Oil Pressure Gauge Repair

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.
 
Dave Hennessey
September 2007

U.S. Gauge is still in business!