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Discussion Starter · #1 ·
I recently dusted my cylinder cause my air filter fell off. I want to run an Athena 480 kit and a cp piston. I also want to be around 12.5:1. My question is what gaskets should I run to get the 98mm 12.75:1 to stay around that compression. I know this topic has been beat to death but I couldn't find what I was looking for. Any help appreciated.
 

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Discussion Starter · #3 ·
I would rather just do it with gaskets. I know it shouldn't be that hard I just don't know all the measurements and how to equate it to compression. On a side note has anyone put a reinforcement ring in there fci intake where the filter clamps on to the tube. Mine was puckering it so I thought it was too tight. I loosened it and my filter fell off and didn't notice it for a few hours. Didn't know if anyone had the same issues.
 

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don't try using thicker gaskets to decrease compression !

you need the tight squish clearance for combustion efficiency
( knock control ) !

that 12.75-1 piston in a latter year (06-08 YFZ) will be around 12.25-1 anyway.

on a well tuned YFZ with proper squish you can run 13-1 on pump premium at sea level , I build them to run constant high rpm for extended periods of time (FT and TT race builds) .

order your cylinder and piston, mock it up with NO base gasket.

measure the piston deck height (very outer edge of piston over wrist pin), to top of cylinder.

measure the head gasket compressed thickness (all three layers are laying together and flat).

subtract the deck clearance from the head gasket thickness.

subtract that sum from .040" , order that size base gasket.

example piston above deck .005" (neg value if adding into a comp calculator)

deck height .005"
head gasket .035"
quench clearance target .040"

.035 - .005 = .030"

.040 - .030 = .010"

base gasket needed .010"

if piston is below deck (most are), add the deck height and the head gasket values together.
 

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Thanks. One question though. You took the .035 -.012 for deck height and head gasket together. Wouldn't you add them? Or am I missing something. Thanks for the reply. Im ordering as I type.

normally you would add them , but with the shorter Athena CYL and no base gasket the piston may stick up above the cyl.

you understand the concept so it will be easy for you to measure it out.

just shoot for .040 piston to head , .037"-.042" would be fine.

I'll go back and add the difference for a positive or negative deck setup !
 

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And it says its a 06+ spec piston so I would assume I would get a true 12.75:1.

not likely, a shelf CP will be lower in the later year YFZ.

unless they recently redesigned and seperated all of the 04-05 and
06-08 YFZ piston lines, the later years are about a .5 point lower.

I haven't came across anything different in the piston comp height on any of the CP shelf pistons I've used.
 

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Discussion Starter · #9 ·
Ok thanks man the help is appreciated. I think the compression will work out fine for me. Im just trying to set it up right and not have to run race gas while still having some decent compression.
 
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