The smaller nitro engines don't have pistion rings. The sleeve has a slight taper to it so the fit is very snug as the pistion reaches the top of the sleeve. As your engine warms up the pistion and sleeve expand alittle and the compression goes down. This is why it is easy to turn it over when it's warm. Some of my engines have soo much compression I have to loosen the glow plug just to start it the first time. After it is warm I can start it very easily. I have even heard of people taking a heat gun to the engine prior to starting to make it easier. (I don't recomend trying this though!)

Your engine should be OK if you still have compression when it's cold. My brother has a few engines that have almost no compression and they still run great.

As for what hull to buy? Hopefully Ron can help. I don't know what one would be best.