Vacuum Gauge Question

flatford8

flatford8

Well-known member
I was digging thru boxes of stuff I’ve collected and found two vacuum gauges, but neither one is zeroed out. If they don’t start at zero they’re junk, right?......
Thanks, Mark
 
flatford8 said:
I was digging thru boxes of stuff I’ve collected and found two vacuum gauges, but neither one is zeroed out. If they don’t start at zero they’re junk, right?......
Thanks, Mark
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If they are just a straight vacuum gauge they should be at zero yes.
Easier to get a replacement one unless these are of special significance.
Vacuum gauges are a useful gauge to have fitted to a flathead .
Phil NZ
 
Sometimes you can take the lens off and take a real small needle nose pliers and tweak the needle, turn it on the shaft slightly, to bring it back to zero.
 
Guess you could start at where it is and do the math. It's like torque wrenches, you sure is really down to within 1lb.
 
Thanks guys!! These aren’t special, just stuff I’ve collected and I’m trying lighten the load…… Mark
 
As with all measuring tools mechanical & electrical age & use renders them inaccurate. Some are designed for recalibration some not. Most if not all can be checked against a standard to insure calibration is correct. In the case of a Vac gauge which can be used for so many engine diagnostics you want accuracy! If they’re not designed to be calibrated & checked or as in your case the needle doesn’t register zero, then smart to throw in the pile to go.
 
You could check them both to see if they read the same after doing the math. If not, junk em, if they do they are probably ok.
 
They are not junk when they don't retrun to zero. They're still good for adjusting idle mixture, setting timing, and engine diagnostics. You just have to know the actual vacuum is slightly lower than indicated.

Sal
 
While the numbers do mean something the needle action tells most of the story. If you use it to adjust timing just keep moving the distributor until you get the highest vacuum reading. You'll see it climb then drop. Same with setting the idle mixture. A fluttering needle means a bad valve.
 
Are these dash gauges or the large face mechanic gauges? Just curious. These are so simple it's hard to imagine anything going wrong with them, or put another way how do they get damaged like that? Maybe they were dropped? I'd be inclined to mechanically zero the pointer and then see what it displays with a known vacuum applied - maybe a MityVac has some utility, or simply see how it compares to a known good gauge.
 
Tinker said:
Guess you could start at where it is and do the math. It's like torque wrenches, you sure is really down to within 1lb.
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Vacuum gauges don't measure in "lbs." They're normally calibrated to measure in "inHg" (inches of mercury). DD
 
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V8COOPMAN said:
Vacuum gauges don't measure in "lbs." They're normally calibrated to measure in "inHg" (inches if mercury). DD
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Thanks Coop. Yes Apples to oranges, vacuum to torque. It is a unit of measure and relative to calibration. If it is off then it is off. Picked up some nice new water temp gauges, they are off by 20 degrees and the other by 40.
 
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