I hate to openly disagree with a poster but 20-30 hours on a TT (or any other engine as a matter of fact) is not a reason for a piston and cylinder liner change. However, a lean running engine (not run 300-400 rpm shy of max lean rpm) may experience such a shortned life.

Besides checking for airleaks and possible fuel line problems, the first thing I'd do is install new glow plugs in your engines-OS A3. It is not unusual for a glow plug that is going bad to cause the top end to drop off a bit.

Also, if you cleaned the engines real good (sonic bath) it is possible this caused your problem. Let me explain that I am assuming you burn a fuel that has some castor in it. It is not unusual for castor deposits to form in the engine that actually provide or improve engine compression. I had a perfectly good engine that ran fine that I "killed" when I cleaned it in antifreeze. After the cleaning the engine had no compression. The gastor gunk built up in the piston/cylinder assembly actually provided most of the compression seal. I wish I had never cleaned it!

If you suspect engine compression, here is a neat little tip that can add a little extra life to you engine (actually two different methods of doing this) without having to buy a new liner.

Method 1: Take a (very) small pipe cutter and cut/notch a 360 degree groove just under the top of the piston.

Method 2: Remove piston and using a hammer and a punch, punch the piston from underneat. (lay piston upside down on a wooden work bench). Install piston in cylinder, squirt a little honing oil and rotate piston to ensure nice fit. Reinstall all components.

Good luck and please let us know if you solved the problem.