I'm trying to put together a list of what I need for my Sei;
It seems to me that my car model (1.1 SX) has to different type of 187A1000 engine: the one with the return pulley (you can see a "hunch" on the timing belt cover) and the one in straight line (so the cover is straightforward).
So it seems to me that my timing belt is the one with 104 teeth (and not the 125 teeth), but I have some problem with water pump: when it's not in a complete kit, I don't know how many teeth the sprocket has.....

From what I can work out, if you have the 104 tooth belt, the older type, you'd have the 46531183 water pump, which from a Google looks to have 20-teeth on the pulley.

You're right, but now I'm not so sure to have sort it right; probably my engine has a timing belt with 125 teeth.
My mistake was caused by this: On mister-auto website, some engine spare parts for the Seicento are listed as ("till frame number 9042883" and "from frame number 9042884").
So I was sure because my frame number is lower than that. But then I discovered that that number is not a frame but an engine number......
So now I'm trying to locate that one on my cylinder block.....

Mister-Auto?
Why not use the 'ePER' website link (at the top of the page) for the correct part numbers/description.

yes, you're right; I've tried that, but it doesn't work with my vin number, as I stated here:
https://www.fiatforum.com/tech-talk/447967-fiatforum-eper-gives-me-error-using-my-vin-number.html

I've tried using the manual search, but is not conclusive: you can't determine your exact timing belt unless you open up the car.....

by the way, just a curiosity: searching in Eper, I've seen that all Seicentos has only two type of air cleaner: square or rectangular. In any model there is a circular air cleaner like mine ("doughnut" filter, is it right?); how it can be? my car has only original pieces, I've bought it new.

Thanks.

well the easiest thing to do would be take the belt off and count the teeth. If it has the wrong waterpump for the belt it destroys the teeth on the belt pretty quickly and the teeth clearly don't fit the waterpump properly, like its visually obvious.

Whats your VIN number, I have a newer eper that i am sure it will work with

Yes, you're right. But at this moment I'm using the car, and certainly I will buy online spare parts; What I'm trying to do is to order all I need reducing wrong pieces and avoiding to stop using the car only to check the right piece.

The user Davren has given me a big hand checking VIN number for me, and I think now I understand what's happened.
The Seicento 1.1 was equipped with two different type of engine over time.

First,
76B2000 engine (also mounted on punto mk1) till number 9042883
104 tooth timing belt, 9.525 tooth pitch, 15 mm wide ( for example Contitech CT973)

second,
76B2000 engine from number 9042884
AND187A1000 engine till number 2934169
125 tooth timing belt, 8mm tooth pitch, 15mm wide (for example Contitech CT997)

third,
187A1000 engine from number 2934170
125 tooth timing belt, 8mm tooth pitch, 22mm wide (for example Contitech CT1114)

The confusing part was that on most sites, you can see the interval of engine number, but is not specified what type of engine. So when you read "from 9.042.883 to 1.290.423" you become a bit confused.....

Water Pump.
Best solution I think is to buy a timing belt+water pump kit.

What I have understood is that for the first case (belt with 107 teeth) a pump with a 20 tooth sprocket is needed. (for example

For the second case a pump with a 24 tooth sprocket is needed. (for example SKF VKPC 82250 is needed). the Fiat OEM code is 46526243. Oem code 46531183 is not really a water pump, but a complete kit (timing belt, pulley, water pump).
I've found this website
http://www.graf.it/en/
where you can put the water pump code of any maker (even the fiat oem code) and it gives you plenty of alternative....

well you certainly done your reading on it lol... just go by engine number and it will be right.