So I'm trying to run a 32c Vittoria Randonneur on a 27mm wide 29er disc rim. I've got one of those beefy thorn-resistant tubes in it. There's a couple spots when I inflated that don't seem to be clinching. Could it be that the tire is too narrow? Is my rim tape too narrow? I've got a 17mm rim tape that only covers the spoke hole area of the rim rather than going from sidewall-to-sidewall. Will it expand and clinch if I just ride on it? What do you guys think?

Is the rim 27mm on the outside or inside?
If the rim is 27mm on the outside (it should be around 21 or 22mm on the inside) then a 32mm tire is just under the minimum limit of 35mm.
If the rim is 27mm on the inside, then the tire is just way too narrow.

Is it next to the valve stem?

Could be the thorn resistent tube is to bulky and preventing the seating of the tire. Try baby powder around the tube and it may slip into place. If that doesn't work make sure valve is pushed in a little when you start to pump tube so it allows tire to seat. If none of that does it , then it must be the tube, to bulky

Since you didn't say how you measured I can't answer whether it's tire/rim width issue. But you can use this tire/rim width chart and decide for yourself. There's some wiggle room on either side, but I feel that there's less on the narrow tire /wide rim side than the other way around.

Other than that it could be too much tube for the tire. You can measure the width of the tube while it's folded flat, and that should be less than 1-1/4 the stated width of the tire. (the math is 1-1/2, but I go smaller to allow for the thickness of the tire wall).

Then if the problem is at the valve, it's a question of your mounting technique. Push the valve into the tire when finished to lift any part of the tube that may be trapped under the bead, then pull it back down to seat the valve before inflating.

If all is right, then it could be that you're using a tire made for a hook edge rim on a straight side rim, or simply that you're not spreading the tire all the way around the rim evenly. If you have a low area someplace, you'll have a high spot somewhere else.

It's inner width is 21mm. Got some pictures. https://smg.photobucket.com/albums/v8...ire%20Seating/ Seems like there's too much tube, as if it's bunching up or something. A little bit of rim tape is actually exposed in the left low spot in pic 2. The rest seems to be seated, even though it's weird. The low spots are staggered. The tube is meant for 700x28-32c. I read that others were having seating problems with this rim also. It's a WTB Laserdisc Trail 29er.

I run 26c's on the same width rim without problem.
First, I remove the valve core and put about? 5 psi in the tire and remove the pump, letting the air out.
That hopefully, allows the tube to straighten out and remove any kinks, folds etc.
Then, I reinstall the valve core.
I put just a couple psi (real soft) in and go around the rim "pinching" the tire near the bead with my thumb & index finger.
If that doesn't do it, I add a bit more pressure and repeat.
Usually, it'll be good and I pump it up.

How much pressure is in the tire in the picture?

This looks more like a classic case of the tire not being equalized all around the rim. If you have low spots in some places, you'll have high spots in others. The tire's bead is of a set circumference, if you move it inward to the hub, the circumference of the rim there is less so it'll be too long and want to bulge out elsewhere.

The thing you need to do is walk the tire away from the high spots toward the low spots. inflate the tire to about 5psi or just enough to fill it out and give it shape. Find the high areas, and push the tire in toward the rim there and push either or both sides in (not to the hub, but away from the rim) so you can slide it, and work it away from the high area to take up the slack. use the same method to push work tire toward the low area to give it the slack it needs so you can work it up the the correct position.

If you're struggling to overcome tire/rim friction, spray or dribble on some weak soap water solution as a lubricant.

If you have reason to pull the tube, compare it's width as I mentioned earlier. Wider tubes can make tire seating more difficult.

Ok, I will try. Thanks guys.

Soap and water worked!

Glad to hear you resolved it. This is another of those things in the category of "Easy------if you know how"