I replaced the cantilever brakes on an old mountain bike with V-brakes.
Problem is that when one side is flush on the other the rim is over 5 mm from the pad.

I swapped the fatter washer in and thin one out on that side, but it's not nearly enough.

With cantilevers the brake shoe can be moved in quite a bit, but there seems to be no way to do that with V-brake blocks.

I'd rather not try to redish the wheel, I've never done more than take bumps out of a wheel.
Is there any other option? Or am I going to have to put the old brakes back?

You need to do a bit more diagnosing first. If the wheel is improperly dished, then you really should sort that out.

Check this:
- with the brake pads mounted in the same configuration(washers/spaces in the same sequence both sides, how are the brake arms aligned?
If they have the same alignment WRT vertical/the fork legs and you still have different amounts of clearance, then there's something going on with the wheel, either dish, or something as simple as improper seating in the dropouts.
-If the brake arms are pointing in different angles, then you should be able to sort it all out by adjusting/balancing the tension on the return spring(s).

- try flipping the wheel over. if the assymmetry change side, then the wheel is at fault. If it stays, it's the brakes.

There are these screws sticking out near the pivot. They help adjust distance from the rim. The further out the screw, the closer the pad to the rim. Careful messing with them though, I've had a few that were rather soft.

M.

Are the spring ends in the same holes in the mounting bosses?

Is each caliper arm mounted the same way? Many mounting posts have three holes for the end of the spring. Usually it is best to start with both springs in the middle hole.

What model V-brake is it? Most have a small screw that adjusts the spring tension of each arm. Screwing in the screw on the side that is flush and/or unscrewing the screw on the other side should center the brake.

The brake shoe washer arrangement is not really designed for centering the brakes, but to adjust the distance between the arms when the brake is engaged.

Unrelated to the centering, did you only change the calipers, or the brake levers too? Cantilever brake levers do not work well with V-brakes.

Use the adjustment screws first. Tighten one side and loosen the other. If that doesn't center them, then you may need to lubricate the posts.

before you do any of the above
make sure the wheel is straight in the dropout

this is the most common cause of mysteriously unbalanced v or canti brakes

As others have said, you center the brake to the rim, not the other way around.

Start by checking that the rim is centered between the seat stays. If not, it could be that the wheel isn't seated properly in the dropout (mount wheels with the bike on the floor, so gravity pushes it all the way into the dropouts), or that the wheel isn't dished, or that the frame is off center.

Once you're satisfied that the wheel is OK and centered (or that's the best you can get it) then center the brake by way of the balancing system provided -- small screws on the arm or an movable cam spring anchor. Some bottom end brakes don't provide for balancing, so you need to distort the spring by forcing the close arm open beyond the normal limit. This is a PIA tiral and error method, but does work if you're careful.

However, before bending springs confirm that your problem isn't something else.

if your brake has an external spring that engages the arm about 1.5" up, friction where the springs leg rubs on the arm can unbalance the action. A drop of oil there, or piece of teflon tubing slipped over the spring makes a world of difference.

Also check that the housing coming to the arm isn't long or short since that can push or tug on the arm restricting it's movement.

I started a similar thread a while ago and got a lot of good answers. I tried everything, and finally swapped the Tektros that were on the bike for some Shimano brakes, and the problem has gone away. They weren't the expensive ones either, I think they cost me about $20 at my LBS. To be fair, the brakes that were on there had gone through some tough times, but they were impossible to fix.

Also, ensure the mounts for the arms are greased.

M.

Very rarely matters. The overwhelming majority of brake arms pivot against a bushing in the arm, not against the post.

Same thing applies, the post or bushing has to be greased or oiled. Whether the bushing is oiled on the inside (post) or the outside (the arm) depends if the design calls for the busing to turn with the brake or not. (some aren't bound either way and will either turn or not depending on where friction is higher).

V-brakes are super easy to setup.

1. Install a properly dished wheel completely in the dropouts (to be sure, flip it over and confirm.)
2. When putting the arms on, make sure you're using the same spring hole on each side.
3. Put the same amount of spacer washers on each pad!
4. Install cable.
5. Use adjustment screws to center.

The only thing that can trip you up is if your housing is too long or too short causing it to push/pull the brake to one side.