Problem tightening bolts securing heater core pipe under dashboard

[Lamar-2.6]

Hi,

I had a coolant leak from the heater core pipe/flange connection under the passenger side (UK model) dashboard, so I ordered a new o-ring to fit to see if that's where the leak was coming from.


https://cdn4.pelicanparts.com/techarticles/Mercedes-190E/44-WATER-Replacing_Your_Heater_Core/images_med/pic09.jpg


I removed the two bolts holding the pipe to the heater core and fitted the new o-ring in place, but now the bolts won't grab back onto the nuts that are held in those grooves on the heater core end. The bolts push all the way in as far as they can go but it's as if the nuts aren't there.

I can't understand what's happened. The bolts are meant to be taken out in order to replace the o-ring if necessary, so they are meant to go back in again, and the nuts are meant to stay in place in those grooves because there is no way to get at them without removing everything as if you were going to replace the heater core. I have a new heater core sitting around just in case I need one, so I can see how the nuts are held in place and they shouldn't be able to fall out once the bolts are removed.

So it's very puzzling. Has anyone had this experience, or can make a guess what's happened here?

Thanks.