Depending on which equivalents list I look at I get two options for the auxillary belt for our 54plate 1.4 a/c Stilo. The most common reference is 5PK1150 but the original on the car is a 5PK1145. When I called at GSF to pick up their equivalent it was a 5PK1152.
Can anyone advise if this small size difference will matter, because I think there is no tensioner on this belt on the 1.4. -and my concern is that there might be some slip.
I seem to recall that the o.e replacement we put on my old 1.2 was extermely tight to get on which makes me think some leaway could be possible.

I would have thought that there was a tensioner. I remember some old engines had the tensioner built into the water pump. The vectras had a spring loaded tensioner. There must be a way of tensioning it. I would take the belt off and take it to a motor factor if you are after and aftermarket aux belt.

Personally, for the cost of an OEM part, I would buy one from FIAT. No hassle and it may cost you a fiver more, but peace of mind none the less.

May not be relivant, but on 2,4 engine the auxilliary drive belt (the one that runs fromt eh AC pump to the alternator) is a stretchy belt which has not sdjusters on the run. I have been after one for ages and have ended up going to FIAT who are also going to fit the thing as the belt needs a special tool to stretch it into place. On the price front, this belt on my 2.4 from Fiat is £11.20, so no great costs involved.