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  1. #1

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    Milling 63m head. How much equals 150 psi??

    I am a second round newbie to this site. I was active on here back in 07 when I had a RXP but now I am giving 2 strokes a go and have read almost every thread you can read over a 63m but still have a few questions.

    I have seen everyone say not to mill the head to get over 150 psi on 91/93 octane. But I have never seen how many thousandths you have to mill to achieve this. Also I do not like running ethanol gasoline so I am considering running 100LL from the airport which is a mile from my house. If I was to do this. How much compression could I run 170-180? and again, how many thousandths have to be shaved off to achieve this compression?


  2. #2
    Yamaha artisan Cutlass's Avatar
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    You won't gain enough performance going over 150psi to justify doing it unless you start doing other power mods like porting. All you're doing is putting more heat into the engine. Stick with the 150psi limit if possible. They "general" rule of thumb is .010" for 10 psi. But you can only mill so far before you have to have the squish band re-cut. You could just mill it down to knock off the lip edge. If you look at the head, you'll see the squish band ends and there is a little lip up to the sealing surface. Mill down that far and it should bump your compression up about 20 ish psi...maybe.
    If you want 150 or higher, then you'll have to send it off to someone who knows what they are doing.

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  4. #3

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    Quote Originally Posted by Cutlass View Post
    You won't gain enough performance going over 150psi to justify doing it unless you start doing other power mods like porting. All you're doing is putting more heat into the engine. Stick with the 150psi limit if possible. They "general" rule of thumb is .010" for 10 psi. But you can only mill so far before you have to have the squish band re-cut. You could just mill it down to knock off the lip edge. If you look at the head, you'll see the squish band ends and there is a little lip up to the sealing surface. Mill down that far and it should bump your compression up about 20 ish psi...maybe.
    If you want 150 or higher, then you'll have to send it off to someone who knows what they are doing.
    Who here can I send it to? also if i wanted to buy another head and have it milled and swap them in a weekend so I am not down for a while. What do 63m heads cost?

  5. #4

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    also why do people running race gas go up to 180-200 from what i have seen if there isnt much gain?

  6. #5
    OzarkYamMonster's Avatar
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    Hi welcome to the Hulk. Now that you made the pass to 2 strokes you just contact the guys (Richard) @ WFO performance ... they are on GH here. They are experts on Yammy 2's Good Luck:

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