|My U.S. Gauge oil pressure gauge used to read way low - maybe 15 or 20 when the pressure was really 40. And zero when the pressure was really 20. The previous owner had installed a T fitting in the oil line and mounted an auxilliary modern gauge on the side of the engine. When putting the bike back together, I decided to see if I could "fix" the gauge.|
|The bezel is just pressed on, and is easily removed. Use a small screwdriver and tap the edge of the bezel. Work around the gauge slowly, and it will open up - as shown here.|
|Two screws hold the guts inside the "can".|
|The backside of the gauge shows the copper (phosphor bronze?) "Bourdon Tube" around the outside, and some of the brass linkages. There is an "adjusting screw" inside the oil inlet - but I have no idea what it does. Mine was about halfway turned in. I "adjusted" it to different positions, and even took it out completely - but it made no difference. In the end, I just put it back as I found it.|
|You can't see it in the picture above, but there is a date code printed on the backside of the face. Mine reads "SEP 33" - so this gauge must have been replaced at some time.
At left is an example of the date code - from a different gauge. Courtesy of Perry Ruiter.
|Here is an image of the workings from the face side - with the face removed - from a U.S. Gauge catalog. Courtesy of Perry Ruiter.|
|Here's a diagram that show how the gauge works. The oil fills the curved copper "Bourdon Tube", and it flexes under the pressure - it tries to strighten itself out. This moves the brass linkages, which turns the "needle". The Bourdon Tube is soldered to the brass gauge frame - be very careful bending the Bourdon Tube so you don't break the solder joint. If your gauge leaks oil - the solder joint is a likely suspect.|
|This side view shows some of the brass linkages.|
Here's What I Did
I did not remove the face. You would have to remove the needle and the the two rivets which hold the face on, and I figured I'd break something if I tried.
The inside of my can had developed some rust, so I cleaned it out and soaked with "PB-Blaster". On reassembly, I smeared a very light coating of white lithium grease on the inside of the can.
My gauge had two problems: (a) the brass linkages were sticky. I blasted the linkages with Blaster, and worked them a little. On reassembly, I put a little white grease on the moving parts. (b) the "Bourdon Tube" was mal-adjusted. If there were no zero-stop for the hand, it probably would have rested at negative-20. By very carefully bending the Bourdon Tube, I was able to get it to "zero" with little-to-no resistance.
To test it, I used compressed air with a rubber-tipped blowing nozzle. I regulated the pressure down to 20 psi, and tried the modern gauge - it read 20. Then I tried the old gauge, and it read 10 psi. Slight tweak on the Bourdon Tube. Then I compared the modern and old gauges at 10, 20, 30, 40, and 50 psi, and they corresponded very closely.
Success! I reassembled the gauge, and put it back in the motorcycle. I haven't started the engine yet, but once I do, I'll report back on how the gauge works in actual operation.
U.S. Gauge is still in business!