Don't mean to discourage you from working on your own gun, but there is one other tool you really need to get and learn to use if you plan to order replacement parts -- a decent digital caliper. Then, when ordering, you can specify exactly the OD, ID lenght, thickness, number of coils, etc. of the spring or any part you may need as well as the size of your compression tube ID, etc.
Too many folks out there aren't knowledgeable enough to order the right part or size, and then they complain that the wrong spring or guide or whatever was sent to them for their gun and want their money back. The springs are so many and so varied, and manufacturers of air rifles, particularly the Chinese guns, seem to change the size and tolerances of the parts they use from time to time. So guys who make springs, guides and piston seals, like Jim Maccari, have a real headache trying to supply springs for air rifles just based on just the model name in many cases. That's why you don't see an extensive list of specific air rifles that custom springs and seals are made for, as it would be assumed, correctly or incorrectly, that the manufacturer guarantees that their replacement spring or seal will fit that brand and model gun. Sometimes it doesn't.
On the other hand, the guys who tune professionally who have lathes and grinders, and drill presses, and all the custom metal working tools needed can work around that sort of problem and fit the parts properly. Plus they have a wealth of experience knowing what works and what doesn't when they order parts. And many (most?) pro tuners make some of their own parts - fitted spring guides, top hats, thrust washers, etc..
So, if you need a replacement part, make sure you get the right size measurements of what you need and better still, try to talk to someone who knows the rifle you are working on and get some solid advice before you order.
