Mountain Gear

Tech FAQ: Zinn on Leaking Valve Stems, Chain Length

Featuring Wayne Stetina and Stan Koziatek

Dear Lennard,
I have a set of square profile Shimano WH-M965 MTB wheels.  I have been running them tubeless, but have been having problems with them leaking air around the valve stem.  The leak is bad enough that I have to add more pressure than needed to offset the leak during a ride.  Extra Stan’s and shaking to get a better seal have not worked.  The seal seems to break once the valve stem is opened to fill the tire with air.

I was wondering if you or anyone else had run across a solution for an additional seal?  I have considered adding a tubular tape over the valve stem hole and then inserting the valve stem through that, or using the tubular tape to tape the valve stem in place and cut an opening in the tape to allow air through.  I’ve also considered placing an o-ring around the valve stem at the top of the rim, which the valve stem nut could tighten against.  Any thoughts would be appreciated.
― Todd

From Shimano R&D director Wayne Stetina:
“Only if Todd has original Shimano tubeless valve with removable seal ring. That can distort when tightened.  Tell him to get new valve stems with permanently attached rubber seals. Also if rim under seal is badly corroded from sealant, may need to use some epoxy under newer valve stem.
— Wayne

From Stan Koziatek, founder of Stan’s NoTubes:
“I would first have him soap the entire tire, rim and around the valve stem area. Liquid dish soap and water.

If the leak is around the valve stem nut have him replace the entire valve with one of my valves (all with removable cores.)

He may be using a floor pump that is real hard to remove. By pulling on the valve he may have cracked the rubber base on the valve.

If soap is bubbling around the core assembly my valve stems have removable cores that can be replaced if they leak in the future.

If he has no leaks around the valve or core assembly he should check the sidewalls of his tires to see if they are leaking.
If his tires are leaking through the sidewalls have him bounce the tire to seal the sidewalls. We found bouncing the tire seals quicker.

Sounds like he may just need a new valve stem.

Putting an O ring around the top of the valve will not do any good. The air must be sealed from the inside or it will leak into the inner rim cavity and leak out around the spokes.
— Stan

And Todd, the one thing I’d add is that corrosion around the valve hole was the root cause of the one time I had this problem, and I found the leak by using soap suds. However, the suds were bubbling at all of the spoke nipples, not at the valve stem! That’s because the valve stem was leaking around its base, where the valve hole in the rim bed was corroded away, but air was not leaking out around the seal where the valve stem exited the lower rim wall. So the air was going into the chamber between the rim walls. Since it was a Mavic wheel, there were no spoke holes in the rim bed, but the spoke nipples were not sealed where they penetrate the lower rim wall, and air was getting out around them.
― Lennard

Dear Lennard,
I put a bashguard on my SLX triple cranks a few months ago for our annual rock-bashing trip to Tahoe.  Somewhat to my surprise, the 32/12 top gear works fine for 95% of what I do on a MTB (although I’ll likely buy Shimano’s 36t ring/derailleur combo at some point), and I find I love being able to fearlessly slide over big obstacles.

But I’m wondering what I should do about chain length when I replace my drivetrain in a few weeks.  I still have the full-length chain for the triple.  That works fine, but should I take out the 10-link difference between the big and middle rings next time I redo my drivetrain, or should I leave it a bit longer for a 2×9?

Wrapping the chain big-to-big and adding two links is exactly what I've done with my own 2X10 made from a triple in the front with a bashguard replacing the outer ring. Photo: Lennard Zinn

I plan to leave it as a 2×9.  And after two years and three chains, it’s time to start fresh, with a new cassette, chainrings, and chain.  I’m just wondering if I can shorten the chain with no issues.  It seems like a tighter chain might shift better, and would be quieter on the downhills.  But I was wondering if one would size up a 2×9 the same way one does a 3×9.  I usually follow the Shimano guidelines and wrap the chain around the biggest ring and cog, and then add two links.
― Steve

Dear Steve,
Wrapping the chain big-to-big and adding two links is exactly what I’ve done with my own 2X10 made from a triple in the front with a bashguard replacing the outer ring. That’s what I’d recommend.

It works great, and, like you, I’m amazed at how I don’t miss the high gears I lost.
― Lennard

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Technical writer Lennard Zinn is a frame builder (,a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn.