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You wouldnt catch me running zero deck height either. You could probably get away with it but that is way to close for my liking. When spending thousands of dollars building an engine I'd rather risk a little power and keep it relaiable than have the piston hit the head and bend the rod.
 

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The only motor maker I've seen that runs a zero deck height is KTM. Not yamaha, not honda, not kawaski, not suzuki, not polaris. Not saying they aren't others out there, I can only say what I have seen.
 

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I remember having this problem and asking with my 12.75:1 cp. Shorter rod is mentioned which is right, but im bored and want to break it down just a little more.

RSR Static Compression Ratio Calculator

Using this and an 04 spec yfz, i dont know the head cc for sure but it doesnt matter. Make up numbers to get to to yamahas rated 11.9 compression.


Bore = 3.7402
stroke= 2.4409
Head gsk = .025
Deck = .03
piston volume = 8.9
head cc = 21.5

you get 11.9

add 1.4 stroke of 06+ .7mm higher up and down. But yamaha shortened the rod 1mm. 1mm- .7mm is now .3mm or .0118 lower in the whole than 04-05

add .0118 to our deck height of .03= .0418. now use .0418 as deck height instead of .03 and look at the compression. now you need to add .25 compression from added stroke no matter what. = 11.59.
Its .3 lower for 06+ but yamaha shows .5 lower... so there must be a .2 loss compression from the head in 06+

final answer a cp 12.75 in an 06 will be .5 compression lower than in an 04-05. cant say for sure what it is in any given engine depends on gasket thickness.

And i dont remember what deck height cp said they use to get there 12.5 in an 04-05. I did ask but dont remember. So final could be less than whats rated depending what deck height they built it around.
 

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:lol 1cc more volume in the 06+ according to the calculator. Anyone want to measure 2 heads to confirm? lol
 

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Pretty good bence.

The 06+ head has unshrouded valves compared to the 04/05 heads so that would account for 1cc volume change in the 06+ heads.
 

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"This was configured with a 22cc head, -.028 deck clearance, -.030 gasket.

Best Regards,

Tony Calvert"
 

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hello guys i want to get a 13.25:1 cp piston for my 2007 yfz450 se but want to stay on pump gas and by the looks of it on this form if i get the 13.25:1 it will be a 12.5 right and will i be able to run 91 on the 13.25:1 cp piston.
 
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