Nuts and bolts do not like being assembled dry, especially if they are tightened firmly. Depending on the parts involved, use either grease (e.g. white lithium or Phil Wood), anti-sieze, or threadlocker (Loctite or equivalent) on the threads. It may also be a good idea to take everything apart once in a while to make sure everything is OK and get a chance to put on some more anti-seize. When you put on the pedals don't forget that the left pedal goes on in the opposite direction (counter-clockwise). Keep your wheel true and your bolts tight!
Cutting the seatpost
If you buy a muni, chances are that the seat post will be too tall for you. The procedure of cutting a seat post is trivial and can be done by anyone with access to a metal saw. Do NOT put any part of the unicycle in a vice! Hold the seat post in your hand and cut. File away the sharp edges. Finally put on some grease (anti-seize may not be a good idea here).
There may be many different sources of noise on a unicycle. The most common are the cranks, the pedals and the spokes.
It is very common that there is a creaking noise from the spokes. If you tighten your spokes evenly with a spokes wrench however, the noise level will most likely subside.
Hold the pedal and wiggle it around. If it moves it may be loose. Look for movement where the pedal attaches to the crank. If it is loose, remove it and apply anti-seize or Loctite and tighten it - hard. Notice that the left pedal goes on in the other direction!
A pedal may also be loose on its spindle. Resolving this is somewhat more difficult. See the pedal section below.
If you hop often, the pedal may start to feel loose. This isnt because it is loose, it is the bearing breaking down.
Hold the crank arm and wiggle it. If you feel a click then the crank arm may be loose. It is easy to detect this if you idle with the pressure on one foot.
Splined hub/crank maintenance
It is very hard to actually break a set of splined cranks but unless they are regularly maintained they will wear and show movement on the spline. This goes for all types and makes of splined crank sets. This is because the splines are not tapered but are straight and rely on the precision of the fit to keep them from moving.
Other cranksets can be maintained in a very similar fashion depending on design.
The spokes make a characteristic sound when they are loose.
To tighten the spokes you will need a spoke wrench. It is of utmost importance to have a high quality spoke wrench that fits snugly onto the nipples! High quality spoke wrenches are made by Park Tools. If you have a low quality spoke wrench it will round your nipples and make it impossible to further adjust the nipples without removing the tire. If you notice that the wrench slips, stop immediately!
To tighten a spoke you have to turn the wrench in the opposite to the direction that would be intuitive, i.e. counterclockwise. Think of the nipple as a screw going onto the spoke and you will understand why. If the wheel is true it may be sufficient to go around the wheel and tighten the spokes one by 1/2 a revolution, or in case they are very loose by a whole revolution or more.
If you have a weak rim, a spoke can sometimes gradually enlarge its hole until it pops out of the rim. The spoke will not stay in place if you try to put it back into the rim. A good temporary fix until you can purchase a better rim is to take the nipple off, insert a washer over the hole, replace the nipple, and tighten the spoke again.
The wheel is not centered in the frame
Several things could be wrong.
1. One side of your fork could be longer than the other, this is unusual, more often the fork legs are not correctly aligned but these can be straightened quite easily. If the fork legs are different lengths this can be easily fixed by putting shims between the top of the bearing and the bottom of the fork. One could use strips of aluminum soda can as shims. The basic idea is to use shims to add length to the shorter side of the frame. Here are the step by step instructions:
Remove the main caps, but leave the wheel in the frame. Spin the wheel. If the wheel is closer to one side of the frame consistently, than try removing the wheel and putting it back in the other direction. If it's off center to the other side, than your wheel is dished, and that's a different problem. If it wobbles back and forth while it spins, than it's out of true. If it looks just like it did before you turned the frame around, than you need to shim it.
Remember the shorter side where the frame is closer to the wheel. Remove the wheel from the frame. Cut out pieces off soda can to fit in the top of the bearing holder. Depending on how bad it is, you could need 7 or 8 pieces. Put the shims in the bearing holder attached to the shorter side of the frame. Put the wheel in the bearing holders on top of the shims.
You may have to bolt the main caps back on to the frame before you know if you've put the correct number if shims in, because they compress a little when you tighten the main caps. Add or remove shims to get the wheel as close to centered as possible.
This been used on several unicycles and it has not caused problems.
2. Your bearings aren't centered on the axle. If your bearings have a press fit like on a Suzue hub, they may have slipped. If you have a Profile hub or something with a similar design, having a different number of spacers on each side of the axle will cause this also. This will probably be addressed by John Childs if you ask him.
3. Your frame could be bent. This is not very common. You may be able to bend you frame back, or possibly use the shim fix. If not, buy a new frame.
Bearing holder tightness
It is important not to overtighten the bearing holders since this may damage the bearings. It is easiest to do this by trial and error. Tighten the bolts on both sides equally using the fingers only and then turn the wheel by hand and observe how freely it spins. Now continue to tighten the bolts evenly until the wheel comes to a stop more quickly. Now loosen the bolts slightly on both sides just to make the wheel spin freely again.
If you have a heavy wheel, the wheel may spin even though the bearings are too tight. If you can flex the frame, then you need to tighten the bolts more.
It is surprisingly easy to damage the threads on the pedals. To avoid this you should always use only your hands for the first few turns as you put on the pedal. Make sure that there is some grease or even better some anti-seize or Loctite on the threads. If the pedal does not go in easily, you may be on your way to crossthreading it! If you have crossthreaded the cranks or the pedal you will most likely need to replace these parts. The pedals should stay tight once they are on.
In case the pins fall out of the pedal you may use regular screws that you get from a hardware store, or real replacement pins from a bike shop. Use Loctite if they get loose. Sometimes the threads take damage when a pin falls out, and there is not much one can do about that, except avoiding pedal grabs.
If you suspect that the pedal is getting loose on its axle, i.e. clicking and feeling loose, it may fall apart any second, and you may not be able to find the balls if they happen to fall out.
Unsealed pedals may become filled with water and mud. In case this happens you have to take off the plastic cap and remove the nuts. As you bring the axle out of the pedal, keep an eye an the balls as they tend to fall out. Fill the pedal with grease and put it back together. It is very easy to overtighten the nuts, check that the pedal spins freely as you tighten the nuts. Most pedals come with little grease since they spin better that way.
A saddle usually either breaks at the seat base or the seat cover develops rips that cause the padding to fall out.
If you know that you are likely to hit the side of the saddle onto concrete, it would be wise to be able to protect it. Either you can buy a KH Fusion seat cover or simply cut off a section a tube and stretch over the saddle. Segments from used 2.125" tubes are known to work. Cut off a segment, and heat it, stretch it, repeat a few times and then try it on. The front sides of the saddle take the most damage, so putting just in front of the seat post is a good idea. It is good if the tube goes all the way to the handle, that way it won't slide back as easily. The tubes will break eventually, but they are easy to replace. Trying to fix a rip in the may not be worth the effort. Maybe glue would work.
If the seat base fails, you either live with it, buy a new seat base or save up for a Carbon Fibre seat base. The Carbon Fibre seat base will in addition make the seat a lot stiffer, so you can exert force directly to the frame.