by Tom Gaylord
Writing as B.B. Pelletier

Part 1
Part 2
Part 3
Part 4
Part 5

RWS Diana 45 air rifleDiana 45 is a large breakbarrel spring rifle.

This report covers:

• Piston head swaged
• Eliminate sloppy tolerances
• Buttons
• Tighter spring guide
• Remove all burrs
• Clean the spring tube
• Lube the spring tube
• Lube the piston, piston liner and mainspring
• Lube the leather piston seal
• Leave the trigger alone

Today, I’ll show you all the things that have been done to the Diana 45 parts; and I’ll clean, lubricate and assemble the rifle. This will be a long report, so I am breaking it into two parts — today and tomorrow.

Piston head swaged
Before we get to the job though, I was asked by one reader to show how the piston head is attached to the piston body. If you want to replace the leather piston seal, the piston head has to be removed — and that isn’t going to be easy, because it is mechanically swaged onto the piston body.

swaged piston head
Looking through the cocking slot, we see the piston head has a groove cut around its base (arrow). The piston body is swaged into this groove by several mechanical punch marks around the end of the piston body. This is a fast way to assemble the head to the body, but the piston head will be difficult to remove.

Eliminate sloppy tolerances
There were two concerns about the internal parts of this spring rifle. The piston was a little loose inside the spring tube, and the spring guide fit the mainspring a little loosely. The body of this Diana 45 piston swells wider at the rear; but even with that swelling, it still had a little play inside the spring tube. Normally, though, the spring guide and spring will keep the piston body centered and out of contact with the inside of the spring tube. However, when the rifle fires, there can be points in the piston’s travel where it bumps into the spring tube for an instant. The swelled spot at the rear of the piston takes care of that on a factory rifle, but we’re looking to remove all possible vibration from this rifle, so that slop has to be eliminated. We did it with three Delrin bearings — so-called “buttons” —  placed 120 degrees apart near the rear of the piston.

Buttons
To install the buttons, the rifle’s owner, Johnny Hill, drilled the piston body just in front of where it swells out. If he had drilled it right at the swelling, the buttons’ surfaces would have to be only a few thousandths of an inch high. As it was, they were only a couple hundredths of an inch high when the hand-fitting was completed.

Johnny reports that the Diana piston is extremely hard — so hard that a steel drill bit does not want to bite into it. He drilled it on a mill and used pilot drills to start the holes. He said it was a real chore to drill those 3 holes, because his bits tried to walk, and it was impossible to center-punch the holes. Johnny is a skilled machinist, so if he’s complaining — I’m listening!

piston hole and button
Three holes were drilled around the circumference of the piston at the rear, just ahead of where it swells. Then, 3 Delrin bearings — called buttons — were tapped in with a hammer.

button
A Delrin button before fitting.

The buttons, which are actually bearings, are made from Delrin — a type of engineering plastic. Their shafts just fit the holes in the piston, and each had to be tapped in with a hammer. No attempt was made to bond them with the piston, because the spring tube will hold them in place.

Each button was then ground or filed down to a few hundredths of an inch higher than the piston body at the swelling. As this work was done, the piston was inserted into the spring tube many times to gauge the progress of the fit. This work was done by eye and feel, alone, but the finished product (the respective height of the 3 buttons) is probably not more than 0.02 inches different.

Grinding the buttons went faster; but for the final fit, a file was used. I was looking for a tight fit that would still let the piston slide in the spring tube. When there’s no play between parts, we call it an interference fit.

fitting button
The button is filed to fit the spring tube.

Tighter spring guide
The other thing I did to eliminate tolerance/vibration was ask Johnny to make a new rear spring guide that tightly fits the new mainspring. In fact, it fits so tight that the mainspring will not slip on. It has to be twisted on in the direction opposite of how it’s wound. That causes the spring coils to open slightly — something they will also do as the spring is compressed during cocking. Airgunsmith Jim Maccari refers to this type of fit as the spring guide being “nailed on.”

The new mainspring was selected to be as close as possible in size to the original spring. It fits inside the black sheet metal liner of the piston quite well, so nothing was needed there. We know that the mainspring will get a little larger on the outside as it compresses, so some room has to be left for that. Now that the buttons were hand-fitted to the spring tube and the new spring guide was made, all the other parts were gathered and the tuneup could begin.

