This Inspection Camera/Endoscope by Nioh fit my small diameter needs at 5.5mm. Most of the other ones on the market are 7mm or more. I needed one that would get down inside a 6.0L Diesel engine.

This camera will certainly go where others will not. However, you loose a lot of light power by going down to this size. With the lights on max, there just isn't enough light to see clearly in a completely dark combustion chamber. As you can see in the video, I had to supplement the light with a high powered CREE LED flashlight. Not a big deal in some situations, but in many engine combustion chambers, there is not going to be a way to effectively get supplemental light in there. That being said, I don't see how more light could be added. The LEDs are decently bright for their size.

The included right angle adapter is pretty much useless in a dark environment because of the same low light condition.

Make sure you know when you buy this that you will need supplemental light. If you are going for a dark environment and you don't needs something this small, definitely go for a larger camera which will have brighter lights. This cameras limitations all have to do with being soo small and therefore I cannot blame it.