This may be a dumb question but it has been blocking progress on my current project . I went to the dealership to grab a couple of these retaining clips at $10 a pop, and I can't figure out how to lock them in place. I've been using needle-nose pliers, and occasionally a hammer and punch, to try to get them in place but to no avail, it's only marring my thermostat housing.

I've seen that you're supposed to push down on the sensor/sender. I've tried that, and wiggling in all directions. Going slow, fast, hammering it, easing it in. None of it works.

The clip will fit easily in the blank space where it's supposed to go without the sensor/sender there. It also clips easily onto the space on the sensor/sender. However once it's in the unit it simply won't go on. It'll go about halfway, then when i touch it it'll slip loose again.

I'd really appreciate some help on this silly little task I can't seem to grasp ... It's the only thing holding me back and it's making me extremely frustrated!

Thank you!

 

Not sure, I have only done 2 of these(so 4 sensors) and they were not that hard to slide in.
I used original clips and only 1 new sensor but it was fine.

Push and hold sensor down and push clip in

Try wiggling the sensor so it goes down as far as possible, they do need some pressure since the only seal is the pressure on the o-ring from the clip holding it in place.

Good look at this part here: How To Replace the Upper and Lower Thermostat Housing on a Ford 4.0L V6 SOHC Engine
Step #5

 

I feel dumb now but after a few hours of frustration the other day, yesterday I managed to insert the clips on the first try. What I did was work one prong of the clip all the way in first, then work the other in - as opposed to forcing the whole clip in at once.