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  #1  
Old 10-16-2019, 05:24 PM
moosepileit moosepileit is offline
 
Join Date: Dec 2010
Location: Floyds Knobs, IN
Posts: 529
Default Odd leak from valve cap.

I cross threaded a tube stem 30 hours ago. The valve cap on the left, with less rubber seal in it, caused a leak. It would thread on 4 turns. The replacement cap, on the right, from a spare tube, threads on 2.5 turns.

The cap on the left held air for 30 hours and more than 50 taxi, takeoff and landings.

Checked tire, tube, wheel, no issues.

Inflated to 40#, removed cap several times and heard a faint momentary sizzle of air- (that stopped).

The cap on the left held air, until it didn't. It threaded on deep enough to depress the stem.

So, if you'e ever cross threaded and fixed tube stem threads, check that your cap looks like the one on the right and doesn't thread on too many turns.

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Last edited by moosepileit : 10-17-2019 at 04:33 AM.
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  #2  
Old 10-16-2019, 05:37 PM
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Mel Mel is offline
 
Join Date: Mar 2005
Location: Dallas area
Posts: 10,397
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The valve cap is a dust cap. It is not designed to hold air. If it is leaking you need to replace the valve stem core, not the cap.
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  #3  
Old 10-16-2019, 06:45 PM
moosepileit moosepileit is offline
 
Join Date: Dec 2010
Location: Floyds Knobs, IN
Posts: 529
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Mel, the valve core is good tight and matches the other tube and my spare.

If it was a core bad I would have had no reason to post. No cap = no leak.

The cap was seating deep enough on the threads to depress the valve stem. The seal in the cap stopped holding air all at once, found it in the hangar mostly flat.

This is for those that remove the cap with the pants on and may have cross threaded a cap replacement. I had rethreaded the stem and all seemed fine. But, the cap would turn 4.5 times and with its seal shape depress the stem.

3 other caps only thread on nicely 2 and a third rotations and CAUSE no leak. The bad cap caused the leak by depressing the stem and considerately gave up its secondary seal at home in the hangar.

It was audible and obvious once I had checked the normal tube pinch and FOD or slippage type leaks. I think it just got cold enough out it lost the secondary seal, but- it was causing the primary valve to open.

No bubbles at the core, no real bubbles even removing the one cap causing leaks. It was an ear find.

Never seen it, never heard of it. Figures.
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Last edited by moosepileit : 10-17-2019 at 04:39 AM.
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  #4  
Old 10-16-2019, 10:34 PM
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Plummit Plummit is offline
 
Join Date: Apr 2010
Location: SoCal
Posts: 895
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I'm confused. You said:

Quote:
Originally Posted by moosepileit View Post
Inflated to 40#, removed cap several times and heard a faint sizzle of air.
If you *removed* the cap and heard air leak, wouldn't that by default indicate a valve core leak???

-Marc
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  #5  
Old 10-17-2019, 04:23 AM
moosepileit moosepileit is offline
 
Join Date: Dec 2010
Location: Floyds Knobs, IN
Posts: 529
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A short hiss. Not a continuing one.

No cap, no leak. Not a stem issue.

One cap screws on 4.5 rotations. It causes a leak.
Two other caps screw on just over twice. No leak.
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Last edited by moosepileit : 10-17-2019 at 04:29 AM.
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  #6  
Old 10-17-2019, 05:47 AM
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emsvitil emsvitil is offline
 
Join Date: Oct 2019
Location: SoCal
Posts: 81
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See if it hisses when slowly putting the cap on.......
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  #7  
Old 10-17-2019, 07:11 AM
PaulvS PaulvS is offline
 
Join Date: Aug 2019
Location: Western Australia
Posts: 26
Default Core problem?

All the valve cores that I have ever seen are screwed into the valve stem so as to have the centre 'plunger' slightly below the rim of the stem. So, even if a valve cap was screwed on as tight as possible, with or without a rubber seal, it would not be able to contact and depress the stem to deflate the tire.
I'm wondering if perhaps this valve core is not exactly the right one for this tube, which is causing it to stick up proud of the stem? There are minor differences between cores, I have noticed.
Just a thought.
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  #8  
Old 10-17-2019, 07:49 AM
moosepileit moosepileit is offline
 
Join Date: Dec 2010
Location: Floyds Knobs, IN
Posts: 529
Default Airstop Tube pinhole found!

After sleeping on the find I decided to air up the Airstop tube to 10psi outside the tire.

I had only done 4 psi before. Both make the tube quite firm.

I found a pinhole that appears to be a deeper pore type molding imperfection. There are several pores like it in the 10/2014 mfd'd tube.

That aside- that cap that i crossthreaded once two years ago is still a problem. The rethreaded cleaned up valve stem and cap combined with almost no black rubber in the bad cap from repeated 4.5 rotation insertions stacked up to make a leak also.

It should not, the stem is centered and leak free on its own. If I put on the bad cap snug it will make a light, short hiss at removal. Dunking it and removing the cap will get a few bubbles. It actually has a secondary seal still.

So, the pinhole leak double look revealed an existing flaw elsewhere, first.

Tire was at 24 psi this am after leaving it at 38 psi last night.

The tires for 4 years have been the "good" add air once at fall and once in spring type that we pay extra for Airstops.

Thanks for the doubts- really. It convinced me to leave the wheel pant off and give it a night.

The pinhole was 1/3 up the side.

IF you have repaired valve stem cap threads and a metal cap, make sure it tightens in less than 3 rotations.
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Last edited by moosepileit : 10-17-2019 at 09:35 AM.
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