Spring gun tuning: Part 1
Spring gun tuning: Part 2 – Building a mainspring compressor
Spring gun tuning: Part 3 – Mainspring compressor continued
Spring gun tuning: Part 4 – Let’s disassemble a gun!
Spring gun tuning: Part 5 – Powerplant disassembly
Spring gun tuning: Part 6 – Disassembly completed
Spring gun tuning: Part 7 – Disassembly of other spring guns
Spring gun tuning: Part 8 – Disassembly of other spring guns, continued
Spring gun tuning: Part 9 – Cleaning and deburring

by B.B. Pelletier

Lube the parts just before you reassemble them so they won’t attract dirt. Use another paper towel-wrapped screwdriver blade or dowel to spread a thin coat of moly grease on the walls of the clean spring cylinder/compression chamber. You’ll be able to see the metal through this coat because it is so thin.

The piston and seal
Spread a thin even coat of moly grease around the piston seal and about a seal’s width back on the body of the piston. Also, coat the rear piston skirt where it flares out wider. Coating the entire HW piston is pointless, because it doesn’t contact the cylinder except where it is larger in the rear. I lube the front of the piston, behind the seal, in case the seal ever melts and there is metal-to-metal contact. Also lube the piston’s cocking slot and the spring guide rod inside the piston. (Note: some guns don’t have guide rods inside the piston) Next, insert the piston into the spring tube, keeping the cocking slot in the piston aligned with the one in the spring tube. Be careful not to cut the seal as it goes into the tube, and don’t worry about the lube that’s scraped off as it goes in. That’s why you lubed the inside of the cylinder, as well.

The piston seal is coated uniformly with a thin layer of moly grease. Note that I also coated the front part of the piston body, as well as the cocking slot.
When the piston is far enough forward, you can insert the sliding link through the cocking slot in the spring tube. Once installed, the sliding link keeps the piston aligned properly. Lube the sides and bottom of the sliding link with moly grease.

When the sliding cocking link is installed, it keeps the piston from rotating. It’s also lubed with moly.
The mainspring
The mainspring must fit inside the piston with a little room to move, but not much. The closer the fit, the less vibration the gun will have.

If there is too much room, a thin piece of strong metal plate such as stainless steel can be bent to line the inside of the piston, taking up space. This plate needs to be as long as the inside of the piston, and it should line the entire inside without overlapping. The mainspring has to fit inside this liner, so measure carefully before cutting. I don’t care for this method, so I search for the tightest-fitting mainspring I can find.

Now lube the mainspring with what Jim Maccari calls velocity tar. It’s an extremely viscous grease that clings to the coils of the spring and dampens vibration. And, to answer a reader’s question from several weeks ago, it doesn’t slow the velocity much at all. The difference in smoothness is worth the 10 f.p.s. you may lose. No other lube works as well as velocity tar. Look at the picture for how much to use.

Beeman used to sell Mainspring Dampening Compound, which was a thick silicone grease, but it did cut velocity significantly. Regular petroleum and lithium greases are simply not up to the task for this application, though they do work well in lower-powered spring rifles such as Diana 27s and IZH 61s.

Slide the mainspring as far into the cylinder as it will go. The spring that sticks out is what the mainspring compressor has to compress before the threads on the end cap start to engage.

This is what a low-powered spring looks like when it’s pushed in as far as it will go. More powerful springs are longer and stick out farther. Note how much velocity tar was used on the spring. The spring guide is installed and also has tar on it.