HF has two models. The more expensive model has a very small head that can go through the spark plug hole so you can inspect the piston/cylinder.
I have both models, but the more expensive one with the smaller head I purchased at Costco (made in the same factory but different brand). I was just at Sam's Club yesterday and they have one right now as well (can't remember the price).
Watch the Harbor Freight sales as they usually have one of these units on sale at least a couple of times a year.
I initially thought I was going to sell the large-diameter head unit that I bought at HF first. But I decided to keep it, because the flexible stem is larger and more rigid - you can extend it out and it will stay - this is really handy when you are trying to inspect inside wall cavities, furnace ducts, and the like.
The smaller-diameter camera head will fit into a spark plug hole just fine, but that stem is much more flexible and won't self-support for more than a few inches.
