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.

 

[Okie]

Oldstroke in the prev post was typing at same time as me. Their is info in the below links as to clipping turns off the long spring types, etc.

Yes, the opening pressure of schrader valves is all over the place and the color on the band is not a true indicator of it's intended use or opening pressure.
I've found that for small engine compression testing and SPARE schrader valves for the gauges needs to be tested in the gauge on a small engine so as you know your spare valves will read correctly BEFORE needed as replacements. Seems most generally that small pieces of carbon gets into the vavle and it leaks back and can sometimes be cleaned.
I've also noticed that several of the so called low pressure schrader valves are overpriced and quite often will produce a different reading when used in the test gauge.

Here is some info that I had on file where I modded some schrader valves and also made some homemade gauges and links to such on this site.
I use two gauges that I know normally reads correct for when I see a small engine that has low compression I check the engine with both gauges. (I've had guys bring me chainsaws that they had overhauled due to low compression on their gauge and compression was not the issue, their gauge was wrong.
Summary: keep a heads up and open mind when checking small engine compression, especially when your gauge indicates low compression..
Here is a ebay number I seen recently that is advertised as a low pressure valve. I do not know for sure that it really is as stated but price is lower than most. 121040848441
Here is some more info about small engine compression testing, making gauges and schrader valve info from my file and links to posts on this site about such.


Link to a video about small engine compression testing 6/23

https://www.arboristsite.com/thread...-04&utm_content=daily_newsletter#post-8020777



Here is a link about modding Schrader valves and homemade testers, actron tester, etc

Compression tester issues:

http://www.arboristsite.com/community/threads/compression-tester-issues.310133/



I got 3 of the Napa 700-9556 compression gauge Schrader valves today after the local NAPA ordered from their warehouse. The come 5 in a box and you can buy them each @ approx. $2.20 each. They worked A-ok. One was defective out of it's sealed package, was not spring loaded on back check due to a broken spring, noticed immediately before attempting install for test..





3 /2018
Napa #700 9556 is obsolete.

Here is a special low pressure schrader valve for compression gauges Milton and Lisle part number's and I seen them on flea bay as still available.

Milton 1251-2 @ about $5 each

Lisle 20100 @ about $7



Update info 12/20

Possible good compression gauge for chainsaws at $40, STANDIX 1023

Mentioned at this link 12/20

https://www.arboristsite.com/commun...mpression-gauge-and-tested-a-few-saws.348043/