Over time I have collected a fair number of new and used 465 / 490 pistons of various oversizes.
Some are OEM Yamaha and some are Wiseco. Wossner also makes pistons for the 465/490 but I dont have any of these.
The Yamaha pistons are cast pistons while the Wiseco's are forged. The cast pistons expand less than forged pistons as they heat up. Yamaha specifies ~2.5 to 3.5 thou clearance in the various 465/490 manuals I have with the larger 490 piston at the upper end of that range.
The forged pistons are claimed to be stronger and lighter but require greater clearance or they will seize when hot. I
had trouble finding clearance specifications for the wiseco but I did find a application note from JE pistons that gave guidance on clearance for their forged pistons. The clearance they spec varies according to bore size and application but for a yz sized piston it seems to be around 4-5 thousands.
This seems to agree with what I see in my IT490 with 1.5mm wiseco piston. Knowing about the greater expansion rate of these pistons has made me more inclined to let the bike warm up before really cranking on it. I have not had any problems with mine but can understand how one might experience seizures if a wiseco was used with Yamaha OEM clearances.
With all these pistons around, I thought I would weigh them to see how much they varied across bore size and between vendors. I used an old triple beam balance that I calibrated against the weight of 10 pennies.
I measured each piston with rings, wrist pin and bearing (the wiseco rings are thinner than the OEM yamaha's).
Here is a table with results I have so far. Columns are listed for Yamaha, Wiseco and Wossner. Rows are for 0 through 8th oversize.
"Yes" means the listed oversize is available but I dont have one to weigh. Otherwise I list the weight in grams. Red indicates the oversize is not available for that particular vendor.
There is one case where I have a yamaha and wiseco piston in the same oversize (490, 2nd over). The wiseco piston kit is about 12 grams lighter than yamaha.