I have just ordered one of those 51816520 clamps from Desira FIAT, for collection tomorrow.
This is the clamp for a 500; it won't fit a 2009 1.2 Panda without modifying it. I once bought one of these, and only found out it was the wrong part when trying to fit it. Read on.
There's some misinformation in this thread.
The confusion has probably arisen because the 60HP 1.2 Panda exhaust, whilst looking identical to the 500 one, is actually a different diameter throughout. None of the sections, clamps or brackets are interchangeable, even though they look very similar.
The 51816520 clip is for the 500 (and possibly the 69HP Panda, and quite a few other Fiats and Alfas). It secures the bottom of the manicat to the bracket bolted onto the gearbox. It has nothing to do with the connection between the front pipe and the manicat; this is done with a separate clamp, welded to the pipe on OEM exhausts and often missing completely after a few years, but easily replaceable with a simple U clamp.
It's important that this clamp is fitted and is in good condition; without it, any movement of the exhaust can stress the manicat and it will likely crack at the exhaust manifold end - awkward and costly to fix.
51816520 has too large an internal diameter (it's loose on the pipe even when fully tightened), and the bolt holes won't line up either - it won't fit a 2009 1.2 Panda without modification.
Whilst the 51816520 clip is readily available, the similar but slightly smaller clip for the Panda (sorry I don't have the part number, perhaps
@koalar can help?) isn't. I replaced one last year, and couldn't source the correct one for the Panda anywhere in the UK, at any price.
What I did was to take a 51816520 and modify it. You need to make a shim to wrap around the pipe to pack out the connection (I used a piece cut from a thin galvanised strip from a carpentry
joist hanger), and you need to elongate the holes in the clamp in the direction towards its centre. I used a dremel with appropriate grinding and cutting accessories.
Make sure the clamp fits freely and doesn't stress the exhaust when you tighten it up, or you may have problems later on. Also make sure it holds the pipe securely - this will need shimming between the pipe and clamp if you use 51816520.
Alternatively you could just make something up from whatever you have lying around, but it needs to be sufficiently rigid to hold the pipe securely without stressing it. The OEM bracket is surprisingly strong.
Given the speed with which these usually rust away, coat it with anything you think might help protect it, but remember it gets quite hot in service. For some reason, it's only the part which attaches to the exhaust that seems to suffer; the bracket attached to the gearbox is usually solid, and even the bolt is likely to be reusable.
I've got a picture of the finished job somewhere; I'll see if I can dig it out.
Edit: found it, and attached it. If you look closely, you'll see that I've elongated the bolt holes toward the pipe side of the clip; there's also a steel shim wrapped around the pipe inside of the clamp, right where you can't see it.
Also, if you fit an aftermarket exhaust, it likely won't come with the integral clamps between the pipe sections, so you'll need to also buy a couple of U clamps of the correct size.
