300?!?!? 350?!?!? No Way!!! I have a DA 100 that doesn't run that hot. My engine is running around 170-180 right now and last time it was checked it was 91 degrees out. I saw Dick's post and agree with him on most things, but on this one I have to disagree. I have found measuring temps very useful. It's helped me identify engines that were having problems, and by moving the laser around I have found hot spots where we found problems with cowl ducting before saving the guy an expensive rebuild. You can only set one so rich before it fouls the motor up. I also diagnosed engines that had a cylinder cutting in and out even though you couldn't quite tell exactly what (or which one) it was it was so intermittent. I did this by measuring (very carefully around that PROP!) the header temps...we found one that was drastically cold. It was obvious that the cylinder wasn't firing right then, but barely noticeable when the engine was running. I also use them to check and make sure the engine is running in the range it should be (the range it 'normally' checks at) if it doesn't then I know there is a problem developing, and I start looking. In the case of this post, it's obvious that using one caught another overheating problem, so I wouldn't say they were useless. It's only a temp tool though, and that's all it can be used for...it's not gonna tell you what's wrong...only the 'ole brain will help there.