hehe, no worries matey.
The powertube is inside your BT4. When you open the marker up you have a black plastic section bolted to one side. That is the powertube, to remove and check it fully, you need to undo two screws on the right hand body shell. If it is damaged it is fairly cheap to replace, but it is worth bearing in mind that you can get an aluminium one to end any future cracking worries.
If you have a look on youtube, there are some good tuts for doing things like the cvx (valve) tape mod and polishing the internals, both of which help efficiency and performance. Some vids go into good detail so you can see every step.
The cvx is the valve inside the powertube, the clearance is quite big between the outer of the cvx and inner of the powertube so a single layer of electrical tape gives a better seal, you must remember to cut the air holes that are on the valve though and don't stretch or overlap the tape.
Polishing the internals is just that, get some wet and dry metal abrasive paper and give the inside of the marker a good seeing to, anywhere there are parts or o-rings etc moving against the inside of the shell. In the factory, during assembly the markers are painted black. this is sprayed on so there is a little overspray, which unfortunately coats the inside of the marker causing unwanted friction. Polishing the internals cuts it down and helps the performance of the marker.
