would be beneficial for comparison purposes.