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Porting / Milling Cylinder Heads for GX340/GX390 & 420/440 Clones



 
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Our Price: $115.00


Product Code: PORTMILL


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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.


Average Rating: 5 of 5 Total Reviews: 1 Write a review »

  0 of 0 people found the following review helpful:
 
Milled Head = 10% Power Increase November 11, 2016
Reviewer: chuck from Gig Harbor, WA United States  
Quality product, with .100 milling and .020 head gasket gave me a 11.5:1 compression ratio and a measured 10% power increase on my 420 cc clone generator engine at 3,600 rpm. Power increase measured in max achievable wattage of the directly coupled generator. I run the generator on natural gas, and without the compression ratio improvement it was short of rated electrical power. Consultation with Geoff was very helpful on developing the right custom solution to my goals.

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