Ok, this is an old thread but having just replaced the thermostat TWICE within a week I'll feel fairly experienced at doing this job?
I'll explain twice at the end but till then points to note are:
o To drain system you have to take main under tray off AND the small front black section linking to the from bumper. This section has screws and 4 "plunger rivets". You have to remove this to get to the radiator drain tap on the lower RHS.
o Battery & batter box have to come out as well as the turbo intercooler to inlet oxbow air hose (humped/bent hose)
o If you are careful there is no need to disturb the intercooler heat exchanger connections
o On the bottom back corner of the thermostat is a stud with a M10 nut which secures two water pipes. Take this nut off.
o Lift the front edge of the water header tank up and extract from securing bracket
o Back to the small metal pipes. Follow the outer RHS black pipe past the thermostat towards the front of the car up to the cylinder head. Remove the M10 bolt securing it to the front of the cyl. head. You can now move the outer pipe to the side. The inner pipe (stainless/silver?) is a little more difficult to unship from the stud. Gentle levering and a little bending of the "braket/tab" is required but once free pipe can be moved down.
o Hoses. There are 4. Three at back of thermostat (1 small, 2 medium) and one large at front. If you have the proper "Hose Pliers" you can remove AND replace the clips on these three hoses. However the big front hose is a remove/destroy and replace job.
o If you have hose pliers then the rear two larger hose clips should be easily remove. The smaller 10mm hose clip access may be obscured by the temperature sensor so you can just remove the sensor to get better access/aspect/approach angle.
o This smaller 10mm hose travels upwards to a pipe across the top of the cylinder head which is secured by three 10mm bolts. I suggest you release these as well to give more play.
o There is also a crankcase recovery tube, next to the oil filler cap. Suggest you unship this as well.
o Finally you may find it beneficial to uncouple the quick release vacuum hose that do to the brake master cylinder.
o The two M13 thermostat bolt can now be accessed with 1/4" drive 13mm sockets. 1/2" are too tight. If you have a good memory then visually note, or take a picture of the exact position of the thermostat before removing the bolts and extracting the thermostat. Spacing is tight, visibility restricted/narrow and thus accurate replacement is much more fiddly.
Right now for the IMPORTANT bit.
If you look at the original Fiat thermostat housing the "O" (actually square ring) sits in a groove and there are pips/clinch points to help retain the ring in the groove.
I used a GATES replacement that lacked these pips but the ring fitted snugly and looked secure and did not drop out.
When refitting the thermostat (on my own) I had several attempts to get the bolts & holes lined up. Eventually got there and then finger tightened the bolts before alternately 1/4 turn tightening down to "bight/stop" point. This is when I discovered/felt that the top bolt had become firm but the bottom wanted to accept more turns. Something didn't feel right but as we all know when you have struggled to where you are at backing out can be the last thing on your mind.
Anyway, torqued up to 2.5daNM (25NM), refilled, bled, test drove, checked bleed/topup etc. and all OK. Checked several days later to find header tank almost empty and fluid all over the top of the gearbox.
Fearing I may have crushed a slipped "O" ring I bought a complete new thermostat and then proceeded to strip down again. Yes the "O" ring had slipped by the smallest of distances. 95%+ was neatly in the grove but a 10% inner edge had been pinched and crushed thus affecting the overall seal quality.
Second time round I did the following.
1) put 6 small dops (with paper clip wire end) of super glue into the thermostat housing groove, fit "O" ring and then leave face down on flat sure under gentle pressure
2) get another pair of hands to help hold the hoses back, hold a mirror, etc. so you can directly align the thermostat without the need to slide around looking for the holes as this sliding/realignment process is most likely to disturb that pesky "O" ring.
Now I could have been unlucky with the first attempt but I have to say the wife's helping expert hands did a good job!