For your info if your new piston has the two little holes on the aft side it's not for a GFR. It looks exactly the same as a GFR piston visually except for the holes, however ASSO say that this type of piston (Crono, CX, Apache etc) should not be fitted to a GFR engine as it is ground differently.
I'll guess that it's finished differently on the forward surface around the exhaust port area. As we know the GFR engine revs a little higher than the other engines and the area at the exhaust port will get a bit hotter hence the larger cooling channels in the barrel and different finish of the piston.
Asso's contract with Gilera for GFR piston's stated that Asso could only make those pistons for Gilera and not a third party which is why even uncle Bob can't get GFR pistons. If your only pottering around it probably won't matter but if your going to be hitting the red line a lot you should be aware!
Could we approach Gilera and ask them to get ASSO to make a small batch of GFR pistons?? I'll have two or three, any one else want one?
The GF piston is marked on the inside with ASSO W 2829
On the outside it has CX and the arrow.
Iam not shure, but i also think it has no holes....
I will check that next weekend.
As far as i remember the conrod of the GFR is somehow shorter as those of the others. So the position of the hole in the piston is also different.
Not much, but there is a sginificant difference.
So be carefull
In my experience both GFR and Crono/Apache pistons are marked ASSO W 2829. The later are marked with CX on the top and have the two holes, a GFR piston is marked with 93 (not CX) and has no holes. The distance from the piston pin to the top of the piston looks exactly the same by eye.
I have drawings for the Italia Motori 125 kart engine which uses the Gilera Crono/Apache engine (with different gearbox) and the distance between centers of the connecting rod is 106mm. There are aftermarket GFR connecting rods on sale and they are the this length? I will see uncle Bob to see if he has a GFR connecting rod to see if this is also 106 between centers.
The height of the barrels on both engines are also the same at 93.1 mm.
On the GFR main jet the book says the main jet is 170 although I have seen an early GFR with a 165 fitted, this early GFR's carb also had a 262ET nozzle but the manual says a GFR should be fitted with a 262 FV nozzle so I guess that bike was fitted with the same carb as a Crono as these specs match that. I would go with the 170 for safety as that whet Gilera say.