Porting / Milling Cylinder Heads for GX340/GX390 & 420/440 Clones
Porting & Milling of the GX340/GX390 & 420/440 Clone cylinder heads is the single best seat-of-the-pants increase in torque you can acheive on the stock cam.
You can add milling to increase static compression ratio. Milling yields approximately a 1cc decrease per ~.007" removed. The reference numbers below are based on a stock engine:
- Stock = 41cc (8.75:1 CR)
- Milled 0.025" = 37cc (9.5:1 CR)
- Milled 0.050" = 33cc (10.2:1 CR) *Max for 87 Octane
- Milled 0.075" = 30cc (10.8:1 CR) *Max for 91 Octane
- Milled 0.100" = 27cc (11.5:1 CR)
- Milled 0.125" = 24cc (12.3:1 CR)
- Milled 0.150" = 21cc (13.3:1 CR) *Maximum Recommended
There is a Core Charge of $15 included in the price, if you return your old head, we will refund your $15 core charge.