<p>I have a <a href="https://www.monoprice.uk/products/monoprice-maker-ultimate-3d-printer-uk" rel="nofollow noreferrer">Monoprice Maker Ultimate 3D Printer</a> and have tried to replace the nozzle.</p>

<p>The nozzles I bought turned out to be too small.</p>

<p><img src="https://i.imgur.com/iYnMTVf.jpg" alt="Imgur"></p>

<p>What are the important specifications of a nozzle?</p>

<ul>
<li>Thread size</li>
<li>Thread length</li>
<li>That plastic tube thing?</li>
</ul>

<p>Monoprice is very bad at publishing the specs, can I work it out with a caliper?</p>

What part fits?
A replacement nozzle needs to fit 3 parameters:
Thread diameter and pitch need to match up, to allow mounting
Thread length should be close to the original to allow secure fastening
The style needs to fit: there are quite some styles of nozzle - most are not lined, yours is PTFE lined to the nozzle (see also
Can the filament tube be outside of the nozzle?
)
Monoprice nozzles are
not
compatible with what is known as Ultimaker Mk8 or E3D style (which you bought). They are Ultimaker Mk10 style.
What's a good nozzle?
Now, what separates a good replacement nozzle from a bad one?
good machining to leave no burs and a smooth interior.
a good inner geometry that allows easy flow
outlet hole is to size
Finding premade replacement parts
As a first measure to not get the wrong replacement parts, make sure to add the manufacturer of your printer to the search and then check the thread diameter if given. In your case, you might have to add Monoprice or Toymaker, as those use this style of nozzle.
Reverse engineering a Nozzle
Now, which measurements do you need to reverse engineer it?
nozzle front pitch angle
hex head flat-to-flat & hight
recess diameter & hight
screw shaft relief diameter & hight
thread outer diameter & length
inner bore diameter at entry (and in case of a lined one: after the step) & corresponding depth of drilling
amount of chamfering
With these, it's possible to do do a CNC model or a sketch of the outside and produce pretty much blanks or shells on a lathe that just need their last little bit of drilling... and here comes the tricky part: till now, all could be accessed from the outside. We are missing one profile though: the last piece of the inner bore geometry.
This one can't easily be measured, but if one can push some plastic in, let it cool and then pull it out, one might get a molding of it, which might allow to reverse engineer a fitting drill for the last piece.