Remove all burrs
This is when you inspect the metal surfaces of the parts and remove all burrs. They’re found around the edges of the cocking slot and at all places where metal has been cut. I inspected the entire rifle and found no burrs anywhere. This isn’t common, though I will say Diana airguns are burr-free more often than many others. The worst guns are the lower-priced Chinese airguns, such as the B3 sidelevers and the inexpensive breakbarrels. In contrast, I’ve never found a burr on a TX200 of any vintage. Finding no burrs to remove from the Diana 45, I moved on to cleaning the rifle.

Clean the spring tube
The first task is to clean the spring tube. Although this rifle is very old, it has never been apart before, and the inside of the tube is still rather clean. To clean it completely, I folded a strip of paper towel over the end of a long dowel rod and attached it with a rubber band. I soaked the paper towel in alcohol and swabbed out the entire spring tube.

Lube the spring tube
Next, I lubed the inside of the spring tube with Air Venturi moly grease by applying it with the same dowel rod method. I spent some time making sure the moly was completely and evenly spread around the inside of the tube.

dowel rod with paper towel moly
I’m applying moly grease to the inside of the spring tube — front to back. The rod is a dowel, and the grease is on a strip of paper towel that’s held on the dowel with a rubber band.

The Delrin buttons will get impregnated with the moly as the piston slides back and forth, and that will make them slide better in the tube. The leather piston seal will burnish some moly into the forward end of the spring tube that serves as the compression chamber. The gun will probably burn grease for a while during the break-in period of this tune, but it will also be lubricated for the next 10 years, minimum.

Lube the piston, piston liner and mainspring
Next, I lubed the piston rod (a rod that runs down the center of the piston) with lithium grease. Then I lubed the outside of the piston liner with the same lithium grease. I also lubed the inside of the mainspring with lithium grease, but I coated the outside of the spring coils with black tar. That will cut any tendency to vibrate. The coating was very thin, because I’ve tightened up all the other tolerances in the powerplant and didn’t need to dampen much vibration. Put on too buch black tar, and you’ll slow down the rifle.

lube piston  rod
I spread lithium grease on the piston rod. This rod keeps the mainspring from kinking too severely when compressed, but it doesn’t have much contact with the spring at any time.

lube piston liner
This is the sheet metal liner that fits inside the piston. Among other things, it prevents the cocking shoe from dropping into the piston. Lube it on the outside with lithium grease.

lube outside mainspring
I lubed the inside of the mainspring with lithium grease, but I lubed the outside that comes in contact with the inside of the piston liner with black tar — a viscous grease that dampens vibration.

Lube the leather piston seal
I also lubed the leather piston seal with lithium grease. That needs some explanation. You know that moly grease was spread on the inside of the spring tube, which includes the compression chamber. Isn’t that enough lubrication for the piston seal? If this was a synthetic seal, then yes, that would be enough. But this seal is leather.

Leather needs to have oil to keep it soft and pliable. That’s how it seas the piston. I used the lithium grease on the leather because I knew it will last (provide oil to the leather seal) for a very long time. I have a Diana 27 that I lubed this way over 15 years ago, and it’s still going strong.

lube piston seal
I applied lithium grease to the outside of the leather piston seal to keep it soft and pliable.

The lithium grease allows all those parts to slide against each other with reduced friction. The piston liner fits the inside of the piston very well, so the lithium grease is all that’s needed. The black tar on the outside of the mainspring dampens any vibration between the mainspring and the piston liner. I have now removed all tolerances from the inside of the mainspring (with the larger spring guide) and from the piston (with the buttons). This thin coat of black tar should finish the job.

Leave the trigger alone
Here’s a question that comes up all the time: Should we try to “tune” the trigger? I say, “No.” Unless you know the Diana 45 trigger unit very well, I advise you to leave it alone. This is not some straightforward trigger from a Mauser. This trigger is complex. There are things going on inside that swarm of parts that would take weeks to fully understand.

I knew from the earlier testing that this rifle’s trigger worked well, and that was good enough for me. How bad would it be if I tuned the rifle, only to put it out of commission by spoiling the trigger?

trigger
The trigger is adjustable. Let that be all you do to it.

I’m going to stop here, because the next thing is to assemble the parts into the rifle. If all goes well, that will complete the tuneup, and I’ll be able to begin testing the gun again. Tomorrow, we’ll finish the job.