Modern Vespa

There are no arrows on the drive belt for direction of drive.
Which way do I install or does it matter?

Look for the seam on the top layer of the belt
If you mount backwards the tail on the seam can peel apart

generally if a belt doesn't have directional arrows common practice is to install it so that you can read the numbers (or letters) as if they are on a page.

basically, align the numbers to be on top, and if they read "normal" then you're good to go.

How is the quality of Dyco belt compared to original one? I saw it is way cheaper.

First of all, it is Dayco.
Next, a buddy bought and installed such a belt to his 250cc Peugeot (Quasar engine) and after 1000kms WOT the belt died and filled the CVT with rubber and fiber.
You know... A new original belt just installed is one of the best feelings in the scooter world. Don't deprive yourself.

Thanks Jah, will stay with original.

I have had this conversation more than once with my local Vespa agent. The GTS service schedule says 15000km to change the belt. We looked at mine then and closed it up again, still with the OEM belt on. At 20000km I told them I wanted to change it and they agreed that discretion was the better part of valour and they put a new OEM belt in. The old one had worn less than 1mm in width and looked as healthy as ever except that a little of the writing was missing and they think that happens when the belt gets further out on the variator and can just touch the casing occasionally. There was no real signs of belt dust or junk in the casing so I am comfortable that we made the right decision. They said they have seen some of the big name after market belts fail before 10000km! That's enough for me.

Are you referring to Dayco, the OEM provider to Ferrari and provider of original drive belts to Piaggio?

If someone put a Dayco belt in a 250cc scooter and it failed in 1000kms, something else was broken or put together wrong, or it was an alternator belt for a 1996 Toyota. Was this buddy using rollers or sliders? Did your buddy buy it on Aliexpress? Did your buddy do the work himself?
If Dayco belts are good enough for Piaggio and Ferrari, they're probably pretty good.

And yes, a lot of OEM Piaggio belts are made by Mitsuboshi. But not all of them.

About the same. Dayco makes some OEM belts for Piaggio. And I read somewhere on this thread that they also are an OEM provider to Ferrari.
What often happens is that people buy the belt because it is cheaper, and they do the work themselves with no previous experience, or don't replace other wear items and have this sort of premature failure.

Stock rollers. Don't ask me about worn sliders or whatever else, I don't know.
He bought it from eBay (Dayco 8203K was (and still is BTW) listed as a replacement of the stock belt PI843963). Yes, he did it himself, just as several times before and several times later. He travels a lot and prefers fresh belts.

The "best" part about Dayco 8203K is that its dimensions differs a bit from the stock belt.

First off, ignore the dimensions given on what appears to be your favorite site for looking up scooter parts. If you have the Piaggio 82941R belt in one hand and the Dayco 8203K belt in the other, and you compare them, they are the same size. Piaggio doesn't publish the dimensions of their belts, so someone measured one and came up with the numbers you see on that site. I see the same nonsense with Kymco belts all the time. The dimensions someone came up with by measuring the belt are not the dimensions the manufacturer might come up with.

An OEM Piaggio belt for a Vespa 300 costs a dealer about $4-5 more than a Dayco belt. And we know Piaggio likes to mark things up a little bit. So the Dayco belt isn't actually a cheap belt. It's sort of like buying NGK spark plugs in NGK boxes instead of Piaggio blue boxes. One is $4 and one is $12. You are paying for the product and not the package someone put it in.

The Dayco belt that fits the Satelis is 8265K or 8203K. They are both the same length and width as the OEM Piaggio belt. 8203K is identical in size to the Piaggio OEM belt, and 8265K is a little thinner, like the Malossi belt. If he was putting it in a Satelis, it was the right belt. And again, Dayco is an OEM provider to Piaggio, Ferrari and Ducati. There is a possibility that if a timing belt breaks when you run a Ferrari wide open, the repairs will be more expensive than when a drive belt breaks on a Vespa. Ferrari probably takes that into consideration when deciding where to source parts.

Then there's the whole Ebay thing. But we'll save that for another day on the soap box.