Schrader Valve Cracking Pressure

[nedsim]

Recent discussions about what's required of a compression tester that'll work on small engines prompted me to do a little test to get a feel for how much pressure is required to overcome the spring pressure on run-of-the-mill Schrader valves. The short answer is, it's all over the place. I tested 5 miscellaneous used valve cores and the pressure varied from 5psi to 80psi -- the pressure where bubbles just started to escape from the valve.

No correlation between the pressure and color of the plastic band.


No correlation between the color of the plastic band and pressure.

 

[Old2stroke]

Recent discussions about what's required of a compression tester that'll work on small engines prompted me to do a little test to get a feel for how much pressure is required to overcome the spring pressure on run-of-the-mill Schrader valves. The short answer is, it's all over the place. I tested 5 miscellaneous used valve cores and the pressure varied from 5psi to 80psi -- the pressure where bubbles just started to escape from the valve.

No correlation between the pressure and color of the plastic band.
View attachment 1234204

No correlation between the color of the plastic band and pressure.

Looks like you are applying pressure in the direction where it shouldn’t leak at all. Not the pop off pressure.

 

[nedsim]

Looks like you are applying pressure in the direction where it shouldn’t leak at all. Not the pop off pressure.

No, that's a valve stem cap with a thru drilled hole on the end of the red tube. The pressure is applied in the same direction it'd be in a compression gauge, or when filling a tire.

 

[Old2stroke]

No, that's a valve stem cap with a thru drilled hole on the end of the red tube. The pressure is applied in the same direction it'd be in a compression gauge, or when filling a tire.

Gotcha, wasn't obvious from the photo. I'm surprised there was that much difference in the valves. I only have 2 of the white band ones and they both tested at about 1psi, others from car and bike tires all seemed to be between 5 and 8psi. I have had some success by clipping a turn or two off the spring on the long stem valves, if you get them down to something less than 5psi, that's good enough for me.