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Discussion Starter · #1 · (Edited)
Little frustrated right now. This is why i always buy new... Bought an engine from a guy. he said the engine barely had 5 hours on it. He said it had a CP 13.5:1 piston on a Athena Big bore. I didn't like the port job he had on it so i sent the head off. I then saw that it had blow-by on the intake side into the coolant passages. He didn't tell me this so I inspected more. I see the piston is sticking over the cylinder... 0.033 inches over (with the cylinder bolted down) to be exact. So now i am off measuring everything to get it into the right specs...

Here are my measurements:

Piston sticking over 0.033 in./ 0.8382 mm
Base Gasket 0.0145 in./ 0.3693 mm
Head Gasket 0.028 in./ 0.7112 mm

That puts me at -0.005 in. / -0.127 mm squish... not happy. that's contact. Inspected everything. Seems to be fine physically...

Where do you even get a base gasket at 0.0145 in/ 0.37 mm???
This guy's engine builder must have tried really hard to mess this thing up.


Anyway. I need a few things.

Just to be clear, squish needs to be 0.030 - 0.040 inches? or millimeters?
Inches is what i have been reading.


For reference, 0.040 inches squish = 1.016 mm squish

So if i stack 2 athena gaskets 2(.67 mm per Terse)= 1.34 mm
that would make my clearance = 0.134 mm (now below the cylinder)
Add a new head gasket = 0.711 mm
That puts me at 0.845 mm or 0.033 inches squish.

Can anyone verify that the athena gaskets are 0.67 mm thick?
Does anyone know what head gasket is 0.71 mm thick?
And where do i order these gaskets?

Like i said i have never seen a base gasket so small, so i have no idea where this guy's engine builder got the gaskets.

Now, another issue.

How do i know what size the big bore is? Does athena make a 470? or just a 480? The reason i ask is, the Cylinder is 97.78 mm and the Piston is 97.2 mm. CP makes a 97 mm 13.5:1 piston, M1031, and a 98 mm 13.5:1 piston, M1051. The only marking i could find is MX98 on the back side, so i would assume this is a 98 mm piston. I just need to know so i can order the right gaskets.
 

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As far as determining your bore size, you´ll likely just have to measure it. A dial caliper can be easily had at a store like Harbor Freight for around $20... maybe less. From there, you´ll just have to convert your findings to mm, unless you buy a metric one. If you buy a standard one, just take your findings in inches and multiply the value by 25.4 to get your measurement in millimeters. Also, I would see if you can get your hands on a spacer, rather than using two base gaskets. I would be concerned about gasket deflection. Athena, to the best of my knowledge only makes the one big bore kit, but it´s advertised as a 470 kit for the 04-05 YFZ because of the shorter stroke crank. The same kit installed on a 2006+ YFZ will net you a 480 because of the longer stroke crank. Good luck, and let us know what you find out.
 

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Discussion Starter · #4 ·
Thanks for the reply. 97.78 mm is the Bore measurement and 97.2 mm is the piston measurement i got as stated. All these readings were taken with a dial caliper. Talked to an engine builder that said he would measure his 480 kit and report back to me.

He said the same thing you did about the 470 Big bore kit, that it just depends on what year model you put it on. 04-05 will be a 470 and 06+ with be 480 like you stated.

He said that since it was over 97 mm that it was probably a 98 mm kit and piston.

I emailed cometic gaskets to see if they could make me a 1.34 mm base gasket and get me a .71 mm head gasket.
 

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Sounds like you pretty much have it covered. I plan on taking my top-end down relatively soon and machining the base of the jug to get the same effect as the SLHG mod without taking away the reliability of the complete gasket. In doing this, I´ll have to set my squish, of course. I haven´t decided whether to go with OEM or Cometic gaskets yet. I´ll probably just go with OEM so I won´t have to recalculate any of my thicknesses. I also plan on doing my headwork at the same time, so I´ll have some tuning to do afterward, as well. Good luck with your build. Let us know what Cometic says.
 

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Discussion Starter · #6 ·
Well just bought the gaskets. They said they can do it all! I emailed them and ordered directly from them instead of the website they sell their stuff on. Quick and easy.

If doing any custom work i recommend Cometic. They were easy to deal with. This will be the first time using cometic so i cannot comment on the reliablilty. From what i have read they work just fine. I always used OEM before this and never had a problem. Can't go wrong either way.
 

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Sounds like you got it all figured out, cometic gaskets are very reliable. The Athena cylinders are .020"-.030" shorter than OEM cylinders on the carb yfz's. You might want to consider getting your cylinder o-ringed if your going to run high compression on a big bore setup.
 

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Discussion Starter · #10 ·
http://www.yfzcentral.com/forum/32-yfz-powertrain/59824-yfz-athena-480cc.html

Terse showed that they were shorter here. So if you wanted to use a different piston, you had to keep in mind that the athena was shorter.

Where do you guys send the cylinder to get o-ringed? what gasket needs to be used after that?

I'll put it together like this, if i blow a head gasket i'll get it o-ringed.
 

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http://www.yfzcentral.com/forum/32-yfz-powertrain/59824-yfz-athena-480cc.html

Terse showed that they were shorter here. So if you wanted to use a different piston, you had to keep in mind that the athena was shorter.

Where do you guys send the cylinder to get o-ringed? what gasket needs to be used after that?

I'll put it together like this, if i blow a head gasket i'll get it o-ringed.
Send it to Racers Edge or MMAD for an O-ring, use the same Cometic 3 layer gasket with the O-ring.
 

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Discussion Starter · #13 ·
Sounds like you got it all figured out, cometic gaskets are very reliable. The Athena cylinders are .020"-.030" shorter than OEM cylinders on the carb yfz's. You might want to consider getting your cylinder o-ringed if your going to run high compression on a big bore setup.
Sent my cylinder to millennium to be replated and o-ringed. They are asking for dimensions to be o-ringed. OD, ID, and depth? What do i tell them?
 
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