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DIY YZ250 Cylinder head mill and repair

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Can anyone give me dimensions of the 2005 YZ250 head? I have the bore diameter, but would like to get the diameter and the depth of the bowl.

 

My plan is to measure my 2006 YZ250 squish, adjust to 0.045-0.050" by installing a used and modified 2005 YZ250 head. I damaged this old head back in 2005 (detonation >> piston broken) and have it just kicking around.

I'd like to mill about 0.025 from the face of the 2005 head (to be confirmed w squish measurement on 2006 head) and then mill the squish area and dome just slightly to cut out the old piston marks (~1 mm marks).

 

Before I jump into this little project, I thought I'd ask for the head dimensions so that I can confirm if cutting 1 mm from the squish area and dome is approximately equal to the volume reduction from cutting 0.025 from the face.

 

I currently run AV100LL at 0-1000' elevation.

 

Thanks,
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This should help out some.  Dimensions are from a couple different heads which I believe are stock and I took the average of the measurements that didn't agree between heads (everything was with .25mm or so).  I would highly recommend using a burette or similar to cc your head before and after cutting.

 

YZ-250T_HEAD_STOCK.png

 

Head.jpg

 

1.jpg

 

 

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 my Plan was to:

 

use a plexiglass plate with a hole in it to CC the head volume . use a graduated cylinder with colored water and grease the glass to seal. then fill the head chamber until no air exists.  read the volume difference.

 

Then cut your .025" off you head and check the volume again. the difference will be what you need to remove.

 

Then start making cuts to clean up the chamber surface and polish the bowl.  You will need to take measurments  to see how much volume is being removed when you get close to target volume

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Do your self a favor and (dont) put a hole in your plate. Go through the spark plug hole to cc the head. It is a lot easier to get the air bubbles out off a dome than on a flat plate. The spark plug hole takes 2.2cc from bottom to top so just subtract 2.2 for the total to top of plug hole. Just put a thin layer of light grease on the cylinder head wear the o-ring rides to seal the flat plat to it.

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 my Plan was to:

 

use a plexiglass plate with a hole in it to CC the head volume . use a graduated cylinder with colored water and grease the glass to seal. then fill the head chamber until no air exists.  read the volume difference.

 

Then cut your .025" off you head and check the volume again. the difference will be what you need to remove.

 

Then start making cuts to clean up the chamber surface and polish the bowl.  You will need to take measurments  to see how much volume is being removed when you get close to target volume

 

If you check your total CCV installed you can figure how much you have to remove when cutting and be precise. When figuring instead of using your current stroke just use .25mm for the stroke. .25mm = .00984". This will tell you how many cc's .010 will remove on a flat cut off the head. I use rubbing alcohol instead of water but dont leave it in the engine as it will choroid the aluminum after a short period of time.

 

I wish you could buy a burret with a big hole so you can use 5w oil. I friend of mine had one he could pressurize with oil. I have no clue where he got it and he has pass on sense but it did work great.

Edited by Zrt1200

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