Yeah a safe temp is 270 but I wouldn't go higher (it also depends on the materials that the sleeve is made of so some engines could do great at that temp and last a long time). When you get up to about 300 degrees the sleeve expands and just doesn't quite shrink all the way back like it normally would after it cools making your engine lose compression sooner. Try to take the sleeve out of a worn out engine that ran at 270+ degrees for its life... its going to be very tough because the sleeve expanded against the engine block and stayed that way... when you overheat an engine so many times you just lose more and more compression. If an engine runs cold the sleeve wears out because material is taken away from it because of the friction of a cold, tight sleeve. Thats why you should run your engine anywhere between 190F to 270F and no higher or lower.