This sat I put in a Zero-Start frost plug heater in my 2000 4.3 4dr 4wd Blazer. Not much room to go trough wheel well even after removing tire. However its done and I hope it doesn't leak. I wonder though, how are the aftermarket heaters that install with an O-ring and butterfly type of clamping device. Different from OEM instalation. I'm thinking about putting one on my S-10 pickup and my mazda.

 

There are only two different designs that I have seen before. One is like you describe with a large o-ring and a butterfly clamp which expands to seat the heater body into the block. The other had two plates which fit into each other and squeezed an o-ring out to seal and hold the unit into the block.