KX Guru Racing

After speaking to Stewart about the window here is the scoop.
Yes, he gained 3hp on the top end windowed vs non windowed.
Remember, this was a heavily modded engine generating a lot of heat.
Why the gain using a window?
Heat dissipation especially on the piston crown...
If your setting a land speed record, desert racing, Kart racing, Supermoto, or similar (Wide open for extended periods) then you need a window.
If your a trail rider or only see wide open throttle for short periods of time then you likely will not need it.
It will not hurt to have one as far as what Stewart could tell.
My piston is windowed and I run mine in the desert, dunes, and Street Tard.
My engine has a lot of hours on it over the last 5 years and no issues.
So, I will continue to run a window just cause it's cool

Here is some measurements for a 89-04 KX500.
From the top edge of the piston to the top of the port .900"
Width: .905"
Height: 1.000"

10-23-20 UPDATE:

So, as with all things Moto... there is more than one way to do something.
Here is some new window specs that have proved to be just the same HP wise.
From the top edge of the piston to the top of the window .970".
Window height 1.160"
Window width .840"