On the quest to find a consistent, smooth writing, least probability of leaking, most cost effective, pen that can write when it gets cold soaked at 17,000 over the Rockies, or heat soaked on the ramp in Arizona, I came across the Fisher Space Pen Company and their pressurized ink cartridges.  These pens (and refills) have thixotropic ink that is hermetically sealed in a pressurized reservoir.  Translation – their ink writes consistently smooth no matter how hot or cold it gets.  I started by ordering one of their Cap-O-Matic space pens.  These have a brass body and include a medium point black (PR4) ink cartridge out the gate.  This pen writes extremely smooth but the slim design required a bit of grip strength be used to grasp its slender body often leading to it slipping out of my hand and landing on the floor boards when grabbing it to write.  I then got my hands on a Safety Orange Space-Tec space pen.  This has a larger diameter plastic body that unfortunately leaves quality by the wayside as it likes to have its ends slide out from the body rendering it useless.  This plastic pen shares the same ink cartridge as its more expensive brass cousin. 

Leaving no stone unturned I decided that while the ink was exceptional the pen bodies available from Fisher left much to be desired so I started to look for an alternative delivery method.  Soon there after I ran across the all metal Zebra F-xMD and metal exterior/plastic action Zebra 701 pen lines.  I also took a look at the Zebra 402 series.

Zebra F-xMD – ($11 Amazon)

• All metal (including the action) 
• Requires no modification to accept Fisher refills
• Accepts PR4 (medium) refills (and any other color/line width in this family of refills)

Zebra 701 ($10 anywhere)

• All metal exterior and plastic action 
• Requires no modification to accept Fisher refills
• Accepts PR4 (medium) refills (and any other color/line width in this family of refills)

Zebra 401 – ($6 per 2 pack anywhere)

• Metal/plastic/rubber construction
• Requires nose to be honed out to #38 drill bit to accept Fisher refills
• Accepts PR4 (medium) refills (and any other color/line width in this family of refills) after modification with drill bit

After flying with these pens for a few years I have deduced the following:

The finer line width of the PR4F refill is my preferred line width.  No stock pen from Fisher comes with this so if you get a Fisher body you will need to order a F class refill for the thinner line width.  This thinner line width fits on a paper log better as well as conserves space on my knee-board while copying clearances.  Simply put, more fits in less space with a thinner width line.  My preferred pen body is the Zebra F-xMD.  Its all metal action clicks just every so slightly smoother than the more readily available Zebra 701.  The Zebra 402 is nice with its rubber comfort grip if you find yourself writing a long letter but when you get hot in the cockpit and a bit of sweat gets on your hands the F-xMD or 701 with their knurled grips are the better choice.  As for the Fisher bodies, the Cap-O-Matic is a solid choice but given its slender design it is harder to handle in the cockpit.  I have ordered a few custom engraved copies of the Cap-O-Matic to hand out as gifts though.   They are a high quality pen.

Conclusion:

For less than $17 you can have the best pen that money can buy, the Zebra F-xMD with a Fisher Space Pens PR4F refill. (SPR4F is the PR4F with the S as a single refill packaging designation).

Zebra F-xMD opened and looking at Fisher SPR4F refill (fine) with stock pens waiting upgrade.

Stock pens disassembled, each with a fresh Fisher refill sitting to the left of their respective stock pens ink for comparison.

Diameter of the Fisher refill tube that must protrude through the nose of each pen nose.

Nearly identical pens with the exception of their internals. 701 is plastic and F-xMD is metal inside.

Honing out the Zebra 402 nose to fit the Fisher refill, otherwise the 402 will not accept the Fisher refill.