replacement piston for yz 125

[A-310]

I just replaced my oem piston with a wiseco piston. I noticed that the oem piston had a hole on the intake side 1/2 inch by 3/4 inch. The new wiseco doesn't have this hole. I ran the bike and it ran well, but had to richen up the main jet two sizes. Anyone know if there was a reason for the hole on the intake side of the oem piston? Thanks

Harling

[Aus_Rider_22]

That is normal.

My Vertex has a rectangular hole with circular corners. Look below to see if it's what your talking about.

[A-310]

ya, that's the same as the one I replaced. The wiseco did not have this hole. I'm just wondering why?

[Aus_Rider_22]

The Wiseco piston you got it forged. It is called a "Prolite" piston.
It is very light from being forged rather than cast (OEM, Vertex)

So Wiseco can make it stronger and more rigid by removing the whole. Yet, the piston is still very light as it is forged.

That's my understanding anyway.

[Crogg]

Originally posted by A-310
I just replaced my oem piston with a wiseco piston. I noticed that the oem piston had a hole on the intake side 1/2 inch by 3/4 inch. The new wiseco doesn't have this hole. I ran the bike and it ran well, but had to richen up the main jet two sizes. Anyone know if there was a reason for the hole on the intake side of the oem piston? Thanks

Harling

The reason for the hole is performance. It is used as an intake port and also a transfer port. It lines up with the keyhole port or rear transfer port. It enhances mainly the midrange and top end of your motor.

[FFRacing11x]

In addition to what Crogg has posted, the position of the boost port determines what part of the powerband it affects. The higher up the skirt the port is positioned, the higher up the rpm range it will help. It is all in the port timing.
if I wanted a topend screamer, I would position the port as high as possible. The stock position on the OEM piston benefits mostly the midrange. I have cut them as close as 2mm below the ring land in my 26hp 80s. Cutting them too large will be like keeping the intake valve open too long on a 4 stroke, and performance will suffer.

[char393]

I have what may well be a stupid question then.

With the Wiseco piston are you forfeiting some midrange to top-end power? I mean, being that it doesn't have the port?

Not trying to get this into a stock vs. aftermarket war, just curious, if you are losing some power, what would be the advantage of an aftermarket piston, such as wiseco...Assuming, that you're not using an oversized piston?

[FFRacing11x]

The Wiseco with no window will be tailored for more bottom end. It can be modified to suit different needs.

[brokenbones]

I believe in the instructions that accompany the Wiseco piston there should be instruction for drilling a hole.

[yzjohn89]

tell me if yall find a 05 yz 125 aftermarket piston, i need one!

[Red_Thumper]

Originally posted by yzjohn89
tell me if yall find a 05 yz 125 aftermarket piston, i need one!

The '05 information is not out from the aftermarket suppliers. Now is about the time they are all replacing catalogs.