combustion chamber

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Nov 3, 2003
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#1
Hello,
I'm in the process of rebuilding my 89 yz 250. I need to know what modifications,if any will need to be done to my cylinder head.I'm using a 92 yz 250 cylinder head with a 1.25mm over bore piston from wiesco in a 89 cylinder.I can tell that the combustion chamber is slightly different,but cannot compare due to the fact that the 89 head is very messed up.Any help would be greatly appreciated.If it matters the bike has a pro circuit exhaust and aftermarket reeds.
Thanks
 
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#2
from what i can tell.The squish band is wider(9.5mm),and the spark plug location is higher(3.5mm higher) on the 92 yz250 cyl. head.I don't have tools to measure the volume.The 89 cylinder head has gone through what looks like major detonation.The squish band area is pretty bad so it makes measurements pretty difficult.I'm pretty new to these types of engines and I would like to know wether this cylinder head is a good choice with the fresh 69.25mm wiesco piston.
 
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#3
the compression would go WAY way down, with the higher plug hole and wider squish band. also, with the squish band wider by 9.5mm, the gasket area will be different, and coolant ports will be aligned different.

go HERE and click on "online parts catelog" to find the part. for relative prices, check out www.ronnies.com but they only go back to 1990. i have had great experiences with them

i really hope that isn't spam~

Rob
 
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#4
Hello Rob,
The squish band is not 9.5 mm larger.It is 9.5mm in total width compared to What looks like 8mm on the old head.The combustion chamber is taller but also seems to be smaller in diameter.The bore of the 92 and the 89 are identical as are the coolant ports . It is impossible for me to know which will give me a higer compression ratio without knowing the volumes of each chamber.I was hoping someone here might know the volumes from the factory.I read a response to a question somewhere and Eric mentioned some particular details that lead me to believe he might know the information.
Thanks for your reply Rob
 

Faded

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#5
Why don't you just cc the heads? Shouldn't take that long or require that much effort.
 
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#6
I need some tools to do the measurement ,plus the old head is pretty messed up.I doubt I would get a usable measurement from the old head to compare.It's not a big deal,I just thought by chance somebody here might have the info.
Thanks for your replies