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Tried to do a ghetto tubeless conversion today with gorilla tape. To my amazement the first wheel went up perfectly first time. However, Sod's law, having problems with the second wheel. Applied the tape and went up first again but this time I've got a sizeable leak from the valve. I've not got the sealant in but checking the tyre under water. Took the tyre off, made a rubber gasket from a small strip of tube, and tried again, but still leaking badly. At this point I inspected it more closely as the air seemed to be coming from one particular point and realised that the raised rim hole which the valve lock nut tightens against has a hairline crack in it which extends vertically 1-2mm (hope that makes sense). Therefore no matter how tight I do the lock nut up, it isn't sealing as air is pissing out of this crack.
Any ideas on whether this is likely to be fixable or not? I haven't tried it with sealant because it looks to me to be way too leaky to seal as it is but I've no experience so perhaps it would seal? Otherwise I might have to go back to tubes for this wheel as I can't see how I can fix the crack.
Having a cracked rim, is slightly more of a concern than if yer tubeless exploits will work.
A crack is probably overstating it. It's almost impossible to see its that small. It's a tiny hairline imperfection in the raised hole that the valve comes through. It's just enough to lose the seal with the lock nut.
put the sealant in
There doesn't need to be a seal at the locknut - the valve has to seal against the tape on the inside of the rim. The hole in the tape that the valve goes through needs to be cut cleanly to give it the best chance.
Air coming out around the valve can be leaking into the rim pretty much anywhere (assuming a double-walled rim) - if the tape is covering the inside of the rim from bead to bead, it's most likely leaking around where the valve goes through the tape. Have you tried tightening the locknut a little more?
Sealant may well cure it.
Do you mean a valve hole reducer with a split in? Like this sort of thing-
[img] 
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If so non-split reducers are available or you could try epoxying that one to fill the gap.
If you actually have a damaged rim drilled for presta, you could drill it out an fit a reducer or there are schrader valves available instead, another option might be applying chemical metal and then filing out the hole to suit...
Got any pictures of the offending items?
As above, you don't need a seal at the locknut- the rim bed should be airtight and the valve should be sealed at the tyre side, so no air should ever get as far as the locknut. Nothing you do there can fix the problem if it is getting there, it means the basic seal of the rim has failed.
Also, air leaking out at the valve hole isn't necessarily leaking into the rim at the valve hole- spoke nipples can seal up the other holes pretty effectively especially once they've been in there for a while, so air that gets into the rim tends to escape by the valve hole.
Unless it's a single-walled rim (or the crack extends to both walls) then that's not where the leak is, just where the air's eacaping.raised rim hole which the valve lock nut tightens against has a hairline crack in it which extends vertically 1-2mm
I'd suggest checking your taping
(Oh, wot he said ^ )
Ok cheers.
I'll recheck the taping tomorrow and see how I get on.
I've screwed up tubeless every way there is and I bet 10p it's the tape or the valve, or the valve and the tape. An air leak almost always is. Usually you'll see a fault but sometimes just rip it out and start over.
You could add sealant and see if it helps but chances are, you just get a rim full of sealant.
oh... What are you using for valves? You can wing it, but I'm a huge fan of these:
http://www.superstarcomponents.com/en/alloy-tubeless-valve.htm
WTB sell the exact same valve. it's that massive conical plug, they're incredibly unpicky about valveholes, tape etc. Basically cheat mode. Also, they're light, and purty.
Ive found stans style vakves to be a little hit or miss. Dt swiss' version gives a much more robust and stable seal between rim tape and valve.
[img] https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcR4NNPYia2LWNK1Eszr0yyboqCk4X1oh60UP7ogWtm1EElucyyo5Q [/img]
Remove cracked rim, lace up with new tubeless ready rim and bingo.
A tubeless ready rim will still need sealing tape.
I'm using valves cut out of an old tube!
I've just ripped out the tape and re-did it, this time with two wraps rather than one as I did last time. I've checked it as best I can and it looked good. Made a tiny hole in the tape and pushed the valve through to try to keep the seal good at the valve. Still leaking so I think it's maybe the ghetto valve. I'll pick up some proper ones and try that.
Try WTB's valves, they have a tapered rubber bung on the inside. Better-sealing with less pressure than DT or Shimano shape. (edit, just read NW post saying the same ..) Available from other brands also, the Superstar one up there ^ is the same shape.
One of my rims has had a small valve-hole crack like that for years, used a cut-out bit of tube as a flat o-ring or gasket and that solved it, or the WTB valves. Even a valve cut from an old tube works fine.
To make hole in rim tape I've had better results melting the hole rather than cutting - just heat up with a lighter a screwdriver or something of comparable diameter to valve and burn through rim tape ..
Try seating the valve in some PVA glue
A tubeless ready rim will still need sealing tape.
Clearly. The difference is that there will be no bodging, building up rim with tape etc,. and it is a lot more likely to just work as intended. And as the rim needs replacing anyway why not?